1 <?xml version="1.0" encoding="UTF-8"?>
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2 <chapter xml:id="opensrf" xmlns="http://docbook.org/ns/docbook" version="5.0" xml:lang="EN"
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3 xmlns:xi="http://www.w3.org/2001/XInclude" xmlns:xlink="http://www.w3.org/1999/xlink">
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5 <title>OpenSRF</title>
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7 <abstract id="_abstract">
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8 <simpara>One of the claimed advantages of
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9 Evergreen over alternative integrated library systems is the underlying Open
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10 Service Request Framework (OpenSRF, pronounced "open surf") architecture. This
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11 article introduces OpenSRF, demonstrates how to build OpenSRF services through
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12 simple code examples, and explains the technical foundations on which OpenSRF
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15 <section id="_introducing_opensrf">
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16 <title>Introducing OpenSRF</title>
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17 <simpara>OpenSRF is a message routing network that offers scalability and failover
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18 support for individual services and entire servers with minimal development and
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19 deployment overhead. You can use OpenSRF to build loosely-coupled applications
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20 that can be deployed on a single server or on clusters of geographically
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21 distributed servers using the same code and minimal configuration changes.
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22 Although copyright statements on some of the OpenSRF code date back to Mike
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23 Rylander’s original explorations in 2000, Evergreen was the first major
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24 application to be developed with, and to take full advantage of, the OpenSRF
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25 architecture starting in 2004. The first official release of OpenSRF was 0.1 in
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26 February 2005 (<ulink url="http://evergreen-ils.org/blog/?p=21">http://evergreen-ils.org/blog/?p=21</ulink>), but OpenSRF’s development
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27 continues a steady pace of enhancement and refinement, with the release of
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28 1.0.0 in October 2008 and the most recent release of 1.2.2 in February 2010.</simpara>
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29 <simpara>OpenSRF is a distinct break from the architectural approach used by previous
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30 library systems and has more in common with modern Web applications. The
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31 traditional "scale-up" approach to serve more transactions is to purchase a
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32 server with more CPUs and more RAM, possibly splitting the load between a Web
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33 server, a database server, and a business logic server. Evergreen, however, is
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34 built on the Open Service Request Framework (OpenSRF) architecture, which
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35 firmly embraces the "scale-out" approach of spreading transaction load over
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36 cheap commodity servers. The <ulink url="http://evergreen-ils.org/blog/?p=56">initial GPLS
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37 PINES hardware cluster</ulink>, while certainly impressive, may have offered the
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38 misleading impression that Evergreen requires a lot of hardware to run.
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39 However, Evergreen and OpenSRF easily scale down to a single server; many
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40 Evergreen libraries run their entire library system on a single server, and
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41 most OpenSRF and Evergreen development occurs on a virtual machine running on a
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42 single laptop or desktop image.</simpara>
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43 <simpara>Another common concern is that the flexibility of OpenSRF’s distributed
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44 architecture makes it complex to configure and to write new applications. This
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45 article demonstrates that OpenSRF itself is an extremely simple architecture on
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46 which one can easily build applications of many kinds – not just library
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47 applications – and that you can use a number of different languages to call and
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48 implement OpenSRF methods with a minimal learning curve. With an application
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49 built on OpenSRF, when you identify a bottleneck in your application’s business
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50 logic layer, you can adjust the number of the processes serving that particular
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51 bottleneck on each of your servers; or if the problem is that your service is
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52 resource-hungry, you could add an inexpensive server to your cluster and
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53 dedicate it to running that resource-hungry service.</simpara>
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54 <simplesect id="_programming_language_support">
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55 <title>Programming language support</title>
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56 <simpara>If you need to develop an entirely new OpenSRF service, you can choose from a
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57 number of different languages in which to implement that service. OpenSRF
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58 client language bindings have been written for C, Java, JavaScript, Perl, and
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59 Python, and service language bindings have been written for C, Perl, and Python.
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60 This article uses Perl examples as a lowest common denominator programming
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61 language. Writing an OpenSRF binding for another language is a relatively small
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62 task if that language offers libraries that support the core technologies on
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63 which OpenSRF depends:</simpara>
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67 <ulink url="http://tools.ietf.org/html/rfc3920">Extensible Messaging and Presence
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68 Protocol</ulink> (XMPP, sometimes referred to as Jabber) - provides the base messaging
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69 infrastructure between OpenSRF clients and services
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74 <ulink url="http://json.org">JavaScript Object Notation</ulink> (JSON) - serializes the content
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75 of each XMPP message in a standardized and concise format
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80 <ulink url="http://memcached.org">memcached</ulink> - provides the caching service
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85 <ulink url="http://tools.ietf.org/html/rfc5424">syslog</ulink> - the standard UNIX logging
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90 <simpara>Unfortunately, the
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91 <ulink url="http://evergreen-ils.org/dokuwiki/doku.php?id=osrf-devel:primer">OpenSRF
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92 reference documentation</ulink>, although augmented by the
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93 <ulink url="http://evergreen-ils.org/dokuwiki/doku.php?id=osrf-devel:terms">OpenSRF
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94 glossary</ulink>, blog posts like <ulink url="http://evergreen-ils.org/blog/?p=36">the description
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95 of OpenSRF and Jabber</ulink>, and even this article, is not a sufficient substitute
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96 for a complete specification on which one could implement a language binding.
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97 The recommended option for would-be developers of another language binding is
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98 to use the Python implementation as the cleanest basis for a port to another
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102 <section id="writing_an_opensrf_service">
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103 <title>Writing an OpenSRF Service</title>
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104 <simpara>Imagine an application architecture in which 10 lines of Perl or Python, using
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105 the data types native to each language, are enough to implement a method that
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106 can then be deployed and invoked seamlessly across hundreds of servers. You
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107 have just imagined developing with OpenSRF – it is truly that simple. Under the
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108 covers, of course, the OpenSRF language bindings do an incredible amount of
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109 work on behalf of the developer. An OpenSRF application consists of one or more
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110 OpenSRF services that expose methods: for example, the <literal>opensrf.simple-text</literal>
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111 <ulink url="http://svn.open-ils.org/trac/OpenSRF/browser/trunk/src/perl/lib/OpenSRF/Application/Demo/SimpleText.pm">demonstration
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112 service</ulink> exposes the <literal>opensrf.simple-text.split()</literal> and
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113 <literal>opensrf.simple-text.reverse()</literal> methods. Each method accepts zero or more
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114 arguments and returns zero or one results. The data types supported by OpenSRF
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115 arguments and results are typical core language data types: strings, numbers,
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116 booleans, arrays, and hashes.</simpara>
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117 <simpara>To implement a new OpenSRF service, perform the following steps:</simpara>
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118 <orderedlist numeration="arabic">
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121 Include the base OpenSRF support libraries
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126 Write the code for each of your OpenSRF methods as separate procedures
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131 Register each method
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136 Add the service definition to the OpenSRF configuration files
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140 <simpara>For example, the following code implements an OpenSRF service. The service
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141 includes one method named <literal>opensrf.simple-text.reverse()</literal> that accepts one
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142 string as input and returns the reversed version of that string:</simpara>
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143 <programlisting language="perl" linenumbering="unnumbered">#!/usr/bin/perl
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145 package OpenSRF::Application::Demo::SimpleText;
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149 use OpenSRF::Application;
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150 use parent qw/OpenSRF::Application/;
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153 my ($self , $conn, $text) = @_;
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154 my $reversed_text = scalar reverse($text);
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155 return $reversed_text;
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158 __PACKAGE__->register_method(
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159 method => 'text_reverse',
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160 api_name => 'opensrf.simple-text.reverse'
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161 );</programlisting>
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162 <simpara>Ten lines of code, and we have a complete OpenSRF service that exposes a single
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163 method and could be deployed quickly on a cluster of servers to meet your
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164 application’s ravenous demand for reversed strings! If you’re unfamiliar with
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165 Perl, the <literal>use OpenSRF::Application; use parent qw/OpenSRF::Application/;</literal>
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166 lines tell this package to inherit methods and properties from the
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167 <literal>OpenSRF::Application</literal> module. For example, the call to
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168 <literal>__PACKAGE__->register_method()</literal> is defined in <literal>OpenSRF::Application</literal> but due to
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169 inheritance is available in this package (named by the special Perl symbol
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170 <literal>__PACKAGE__</literal> that contains the current package name). The <literal>register_method()</literal>
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171 procedure is how we introduce a method to the rest of the OpenSRF world.</simpara>
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172 <simplesect id="serviceRegistration">
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173 <title>Registering a service with the OpenSRF configuration files</title>
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174 <simpara>Two files control most of the configuration for OpenSRF:</simpara>
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178 <literal>opensrf.xml</literal> contains the configuration for the service itself, as well as
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179 a list of which application servers in your OpenSRF cluster should start
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185 <literal>opensrf_core.xml</literal> (often referred to as the "bootstrap configuration"
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186 file) contains the OpenSRF networking information, including the XMPP server
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187 connection credentials for the public and private routers. You only need to touch
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188 this for a new service if the new service needs to be accessible via the
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193 <simpara>Begin by defining the service itself in <literal>opensrf.xml</literal>. To register the
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194 <literal>opensrf.simple-text</literal> service, add the following section to the <literal><apps></literal>
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195 element (corresponding to the XPath <literal>/opensrf/default/apps/</literal>):</simpara>
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196 <programlisting language="xml" linenumbering="unnumbered"><apps>
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197 <opensrf.simple-text> <co id="CO1-1"/>
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198 <keepalive>3</keepalive><co id="CO1-2"/>
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199 <stateless>1</stateless><co id="CO1-3"/>
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200 <language>perl</language><co id="CO1-4"/>
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201 <implementation>OpenSRF::Application::Demo::SimpleText</implementation><co id="CO1-5"/>
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202 <max_requests>100</max_requests><co id="CO1-6"/>
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203 <unix_config>
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204 <max_requests>1000</max_requests> <co id="CO1-7"/>
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205 <unix_log>opensrf.simple-text_unix.log</unix_log> <co id="CO1-8"/>
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206 <unix_sock>opensrf.simple-text_unix.sock</unix_sock><co id="CO1-9"/>
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207 <unix_pid>opensrf.simple-text_unix.pid</unix_pid> <co id="CO1-10"/>
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208 <min_children>5</min_children> <co id="CO1-11"/>
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209 <max_children>15</max_children><co id="CO1-12"/>
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210 <min_spare_children>2</min_spare_children><co id="CO1-13"/>
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211 <max_spare_children>5</max_spare_children> <co id="CO1-14"/>
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212 </unix_config>
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213 </opensrf.simple-text>
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215 <!-- other OpenSRF services registered here... -->
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216 </apps></programlisting>
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218 <callout arearefs="CO1-1">
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220 The element name is the name that the OpenSRF control scripts use to refer
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224 <callout arearefs="CO1-2">
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226 The <literal><keepalive></literal> element specifies the interval (in seconds) between
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227 checks to determine if the service is still running.
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230 <callout arearefs="CO1-3">
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232 The <literal><stateless></literal> element specifies whether OpenSRF clients can call
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233 methods from this service without first having to create a connection to a
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234 specific service backend process for that service. If the value is <literal>1</literal>, then
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235 the client can simply issue a request and the router will forward the request
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236 to an available service and the result will be returned directly to the client.
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239 <callout arearefs="CO1-4">
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241 The <literal><language></literal> element specifies the programming language in which the
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242 service is implemented.
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245 <callout arearefs="CO1-5">
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247 The <literal><implementation></literal> element pecifies the name of the library or module
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248 in which the service is implemented.
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251 <callout arearefs="CO1-6">
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253 (C implementations only): The <literal><max_requests></literal> element, as a direct child
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254 of the service element name, specifies the maximum number of requests a process
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255 serves before it is killed and replaced by a new process.
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258 <callout arearefs="CO1-7">
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260 (Perl implementations only): The <literal><max_requests></literal> element, as a direct
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261 child of the <literal><unix_config></literal> element, specifies the maximum number of requests
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262 a process serves before it is killed and replaced by a new process.
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265 <callout arearefs="CO1-8">
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267 The <literal><unix_log></literal> element specifies the name of the log file for
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268 language-specific log messages such as syntax warnings.
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271 <callout arearefs="CO1-9">
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273 The <literal><unix_sock></literal> element specifies the name of the UNIX socket used for
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274 inter-process communications.
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277 <callout arearefs="CO1-10">
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279 The <literal><unix_pid></literal> element specifies the name of the PID file for the
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280 master process for the service.
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283 <callout arearefs="CO1-11">
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285 The <literal><min_children></literal> element specifies the minimum number of child
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286 processes that should be running at any given time.
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289 <callout arearefs="CO1-12">
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291 The <literal><max_children></literal> element specifies the maximum number of child
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292 processes that should be running at any given time.
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295 <callout arearefs="CO1-13">
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297 The <literal><min_spare_children></literal> element specifies the minimum number of idle
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298 child processes that should be available to handle incoming requests. If there
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299 are fewer than this number of spare child processes, new processes will be
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303 <callout arearefs="CO1-14">
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305 The`<max_spare_children>` element specifies the maximum number of idle
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306 child processes that should be available to handle incoming requests. If there
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307 are more than this number of spare child processes, the extra processes will be
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312 <simpara>To make the service accessible via the public router, you must also
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313 edit the <literal>opensrf_core.xml</literal> configuration file to add the service to the list
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314 of publicly accessible services:</simpara>
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315 <formalpara><title>Making a service publicly accessible in <literal>opensrf_core.xml</literal></title><para>
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316 <programlisting language="xml" linenumbering="unnumbered"><router><co id="CO2-1"/>
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317 <!-- This is the public router. On this router, we only register applications
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318 which should be accessible to everyone on the opensrf network -->
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319 <name>router</name>
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320 <domain>public.localhost</domain><co id="CO2-2"/>
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322 <service>opensrf.math</service>
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323 <service>opensrf.simple-text</service> <co id="CO2-3"/>
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325 </router></programlisting>
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326 </para></formalpara>
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328 <callout arearefs="CO2-1">
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330 This section of the <literal>opensrf_core.xml</literal> file is located at XPath
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331 <literal>/config/opensrf/routers/</literal>.
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334 <callout arearefs="CO2-2">
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336 <literal>public.localhost</literal> is the canonical public router domain in the OpenSRF
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337 installation instructions.
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340 <callout arearefs="CO2-3">
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342 Each <literal><service></literal> element contained in the <literal><services></literal> element
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343 offers their services via the public router as well as the private router.
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347 <simpara>Once you have defined the new service, you must restart the OpenSRF Router
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348 to retrieve the new configuration and start or restart the service itself.</simpara>
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349 <simpara>Complete working examples of the <link linkend="opensrf-core-xml">opensrf_core.xml</link> and
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350 <link linkend="opensrf-xml">opensrf.xml</link> configuration files are included with this article
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351 for your reference.</simpara>
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353 <simplesect id="_calling_an_opensrf_method">
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354 <title>Calling an OpenSRF method</title>
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355 <simpara>OpenSRF clients in any supported language can invoke OpenSRF services in any
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356 supported language. So let’s see a few examples of how we can call our fancy
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357 new <literal>opensrf.simple-text.reverse()</literal> method:</simpara>
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358 <simplesect id="_calling_opensrf_methods_from_the_srfsh_client">
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359 <title>Calling OpenSRF methods from the srfsh client</title>
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360 <simpara><literal>srfsh</literal> is a command-line tool installed with OpenSRF that you can use to call
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361 OpenSRF methods. To call an OpenSRF method, issue the <literal>request</literal> command and
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362 pass the OpenSRF service and method name as the first two arguments; then pass
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363 one or more JSON objects delimited by commas as the arguments to the method
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364 being invoked.</simpara>
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365 <simpara>The following example calls the <literal>opensrf.simple-text.reverse</literal> method of the
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366 <literal>opensrf.simple-text</literal> OpenSRF service, passing the string <literal>"foobar"</literal> as the
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367 only method argument:</simpara>
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368 <programlisting language="sh" linenumbering="unnumbered">$ srfsh
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369 srfsh # request opensrf.simple-text opensrf.simple-text.reverse "foobar"
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371 Received Data: "raboof"
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373 =------------------------------------
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374 Request Completed Successfully
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375 Request Time in seconds: 0.016718
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376 =------------------------------------</programlisting>
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378 <simplesect id="opensrfIntrospection">
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379 <title>Getting documentation for OpenSRF methods from the srfsh client</title>
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380 <simpara>The <literal>srfsh</literal> client also gives you command-line access to retrieving metadata
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381 about OpenSRF services and methods. For a given OpenSRF method, for example,
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382 you can retrieve information such as the minimum number of required arguments,
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383 the data type and a description of each argument, the package or library in
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384 which the method is implemented, and a description of the method. To retrieve
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385 the documentation for an opensrf method from <literal>srfsh</literal>, issue the <literal>introspect</literal>
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386 command, followed by the name of the OpenSRF service and (optionally) the
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387 name of the OpenSRF method. If you do not pass a method name to the <literal>introspect</literal>
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388 command, <literal>srfsh</literal> lists all of the methods offered by the service. If you pass
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389 a partial method name, <literal>srfsh</literal> lists all of the methods that match that portion
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390 of the method name.</simpara>
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391 <note><simpara>The quality and availability of the descriptive information for each
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392 method depends on the developer to register the method with complete and
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393 accurate information. The quality varies across the set of OpenSRF and
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394 Evergreen APIs, although some effort is being put towards improving the
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395 state of the internal documentation.</simpara></note>
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396 <programlisting language="sh" linenumbering="unnumbered">srfsh# introspect opensrf.simple-text "opensrf.simple-text.reverse"
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397 --> opensrf.simple-text
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400 "__c":"opensrf.simple-text",
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403 "stream":0, <co id="CO3-1"/>
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404 "object_hint":"OpenSRF_Application_Demo_SimpleText",
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406 "package":"OpenSRF::Application::Demo::SimpleText", <co id="CO3-2"/>
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407 "api_name":"opensrf.simple-text.reverse",<co id="CO3-3"/>
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408 "server_class":"opensrf.simple-text",
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409 "signature":{ <co id="CO3-4"/>
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410 "params":[ <co id="CO3-5"/>
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412 "desc":"The string to reverse",
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417 "desc":"Returns the input string in reverse order\n", <co id="CO3-6"/>
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418 "return":{ <co id="CO3-7"/>
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419 "desc":"Returns the input string in reverse order",
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423 "method":"text_reverse", <co id="CO3-8"/>
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424 "argc":1 <co id="CO3-9"/>
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428 <callout arearefs="CO3-1">
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430 <literal>stream</literal> denotes whether the method supports streaming responses or not.
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433 <callout arearefs="CO3-2">
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435 <literal>package</literal> identifies which package or library implements the method.
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438 <callout arearefs="CO3-3">
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440 <literal>api_name</literal> identifies the name of the OpenSRF method.
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443 <callout arearefs="CO3-4">
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445 <literal>signature</literal> is a hash that describes the parameters for the method.
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448 <callout arearefs="CO3-5">
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450 <literal>params</literal> is an array of hashes describing each parameter in the method;
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451 each parameter has a description (<literal>desc</literal>), name (<literal>name</literal>), and type (<literal>type</literal>).
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454 <callout arearefs="CO3-6">
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456 <literal>desc</literal> is a string that describes the method itself.
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459 <callout arearefs="CO3-7">
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461 <literal>return</literal> is a hash that describes the return value for the method; it
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462 contains a description of the return value (<literal>desc</literal>) and the type of the
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463 returned value (<literal>type</literal>).
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466 <callout arearefs="CO3-8">
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468 <literal>method</literal> identifies the name of the function or method in the source
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472 <callout arearefs="CO3-9">
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474 <literal>argc</literal> is an integer describing the minimum number of arguments that
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475 must be passed to this method.
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480 <simplesect id="_calling_opensrf_methods_from_perl_applications">
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481 <title>Calling OpenSRF methods from Perl applications</title>
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482 <simpara>To call an OpenSRF method from Perl, you must connect to the OpenSRF service,
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483 issue the request to the method, and then retrieve the results.</simpara>
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484 <programlisting language="perl" linenumbering="unnumbered">#/usr/bin/perl
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486 use OpenSRF::AppSession;
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487 use OpenSRF::System;
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489 OpenSRF::System->bootstrap_client(config_file => '/openils/conf/opensrf_core.xml');<co id="CO4-1"/>
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491 my $session = OpenSRF::AppSession->create("opensrf.simple-text");<co id="CO4-2"/>
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493 print "substring: Accepts a string and a number as input, returns a string\n";
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494 my $result = $session->request("opensrf.simple-text.substring", "foobar", 3);<co id="CO4-3"/>
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495 my $request = $result->gather(); <co id="CO4-4"/>
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496 print "Substring: $request\n\n";
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498 print "split: Accepts two strings as input, returns an array of strings\n";
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499 $request = $session->request("opensrf.simple-text.split", "This is a test", " ");<co id="CO4-5"/>
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500 my $output = "Split: [";
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502 while ($element = $request->recv()) { <co id="CO4-6"/>
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503 $output .= $element->content . ", "; <co id="CO4-7"/>
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505 $output =~ s/, $/]/;
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506 print $output . "\n\n";
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508 print "statistics: Accepts an array of strings as input, returns a hash\n";
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509 my @many_strings = [
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510 "First I think I'll have breakfast",
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511 "Then I think that lunch would be nice",
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512 "And then seventy desserts to finish off the day"
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515 $result = $session->request("opensrf.simple-text.statistics", @many_strings); <co id="CO4-8"/>
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516 $request = $result->gather(); <co id="CO4-9"/>
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517 print "Length: " . $result->{'length'} . "\n";
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518 print "Word count: " . $result->{'word_count'} . "\n";
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520 $session->disconnect(); <co id="CO4-10"/></programlisting>
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522 <callout arearefs="CO4-1">
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524 The <literal>OpenSRF::System->bootstrap_client()</literal> method reads the OpenSRF
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525 configuration information from the indicated file and creates an XMPP client
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526 connection based on that information.
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529 <callout arearefs="CO4-2">
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531 The <literal>OpenSRF::AppSession->create()</literal> method accepts one argument - the name
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532 of the OpenSRF service to which you want to want to make one or more requests -
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533 and returns an object prepared to use the client connection to make those
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537 <callout arearefs="CO4-3">
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539 The <literal>OpenSRF::AppSession->request()</literal> method accepts a minimum of one
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540 argument - the name of the OpenSRF method to which you want to make a request -
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541 followed by zero or more arguments to pass to the OpenSRF method as input
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542 values. This example passes a string and an integer to the
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543 <literal>opensrf.simple-text.substring</literal> method defined by the <literal>opensrf.simple-text</literal>
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547 <callout arearefs="CO4-4">
\r
549 The <literal>gather()</literal> method, called on the result object returned by the
\r
550 <literal>request()</literal> method, iterates over all of the possible results from the result
\r
551 object and returns a single variable.
\r
554 <callout arearefs="CO4-5">
\r
556 This <literal>request()</literal> call passes two strings to the <literal>opensrf.simple-text.split</literal>
\r
557 method defined by the <literal>opensrf.simple-text</literal> OpenSRF service and returns (via
\r
558 <literal>gather()</literal>) a reference to an array of results.
\r
561 <callout arearefs="CO4-6">
\r
563 The <literal>opensrf.simple-text.split()</literal> method is a streaming method that
\r
564 returns an array of results with one element per <literal>recv()</literal> call on the
\r
565 result object. We could use the <literal>gather()</literal> method to retrieve all of the
\r
566 results in a single array reference, but instead we simply iterate over
\r
567 the result variable until there are no more results to retrieve.
\r
570 <callout arearefs="CO4-7">
\r
572 While the <literal>gather()</literal> convenience method returns only the content of the
\r
573 complete set of results for a given request, the <literal>recv()</literal> method returns an
\r
574 OpenSRF result object with <literal>status</literal>, <literal>statusCode</literal>, and <literal>content</literal> fields as
\r
575 we saw in <link linkend="OpenSRFOverHTTP">the HTTP results example</link>.
\r
578 <callout arearefs="CO4-8">
\r
580 This <literal>request()</literal> call passes an array to the
\r
581 <literal>opensrf.simple-text.statistics</literal> method defined by the <literal>opensrf.simple-text</literal>
\r
585 <callout arearefs="CO4-9">
\r
587 The result object returns a hash reference via <literal>gather()</literal>. The hash
\r
588 contains the <literal>length</literal> and <literal>word_count</literal> keys we defined in the method.
\r
591 <callout arearefs="CO4-10">
\r
593 The <literal>OpenSRF::AppSession->disconnect()</literal> method closes the XMPP client
\r
594 connection and cleans up resources associated with the session.
\r
600 <simplesect id="_accepting_and_returning_more_interesting_data_types">
\r
601 <title>Accepting and returning more interesting data types</title>
\r
602 <simpara>Of course, the example of accepting a single string and returning a single
\r
603 string is not very interesting. In real life, our applications tend to pass
\r
604 around multiple arguments, including arrays and hashes. Fortunately, OpenSRF
\r
605 makes that easy to deal with; in Perl, for example, returning a reference to
\r
606 the data type does the right thing. In the following example of a method that
\r
607 returns a list, we accept two arguments of type string: the string to be split,
\r
608 and the delimiter that should be used to split the string.</simpara>
\r
609 <formalpara><title>Basic text splitting method</title><para>
\r
610 <programlisting language="perl" linenumbering="unnumbered">sub text_split {
\r
614 my $delimiter = shift || ' ';
\r
616 my @split_text = split $delimiter, $text;
\r
617 return \@split_text;
\r
620 __PACKAGE__->register_method(
\r
621 method => 'text_split',
\r
622 api_name => 'opensrf.simple-text.split'
\r
623 );</programlisting>
\r
624 </para></formalpara>
\r
625 <simpara>We simply return a reference to the list, and OpenSRF does the rest of the work
\r
626 for us to convert the data into the language-independent format that is then
\r
627 returned to the caller. As a caller of a given method, you must rely on the
\r
628 documentation used to register to determine the data structures - if the developer has
\r
629 added the appropriate documentation.</simpara>
\r
631 <simplesect id="_accepting_and_returning_evergreen_objects">
\r
632 <title>Accepting and returning Evergreen objects</title>
\r
633 <simpara>OpenSRF is agnostic about objects; its role is to pass JSON back and forth
\r
634 between OpenSRF clients and services, and it allows the specific clients and
\r
635 services to define their own semantics for the JSON structures. On top of that
\r
636 infrastructure, Evergreen offers the fieldmapper: an object-relational mapper
\r
637 that provides a complete definition of all objects, their properties, their
\r
638 relationships to other objects, the permissions required to create, read,
\r
639 update, or delete objects of that type, and the database table or view on which
\r
640 they are based.</simpara>
\r
641 <simpara>The Evergreen fieldmapper offers a great deal of convenience for working with
\r
642 complex system objects beyond the basic mapping of classes to database
\r
643 schemas. Although the result is passed over the wire as a JSON object
\r
644 containing the indicated fields, fieldmapper-aware clients then turn those
\r
645 JSON objects into native objects with setter / getter methods for each field.</simpara>
\r
646 <simpara>All of this metadata about Evergreen objects is defined in the
\r
647 fieldmapper configuration file (<literal>/openils/conf/fm_IDL.xml</literal>), and access to
\r
648 these classes is provided by the <literal>open-ils.cstore</literal>, <literal>open-ils.pcrud</literal>, and
\r
649 <literal>open-ils.reporter-store</literal> OpenSRF services which parse the fieldmapper
\r
650 configuration file and dynamically register OpenSRF methods for creating,
\r
651 reading, updating, and deleting all of the defined classes.</simpara>
\r
652 <formalpara><title>Example fieldmapper class definition for "Open User Summary"</title><para>
\r
653 <programlisting language="xml" linenumbering="unnumbered"><class id="mous" controller="open-ils.cstore open-ils.pcrud"
\r
654 oils_obj:fieldmapper="money::open_user_summary"
\r
655 oils_persist:tablename="money.open_usr_summary"
\r
656 reporter:label="Open User Summary"> <co id="CO5-1"/>
\r
657 <fields oils_persist:primary="usr" oils_persist:sequence=""> <co id="CO5-2"/>
\r
658 <field name="balance_owed" reporter:datatype="money" /> <co id="CO5-3"/>
\r
659 <field name="total_owed" reporter:datatype="money" />
\r
660 <field name="total_paid" reporter:datatype="money" />
\r
661 <field name="usr" reporter:datatype="link"/>
\r
664 <link field="usr" reltype="has_a" key="id" map="" class="au"/><co id="CO5-4"/>
\r
666 <permacrud xmlns="http://open-ils.org/spec/opensrf/IDL/permacrud/v1"><co id="CO5-5"/>
\r
668 <retrieve permission="VIEW_USER"><co id="CO5-6"/>
\r
669 <context link="usr" field="home_ou"/><co id="CO5-7"/>
\r
673 </class></programlisting>
\r
674 </para></formalpara>
\r
676 <callout arearefs="CO5-1">
\r
678 The <literal><class></literal> element defines the class:
\r
683 The <literal>id</literal> attribute defines the <emphasis>class hint</emphasis> that identifies the class both
\r
684 elsewhere in the fieldmapper configuration file, such as in the value of the
\r
685 <literal>field</literal> attribute of the <literal><link></literal> element, and in the JSON object itself when
\r
686 it is instantiated. For example, an "Open User Summary" JSON object would have
\r
687 the top level property of <literal>"__c":"mous"</literal>.
\r
692 The <literal>controller</literal> attribute identifies the services that have direct access
\r
693 to this class. If <literal>open-ils.pcrud</literal> is not listed, for example, then there is
\r
694 no means to directly access members of this class through a public service.
\r
699 The <literal>oils_obj:fieldmapper</literal> attribute defines the name of the Perl
\r
700 fieldmapper class that will be dynamically generated to provide setter and
\r
701 getter methods for instances of the class.
\r
706 The <literal>oils_persist:tablename</literal> attribute identifies the schema name and table
\r
707 name of the database table that stores the data that represents the instances
\r
708 of this class. In this case, the schema is <literal>money</literal> and the table is
\r
709 <literal>open_usr_summary</literal>.
\r
714 The <literal>reporter:label</literal> attribute defines a human-readable name for the class
\r
715 used in the reporting interface to identify the class. These names are defined
\r
716 in English in the fieldmapper configuration file; however, they are extracted
\r
717 so that they can be translated and served in the user’s language of choice.
\r
722 <callout arearefs="CO5-2">
\r
724 The <literal><fields></literal> element lists all of the fields that belong to the object.
\r
729 The <literal>oils_persist:primary</literal> attribute identifies the field that acts as the
\r
730 primary key for the object; in this case, the field with the name <literal>usr</literal>.
\r
735 The <literal>oils_persist:sequence</literal> attribute identifies the sequence object
\r
736 (if any) in this database provides values for new instances of this class. In
\r
737 this case, the primary key is defined by a field that is linked to a different
\r
738 table, so no sequence is used to populate these instances.
\r
743 <callout arearefs="CO5-3">
\r
745 Each <literal><field></literal> element defines a single field with the following attributes:
\r
750 The <literal>name</literal> attribute identifies the column name of the field in the
\r
751 underlying database table as well as providing a name for the setter / getter
\r
752 method that can be invoked in the JSON or native version of the object.
\r
757 The <literal>reporter:datatype</literal> attribute defines how the reporter should treat
\r
758 the contents of the field for the purposes of querying and display.
\r
763 The <literal>reporter:label</literal> attribute can be used to provide a human-readable name
\r
764 for each field; without it, the reporter falls back to the value of the <literal>name</literal>
\r
770 <callout arearefs="CO5-4">
\r
772 The <literal><links></literal> element contains a set of zero or more <literal><link></literal> elements,
\r
773 each of which defines a relationship between the class being described and
\r
779 The <literal>field</literal> attribute identifies the field named in this class that links
\r
780 to the external class.
\r
785 The <literal>reltype</literal> attribute identifies the kind of relationship between the
\r
786 classes; in the case of <literal>has_a</literal>, each value in the <literal>usr</literal> field is guaranteed
\r
787 to have a corresponding value in the external class.
\r
792 The <literal>key</literal> attribute identifies the name of the field in the external
\r
793 class to which this field links.
\r
798 The rarely-used <literal>map</literal> attribute identifies a second class to which
\r
799 the external class links; it enables this field to define a direct
\r
800 relationship to an external class with one degree of separation, to
\r
801 avoid having to retrieve all of the linked members of an intermediate
\r
802 class just to retrieve the instances from the actual desired target class.
\r
807 The <literal>class</literal> attribute identifies the external class to which this field
\r
813 <callout arearefs="CO5-5">
\r
815 The <literal><permacrud></literal> element defines the permissions that must have been
\r
816 granted to a user to operate on instances of this class.
\r
819 <callout arearefs="CO5-6">
\r
821 The <literal><retrieve></literal> element is one of four possible children of the
\r
822 <literal><actions></literal> element that define the permissions required for each action:
\r
823 create, retrieve, update, and delete.
\r
828 The <literal>permission</literal> attribute identifies the name of the permission that must
\r
829 have been granted to the user to perform the action.
\r
834 The <literal>contextfield</literal> attribute, if it exists, defines the field in this class
\r
835 that identifies the library within the system for which the user must have
\r
836 prvileges to work. If a user has been granted a given permission, but has not been
\r
837 granted privileges to work at a given library, they can not perform the action
\r
843 <callout arearefs="CO5-7">
\r
845 The rarely-used <literal><context></literal> element identifies a linked field (<literal>link</literal>
\r
846 attribute) in this class which links to an external class that holds the field
\r
847 (<literal>field</literal> attribute) that identifies the library within the system for which the
\r
848 user must have privileges to work.
\r
852 <simpara>When you retrieve an instance of a class, you can ask for the result to
\r
853 <emphasis>flesh</emphasis> some or all of the linked fields of that class, so that the linked
\r
854 instances are returned embedded directly in your requested instance. In that
\r
855 same request you can ask for the fleshed instances to in turn have their linked
\r
856 fields fleshed. By bundling all of this into a single request and result
\r
857 sequence, you can avoid the network overhead of requiring the client to request
\r
858 the base object, then request each linked object in turn.</simpara>
\r
859 <simpara>You can also iterate over a collection of instances and set the automatically
\r
860 generated <literal>isdeleted</literal>, <literal>isupdated</literal>, or <literal>isnew</literal> properties to indicate that
\r
861 the given instance has been deleted, updated, or created respectively.
\r
862 Evergreen can then act in batch mode over the collection to perform the
\r
863 requested actions on any of the instances that have been flagged for action.</simpara>
\r
865 <simplesect id="_returning_streaming_results">
\r
866 <title>Returning streaming results</title>
\r
867 <simpara>In the previous implementation of the <literal>opensrf.simple-text.split</literal> method, we
\r
868 returned a reference to the complete array of results. For small values being
\r
869 delivered over the network, this is perfectly acceptable, but for large sets of
\r
870 values this can pose a number of problems for the requesting client. Consider a
\r
871 service that returns a set of bibliographic records in response to a query like
\r
872 "all records edited in the past month"; if the underlying database is
\r
873 relatively active, that could result in thousands of records being returned as
\r
874 a single network request. The client would be forced to block until all of the
\r
875 results are returned, likely resulting in a significant delay, and depending on
\r
876 the implementation, correspondingly large amounts of memory might be consumed
\r
877 as all of the results are read from the network in a single block.</simpara>
\r
878 <simpara>OpenSRF offers a solution to this problem. If the method returns results that
\r
879 can be divided into separate meaningful units, you can register the OpenSRF
\r
880 method as a streaming method and enable the client to loop over the results one
\r
881 unit at a time until the method returns no further results. In addition to
\r
882 registering the method with the provided name, OpenSRF also registers an additional
\r
883 method with <literal>.atomic</literal> appended to the method name. The <literal>.atomic</literal> variant gathers
\r
884 all of the results into a single block to return to the client, giving the caller
\r
885 the ability to choose either streaming or atomic results from a single method
\r
886 definition.</simpara>
\r
887 <simpara>In the following example, the text splitting method has been reimplemented to
\r
888 support streaming; very few changes are required:</simpara>
\r
889 <formalpara><title>Text splitting method - streaming mode</title><para>
\r
890 <programlisting language="perl" linenumbering="unnumbered">sub text_split {
\r
894 my $delimiter = shift || ' ';
\r
896 my @split_text = split $delimiter, $text;
\r
897 foreach my $string (@split_text) { <co id="CO6-1"/>
\r
898 $conn->respond($string);
\r
903 __PACKAGE__->register_method(
\r
904 method => 'text_split',
\r
905 api_name => 'opensrf.simple-text.split',
\r
906 stream => 1<co id="CO6-2"/>
\r
907 );</programlisting>
\r
908 </para></formalpara>
\r
910 <callout arearefs="CO6-1">
\r
912 Rather than returning a reference to the array, a streaming method loops
\r
913 over the contents of the array and invokes the <literal>respond()</literal> method of the
\r
914 connection object on each element of the array.
\r
917 <callout arearefs="CO6-2">
\r
919 Registering the method as a streaming method instructs OpenSRF to also
\r
920 register an atomic variant (<literal>opensrf.simple-text.split.atomic</literal>).
\r
925 <simplesect id="_error_warning_info_debug">
\r
926 <title>Error! Warning! Info! Debug!</title>
\r
927 <simpara>As hard as it may be to believe, it is true: applications sometimes do not
\r
928 behave in the expected manner, particularly when they are still under
\r
929 development. The service language bindings for OpenSRF include integrated
\r
930 support for logging messages at the levels of ERROR, WARNING, INFO, DEBUG, and
\r
931 the extremely verbose INTERNAL to either a local file or to a syslogger
\r
932 service. The destination of the log files, and the level of verbosity to be
\r
933 logged, is set in the <literal>opensrf_core.xml</literal> configuration file. To add logging to
\r
934 our Perl example, we just have to add the <literal>OpenSRF::Utils::Logger</literal> package to our
\r
935 list of used Perl modules, then invoke the logger at the desired logging level.</simpara>
\r
936 <simpara>You can include many calls to the OpenSRF logger; only those that are higher
\r
937 than your configured logging level will actually hit the log. The following
\r
938 example exercises all of the available logging levels in OpenSRF:</simpara>
\r
939 <programlisting language="perl" linenumbering="unnumbered">use OpenSRF::Utils::Logger;
\r
940 my $logger = OpenSRF::Utils::Logger;
\r
941 # some code in some function
\r
943 $logger->error("Hmm, something bad DEFINITELY happened!");
\r
944 $logger->warn("Hmm, something bad might have happened.");
\r
945 $logger->info("Something happened.");
\r
946 $logger->debug("Something happened; here are some more details.");
\r
947 $logger->internal("Something happened; here are all the gory details.")
\r
949 <simpara>If you call the mythical OpenSRF method containing the preceding OpenSRF logger
\r
950 statements on a system running at the default logging level of INFO, you will
\r
951 only see the INFO, WARN, and ERR messages, as follows:</simpara>
\r
952 <formalpara><title>Results of logging calls at the default level of INFO</title><para>
\r
953 <screen>[2010-03-17 22:27:30] opensrf.simple-text [ERR :5681:SimpleText.pm:277:] Hmm, something bad DEFINITELY happened!
\r
954 [2010-03-17 22:27:30] opensrf.simple-text [WARN:5681:SimpleText.pm:278:] Hmm, something bad might have happened.
\r
955 [2010-03-17 22:27:30] opensrf.simple-text [INFO:5681:SimpleText.pm:279:] Something happened.</screen>
\r
956 </para></formalpara>
\r
957 <simpara>If you then increase the the logging level to INTERNAL (5), the logs will
\r
958 contain much more information, as follows:</simpara>
\r
959 <formalpara><title>Results of logging calls at the default level of INTERNAL</title><para>
\r
960 <screen>[2010-03-17 22:48:11] opensrf.simple-text [ERR :5934:SimpleText.pm:277:] Hmm, something bad DEFINITELY happened!
\r
961 [2010-03-17 22:48:11] opensrf.simple-text [WARN:5934:SimpleText.pm:278:] Hmm, something bad might have happened.
\r
962 [2010-03-17 22:48:11] opensrf.simple-text [INFO:5934:SimpleText.pm:279:] Something happened.
\r
963 [2010-03-17 22:48:11] opensrf.simple-text [DEBG:5934:SimpleText.pm:280:] Something happened; here are some more details.
\r
964 [2010-03-17 22:48:11] opensrf.simple-text [INTL:5934:SimpleText.pm:281:] Something happened; here are all the gory details.
\r
965 [2010-03-17 22:48:11] opensrf.simple-text [ERR :5934:SimpleText.pm:283:] Resolver did not find a cache hit
\r
966 [2010-03-17 22:48:21] opensrf.simple-text [INTL:5934:Cache.pm:125:] Stored opensrf.simple-text.test_cache.masaa => "here" in memcached server
\r
967 [2010-03-17 22:48:21] opensrf.simple-text [DEBG:5934:Application.pm:579:] Coderef for [OpenSRF::Application::Demo::SimpleText::test_cache]...
\r
968 [2010-03-17 22:48:21] opensrf.simple-text [DEBG:5934:Application.pm:586:] A top level Request object is responding de nada
\r
969 [2010-03-17 22:48:21] opensrf.simple-text [DEBG:5934:Application.pm:190:] Method duration for [opensrf.simple-text.test_cache]: 10.005
\r
970 [2010-03-17 22:48:21] opensrf.simple-text [INTL:5934:AppSession.pm:780:] Calling queue_wait(0)
\r
971 [2010-03-17 22:48:21] opensrf.simple-text [INTL:5934:AppSession.pm:769:] Resending...0
\r
972 [2010-03-17 22:48:21] opensrf.simple-text [INTL:5934:AppSession.pm:450:] In send
\r
973 [2010-03-17 22:48:21] opensrf.simple-text [DEBG:5934:AppSession.pm:506:] AppSession sending RESULT to opensrf@private.localhost/...
\r
974 [2010-03-17 22:48:21] opensrf.simple-text [DEBG:5934:AppSession.pm:506:] AppSession sending STATUS to opensrf@private.localhost/...
\r
976 </para></formalpara>
\r
977 <simpara>To see everything that is happening in OpenSRF, try leaving your logging level
\r
978 set to INTERNAL for a few minutes - just ensure that you have a lot of free disk
\r
979 space available if you have a moderately busy system!</simpara>
\r
981 <simplesect id="_caching_results_one_secret_of_scalability">
\r
982 <title>Caching results: one secret of scalability</title>
\r
983 <simpara>If you have ever used an application that depends on a remote Web service
\r
984 outside of your control — say, if you need to retrieve results from a
\r
985 microblogging service — you know the pain of latency and dependability (or the
\r
986 lack thereof). To improve the response time for OpenSRF services, you can take
\r
987 advantage of the support offered by the <literal>OpenSRF::Utils::Cache</literal> module for
\r
988 communicating with a local instance or cluster of <literal>memcache</literal> daemons to store
\r
989 and retrieve persistent values. The following example demonstrates caching
\r
990 by sleeping for 10 seconds the first time it receives a given cache key and
\r
991 cannot retrieve a corresponding value from the cache:</simpara>
\r
992 <formalpara><title>Simple caching OpenSRF service</title><para>
\r
993 <programlisting language="perl" linenumbering="unnumbered">use OpenSRF::Utils::Cache;<co id="CO7-1"/>
\r
997 my $test_key = shift;
\r
998 my $cache = OpenSRF::Utils::Cache->new('global'); <co id="CO7-2"/>
\r
999 my $cache_key = "opensrf.simple-text.test_cache.$test_key"; <co id="CO7-3"/>
\r
1000 my $result = $cache->get_cache($cache_key) || undef; <co id="CO7-4"/>
\r
1002 $logger->info("Resolver found a cache hit");
\r
1005 sleep 10; <co id="CO7-5"/>
\r
1006 my $cache_timeout = 300; <co id="CO7-6"/>
\r
1007 $cache->put_cache($cache_key, "here", $cache_timeout); <co id="CO7-7"/>
\r
1008 return "There was no cache hit.";
\r
1009 }</programlisting>
\r
1010 </para></formalpara>
\r
1012 <callout arearefs="CO7-1">
\r
1014 The OpenSRF::Utils::Cache module provides access to the built-in caching
\r
1015 support in OpenSRF.
\r
1018 <callout arearefs="CO7-2">
\r
1020 The constructor for the cache object accepts a single argument to define
\r
1021 the cache type for the object. Each cache type can use a separate <literal>memcache</literal>
\r
1022 server to keep the caches separated. Most Evergreen services use the <literal>global</literal>
\r
1023 cache, while the <literal>anon</literal> cache is used for Web sessions.
\r
1026 <callout arearefs="CO7-3">
\r
1028 The cache key is simply a string that uniquely identifies the value you
\r
1029 want to store or retrieve. This line creates a cache key based on the OpenSRF
\r
1030 method name and request input value.
\r
1033 <callout arearefs="CO7-4">
\r
1035 The <literal>get_cache()</literal> method checks to see if the cache key already exists. If
\r
1036 a matching key is found, the service immediately returns the stored value.
\r
1039 <callout arearefs="CO7-5">
\r
1041 If the cache key does not exist, the code sleeps for 10 seconds to
\r
1042 simulate a call to a slow remote Web service or an intensive process.
\r
1045 <callout arearefs="CO7-6">
\r
1047 The <literal>$cache_timeout</literal> variable represents a value for the lifetime of the
\r
1048 cache key in seconds.
\r
1051 <callout arearefs="CO7-7">
\r
1053 After the code retrieves its value (or, in the case of this example,
\r
1054 finishes sleeping), it creates the cache entry by calling the <literal>put_cache()</literal>
\r
1055 method. The method accepts three arguments: the cache key, the value to be
\r
1056 stored ("here"), and the timeout value in seconds to ensure that we do not
\r
1057 return stale data on subsequent calls.
\r
1062 <simplesect id="_initializing_the_service_and_its_children_child_labour">
\r
1063 <title>Initializing the service and its children: child labour</title>
\r
1064 <simpara>When an OpenSRF service is started, it looks for a procedure called
\r
1065 <literal>initialize()</literal> to set up any global variables shared by all of the children of
\r
1066 the service. The <literal>initialize()</literal> procedure is typically used to retrieve
\r
1067 configuration settings from the <literal>opensrf.xml</literal> file.</simpara>
\r
1068 <simpara>An OpenSRF service spawns one or more children to actually do the work
\r
1069 requested by callers of the service. For every child process an OpenSRF service
\r
1070 spawns, the child process clones the parent environment and then each child
\r
1071 process runs the <literal>child_init()</literal> process (if any) defined in the OpenSRF service
\r
1072 to initialize any child-specific settings.</simpara>
\r
1073 <simpara>When the OpenSRF service kills a child process, it invokes the <literal>child_exit()</literal>
\r
1074 procedure (if any) to clean up any resources associated with the child process.
\r
1075 Similarly, when the OpenSRF service is stopped, it calls the <literal>DESTROY()</literal>
\r
1076 procedure to clean up any remaining resources.</simpara>
\r
1078 <simplesect id="_retrieving_configuration_settings">
\r
1079 <title>Retrieving configuration settings</title>
\r
1080 <simpara>The settings for OpenSRF services are maintained in the <literal>opensrf.xml</literal> XML
\r
1081 configuration file. The structure of the XML document consists of a root
\r
1082 element <literal><opensrf></literal> containing two child elements:</simpara>
\r
1086 The <literal><default></literal> element contains an <literal><apps></literal> element describing all
\r
1087 OpenSRF services running on this system — see <xref linkend="serviceRegistration"/> --, as
\r
1088 well as any other arbitrary XML descriptions required for global configuration
\r
1089 purposes. For example, Evergreen uses this section for email notification and
\r
1090 inter-library patron privacy settings.
\r
1095 The <literal><hosts></literal> element contains one element per host that participates in
\r
1096 this OpenSRF system. Each host element must include an <literal><activeapps></literal> element
\r
1097 that lists all of the services to start on this host when the system starts
\r
1098 up. Each host element can optionally override any of the default settings.
\r
1102 <simpara>OpenSRF includes a service named <literal>opensrf.settings</literal> to provide distributed
\r
1103 cached access to the configuration settings with a simple API:</simpara>
\r
1107 <literal>opensrf.settings.default_config.get</literal> accepts zero arguments and returns
\r
1108 the complete set of default settings as a JSON document.
\r
1113 <literal>opensrf.settings.host_config.get</literal> accepts one argument (hostname) and
\r
1114 returns the complete set of settings, as customized for that hostname, as a
\r
1120 <literal>opensrf.settings.xpath.get</literal> accepts one argument (an
\r
1121 <ulink url="http://www.w3.org/TR/xpath/">XPath</ulink> expression) and returns the portion of
\r
1122 the configuration file that matches the expression as a JSON document.
\r
1126 <simpara>For example, to determine whether an Evergreen system uses the opt-in
\r
1127 support for sharing patron information between libraries, you could either
\r
1128 invoke the <literal>opensrf.settings.default_config.get</literal> method and parse the
\r
1129 JSON document to determine the value, or invoke the <literal>opensrf.settings.xpath.get</literal>
\r
1130 method with the XPath <literal>/opensrf/default/share/user/opt_in</literal> argument to
\r
1131 retrieve the value directly.</simpara>
\r
1132 <simpara>In practice, OpenSRF includes convenience libraries in all of its client
\r
1133 language bindings to simplify access to configuration values. C offers
\r
1134 osrfConfig.c, Perl offers <literal>OpenSRF::Utils::SettingsClient</literal>, Java offers
\r
1135 <literal>org.opensrf.util.SettingsClient</literal>, and Python offers <literal>osrf.set</literal>. These
\r
1136 libraries locally cache the configuration file to avoid network roundtrips for
\r
1137 every request and enable the developer to request specific values without
\r
1138 having to manually construct XPath expressions.</simpara>
\r
1141 <section id="_getting_under_the_covers_with_opensrf">
\r
1142 <title>OpenSRF Communication Flows</title>
\r
1143 <simpara>Now that you have seen that it truly is easy to create an OpenSRF service, we
\r
1144 can take a look at what is going on under the covers to make all of this work
\r
1145 for you.</simpara>
\r
1146 <simplesect id="_get_on_the_messaging_bus_safely">
\r
1147 <title>Get on the messaging bus - safely</title>
\r
1148 <simpara>One of the core innovations of OpenSRF was to use the Extensible Messaging and
\r
1149 Presence Protocol (XMPP, more colloquially known as Jabber) as the messaging
\r
1150 bus that ties OpenSRF services together across servers. XMPP is an "XML
\r
1151 protocol for near-real-time messaging, presence, and request-response services"
\r
1152 (<ulink url="http://www.ietf.org/rfc/rfc3920.txt">http://www.ietf.org/rfc/rfc3920.txt</ulink>) that OpenSRF relies on to handle most of
\r
1153 the complexity of networked communications. OpenSRF requres an XMPP server
\r
1154 that supports multiple domains such as <ulink url="http://www.ejabberd.im/">ejabberd</ulink>.
\r
1155 Multiple domain support means that a single server can support XMPP virtual
\r
1156 hosts with separate sets of users and access privileges per domain. By
\r
1157 routing communications through separate public and private XMPP domains,
\r
1158 OpenSRF services gain an additional layer of security.</simpara>
\r
1159 <simpara>The <ulink url="http://evergreen-ils.org/dokuwiki/doku.php?id=opensrf:1.2:install">OpenSRF
\r
1160 installation documentation</ulink> instructs you to create two separate hostnames
\r
1161 (<literal>private.localhost</literal> and <literal>public.localhost</literal>) to use as XMPP domains. OpenSRF
\r
1162 can control access to its services based on the domain of the client and
\r
1163 whether a given service allows access from clients on the public domain. When
\r
1164 you start OpenSRF, the first XMPP clients that connect to the XMPP server are
\r
1165 the OpenSRF public and private <emphasis>routers</emphasis>. OpenSRF routers maintain a list of
\r
1166 available services and connect clients to available services. When an OpenSRF
\r
1167 service starts, it establishes a connection to the XMPP server and registers
\r
1168 itself with the private router. The OpenSRF configuration contains a list of
\r
1169 public OpenSRF services, each of which must also register with the public
\r
1172 <simplesect id="_opensrf_communication_flows_over_xmpp">
\r
1173 <title>OpenSRF communication flows over XMPP</title>
\r
1174 <simpara>In a minimal OpenSRF deployment, two XMPP users named "router" connect to the
\r
1175 XMPP server, with one connected to the private XMPP domain and one connected to
\r
1176 the public XMPP domain. Similarly, two XMPP users named "opensrf" connect to
\r
1177 the XMPP server via the private and public XMPP domains. When an OpenSRF
\r
1178 service is started, it uses the "opensrf" XMPP user to advertise its
\r
1179 availability with the corresponding router on that XMPP domain; the XMPP server
\r
1180 automatically assigns a Jabber ID (<emphasis>JID</emphasis>) based on the client hostname to each
\r
1181 service’s listener process and each connected drone process waiting to carry
\r
1182 out requests. When an OpenSRF router receives a request to invoke a method on a
\r
1183 given service, it connects the requester to the next available listener in the
\r
1184 list of registered listeners for that service.</simpara>
\r
1185 <simpara>Services and clients connect to the XMPP server using a single set of XMPP
\r
1186 client credentials (for example, <literal>opensrf@private.localhost</literal>), but use XMPP
\r
1187 resource identifiers to differentiate themselves in the JID for each
\r
1188 connection. For example, the JID for a copy of the <literal>opensrf.simple-text</literal>
\r
1189 service with process ID <literal>6285</literal> that has connected to the <literal>private.localhost</literal>
\r
1190 domain using the <literal>opensrf</literal> XMPP client credentials could be
\r
1191 <literal>opensrf@private.localhost/opensrf.simple-text_drone_at_localhost_6285</literal>. By
\r
1192 convention, the user name for OpenSRF clients is <literal>opensrf</literal>, and the user name
\r
1193 for OpenSRF routers is <literal>router</literal>, so the XMPP server for OpenSRF will have four
\r
1194 separate users registered:
\r
1195 * <literal>opensrf@private.localhost</literal> is an OpenSRF client that connects with these
\r
1196 credentials and which can access any OpenSRF service.
\r
1197 * <literal>opensrf@public.localhost</literal> is an OpenSRF client that connects with these
\r
1198 credentials and which can only access OpenSRF services that have registered
\r
1199 with the public router.
\r
1200 * <literal>router@private.localhost</literal> is the private OpenSRF router with which all
\r
1201 services register.
\r
1202 * <literal>router@public.localhost</literal> is the public OpenSRF router with which only
\r
1203 services that must be publicly accessible register.</simpara>
\r
1204 <simpara>All OpenSRF services automatically register themselves with the private XMPP
\r
1205 domain, but only those services that register themselves with the public XMPP
\r
1206 domain can be invoked from public OpenSRF clients. The OpenSRF client and
\r
1207 router user names, passwords, and domain names, along with the list of services
\r
1208 that should be public, are contained in the <literal>opensrf_core.xml</literal> configuration
\r
1211 <simplesect id="OpenSRFOverHTTP">
\r
1212 <title>OpenSRF communication flows over HTTP</title>
\r
1213 <simpara>In some contexts, access to a full XMPP client is not a practical option. For
\r
1214 example, while XMPP clients have been implemented in JavaScript, you might
\r
1215 be concerned about browser compatibility and processing overhead - or you might
\r
1216 want to issue OpenSRF requests from the command line with <literal>curl</literal>. Fortunately,
\r
1217 any OpenSRF service registered with the public router is accessible via the
\r
1218 OpenSRF HTTP Translator. The OpenSRF HTTP Translator implements the
\r
1219 <ulink url="http://www.open-ils.org/dokuwiki/doku.php?id=opensrf_over_http">OpenSRF-over-HTTP
\r
1220 proposed specification</ulink> as an Apache module that translates HTTP requests into
\r
1221 OpenSRF requests and returns OpenSRF results as HTTP results to the initiating
\r
1222 HTTP client.</simpara>
\r
1223 <formalpara><title>Issuing an HTTP POST request to an OpenSRF method via the OpenSRF HTTP Translator</title><para>
\r
1224 <programlisting language="bash" linenumbering="unnumbered"># curl request broken up over multiple lines for legibility
\r
1225 curl -H "X-OpenSRF-service: opensrf.simple-text"<co id="CO8-1"/>
\r
1226 --data 'osrf-msg=[ \<co id="CO8-2"/>
\r
1227 {"__c":"osrfMessage","__p":{"threadTrace":0,"locale":"en-CA", <co id="CO8-3"/>
\r
1228 "type":"REQUEST","payload": {"__c":"osrfMethod","__p":
\r
1229 {"method":"opensrf.simple-text.reverse","params":["foobar"]}
\r
1232 http://localhost/osrf-http-translator <co id="CO8-4"/></programlisting>
\r
1233 </para></formalpara>
\r
1235 <callout arearefs="CO8-1">
\r
1237 The <literal>X-OpenSRF-service</literal> header identifies the OpenSRF service of interest.
\r
1240 <callout arearefs="CO8-2">
\r
1242 The POST request consists of a single parameter, the <literal>osrf-msg</literal> value,
\r
1243 which contains a JSON array.
\r
1246 <callout arearefs="CO8-3">
\r
1248 The first object is an OpenSRF message (<literal>"__c":"osrfMessage"</literal>) with a set of
\r
1249 parameters (<literal>"__p":{}</literal>).
\r
1254 The identifier for the request (<literal>"threadTrace":0</literal>); this value is echoed
\r
1255 back in the result.
\r
1260 The message type (<literal>"type":"REQUEST"</literal>).
\r
1265 The locale for the message; if the OpenSRF method is locale-sensitive, it
\r
1266 can check the locale for each OpenSRF request and return different information
\r
1267 depending on the locale.
\r
1272 The payload of the message (<literal>"payload":{}</literal>) containing the OpenSRF method
\r
1273 request (<literal>"__c":"osrfMethod"</literal>) and its parameters (<literal>"__p:"{}</literal>).
\r
1278 The method name for the request (<literal>"method":"opensrf.simple-text.reverse"</literal>).
\r
1283 A set of JSON parameters to pass to the method (<literal>"params":["foobar"]</literal>); in
\r
1284 this case, a single string <literal>"foobar"</literal>.
\r
1291 <callout arearefs="CO8-4">
\r
1293 The URL on which the OpenSRF HTTP translator is listening,
\r
1294 <literal>/osrf-http-translator</literal> is the default location in the Apache example
\r
1295 configuration files shipped with the OpenSRF source, but this is configurable.
\r
1299 <formalpara><title>Results from an HTTP POST request to an OpenSRF method via the OpenSRF HTTP Translator</title><para>
\r
1300 <programlisting language="bash" linenumbering="unnumbered"># HTTP response broken up over multiple lines for legibility
\r
1301 [{"__c":"osrfMessage","__p": <co id="CO9-1"/>
\r
1302 {"threadTrace":0, "payload": <co id="CO9-2"/>
\r
1303 {"__c":"osrfResult","__p": <co id="CO9-3"/>
\r
1304 {"status":"OK","content":"raboof","statusCode":200} <co id="CO9-4"/>
\r
1305 },"type":"RESULT","locale":"en-CA" <co id="CO9-5"/>
\r
1308 {"__c":"osrfMessage","__p": <co id="CO9-6"/>
\r
1309 {"threadTrace":0,"payload": <co id="CO9-7"/>
\r
1310 {"__c":"osrfConnectStatus","__p": <co id="CO9-8"/>
\r
1311 {"status":"Request Complete","statusCode":205}<co id="CO9-9"/>
\r
1312 },"type":"STATUS","locale":"en-CA" <co id="CO9-10"/>
\r
1314 }]</programlisting>
\r
1315 </para></formalpara>
\r
1317 <callout arearefs="CO9-1">
\r
1319 The OpenSRF HTTP Translator returns an array of JSON objects in its
\r
1320 response. Each object in the response is an OpenSRF message
\r
1321 (<literal>"__c":"osrfMessage"</literal>) with a collection of response parameters (<literal>"__p":</literal>).
\r
1324 <callout arearefs="CO9-2">
\r
1326 The OpenSRF message identifier (<literal>"threadTrace":0</literal>) confirms that this
\r
1327 message is in response to the request matching the same identifier.
\r
1330 <callout arearefs="CO9-3">
\r
1332 The message includes a payload JSON object (<literal>"payload":</literal>) with an OpenSRF
\r
1333 result for the request (<literal>"__c":"osrfResult"</literal>).
\r
1336 <callout arearefs="CO9-4">
\r
1338 The result includes a status indicator string (<literal>"status":"OK"</literal>), the content
\r
1339 of the result response - in this case, a single string "raboof"
\r
1340 (<literal>"content":"raboof"</literal>) - and an integer status code for the request
\r
1341 (<literal>"statusCode":200</literal>).
\r
1344 <callout arearefs="CO9-5">
\r
1346 The message also includes the message type (<literal>"type":"RESULT"</literal>) and the
\r
1347 message locale (<literal>"locale":"en-CA"</literal>).
\r
1350 <callout arearefs="CO9-6">
\r
1352 The second message in the set of results from the response.
\r
1355 <callout arearefs="CO9-7">
\r
1357 Again, the message identifier confirms that this message is in response to
\r
1358 a particular request.
\r
1361 <callout arearefs="CO9-8">
\r
1363 The payload of the message denotes that this message is an
\r
1364 OpenSRF connection status message (<literal>"__c":"osrfConnectStatus"</literal>), with some
\r
1365 information about the particular OpenSRF connection that was used for this
\r
1369 <callout arearefs="CO9-9">
\r
1371 The response parameters for an OpenSRF connection status message include a
\r
1372 verbose status (<literal>"status":"Request Complete"</literal>) and an integer status code for
\r
1373 the connection status (`"statusCode":205).
\r
1376 <callout arearefs="CO9-10">
\r
1378 The message also includes the message type (<literal>"type":"RESULT"</literal>) and the
\r
1379 message locale (<literal>"locale":"en-CA"</literal>).
\r
1383 <tip><simpara>Before adding a new public OpenSRF service, ensure that it does
\r
1384 not introduce privilege escalation or unchecked access to data. For example,
\r
1385 the Evergreen <literal>open-ils.cstore</literal> private service is an object-relational mapper
\r
1386 that provides read and write access to the entire Evergreen database, so it
\r
1387 would be catastrophic to expose that service publicly. In comparison, the
\r
1388 Evergreen <literal>open-ils.pcrud</literal> public service offers the same functionality as
\r
1389 <literal>open-ils.cstore</literal> to any connected HTTP client or OpenSRF client, but the
\r
1390 additional authentication and authorization layer in <literal>open-ils.pcrud</literal> prevents
\r
1391 unchecked access to Evergreen’s data.</simpara></tip>
\r
1393 <simplesect id="_stateless_and_stateful_connections">
\r
1394 <title>Stateless and stateful connections</title>
\r
1395 <simpara>OpenSRF supports both <emphasis>stateless</emphasis> and <emphasis>stateful</emphasis> connections. When an OpenSRF
\r
1396 client issues a <literal>REQUEST</literal> message in a <emphasis>stateless</emphasis> connection, the router
\r
1397 forwards the request to the next available service and the service returns the
\r
1398 result directly to the client.</simpara>
\r
1399 <formalpara><title>REQUEST flow in a stateless connection</title><para><inlinemediaobject>
\r
1401 <imagedata fileref="REQUEST.png"/>
\r
1403 <textobject><phrase>REQUEST flow in a stateless connection</phrase></textobject>
\r
1404 </inlinemediaobject></para></formalpara>
\r
1405 <simpara>When an OpenSRF client issues a <literal>CONNECT</literal> message to create a <emphasis>stateful</emphasis> conection, the
\r
1406 router returns the Jabber ID of the next available service to the client so
\r
1407 that the client can issue one or more <literal>REQUEST</literal> message directly to that
\r
1408 particular service and the service will return corresponding <literal>RESULT</literal> messages
\r
1409 directly to the client. Until the client issues a <literal>DISCONNECT</literal> message, that
\r
1410 particular service is only available to the requesting client. Stateful connections
\r
1411 are useful for clients that need to make many requests from a particular service,
\r
1412 as it avoids the intermediary step of contacting the router for each request, as
\r
1413 well as for operations that require a controlled sequence of commands, such as a
\r
1414 set of database INSERT, UPDATE, and DELETE statements within a transaction.</simpara>
\r
1415 <formalpara><title>CONNECT, REQUEST, and DISCONNECT flow in a stateful connection</title><para><inlinemediaobject>
\r
1417 <imagedata fileref="CONNECT.png"/>
\r
1419 <textobject><phrase>CONNECT</phrase></textobject>
\r
1420 </inlinemediaobject></para></formalpara>
\r
1422 <simplesect id="_message_body_format">
\r
1423 <title>Message body format</title>
\r
1424 <simpara>OpenSRF was an early adopter of JavaScript Object Notation (JSON). While XMPP
\r
1425 is an XML protocol, the Evergreen developers recognized that the compactness of
\r
1426 the JSON format offered a significant reduction in bandwidth for the volume of
\r
1427 messages that would be generated in an application of that size. In addition,
\r
1428 the ability of languages such as JavaScript, Perl, and Python to generate
\r
1429 native objects with minimal parsing offered an attractive advantage over
\r
1430 invoking an XML parser for every message. Instead, the body of the XMPP message
\r
1431 is a simple JSON structure. For a simple request, like the following example
\r
1432 that simply reverses a string, it looks like a significant overhead: but we get
\r
1433 the advantages of locale support and tracing the request from the requester
\r
1434 through the listener and responder (drone).</simpara>
\r
1435 <formalpara><title>A request for opensrf.simple-text.reverse("foobar"):</title><para>
\r
1436 <programlisting language="xml" linenumbering="unnumbered"><message from='router@private.localhost/opensrf.simple-text'
\r
1437 to='opensrf@private.localhost/opensrf.simple-text_listener_at_localhost_6275'
\r
1438 router_from='opensrf@private.localhost/_karmic_126678.3719_6288'
\r
1439 router_to='' router_class='' router_command='' osrf_xid=''
\r
1441 <thread>1266781414.366573.12667814146288</thread>
\r
1444 {"__c":"osrfMessage","__p":
\r
1445 {"threadTrace":"1","locale":"en-US","type":"REQUEST","payload":
\r
1446 {"__c":"osrfMethod","__p":
\r
1447 {"method":"opensrf.simple-text.reverse","params":["foobar"]}
\r
1453 </message></programlisting>
\r
1454 </para></formalpara>
\r
1455 <formalpara><title>A response from opensrf.simple-text.reverse("foobar")</title><para>
\r
1456 <programlisting language="xml" linenumbering="unnumbered"><message from='opensrf@private.localhost/opensrf.simple-text_drone_at_localhost_6285'
\r
1457 to='opensrf@private.localhost/_karmic_126678.3719_6288'
\r
1458 router_command='' router_class='' osrf_xid=''
\r
1460 <thread>1266781414.366573.12667814146288</thread>
\r
1463 {"__c":"osrfMessage","__p":
\r
1464 {"threadTrace":"1","payload":
\r
1465 {"__c":"osrfResult","__p":
\r
1466 {"status":"OK","content":"raboof","statusCode":200}
\r
1467 } ,"type":"RESULT","locale":"en-US"}
\r
1469 {"__c":"osrfMessage","__p":
\r
1470 {"threadTrace":"1","payload":
\r
1471 {"__c":"osrfConnectStatus","__p":
\r
1472 {"status":"Request Complete","statusCode":205}
\r
1473 },"type":"STATUS","locale":"en-US"}
\r
1477 </message></programlisting>
\r
1478 </para></formalpara>
\r
1479 <simpara>The content of the <literal><body></literal> element of the OpenSRF request and result should
\r
1480 look familiar; they match the structure of the <link linkend="OpenSRFOverHTTP">OpenSRF over HTTP examples</link> that we previously dissected.</simpara>
\r
1482 <simplesect id="_registering_opensrf_methods_in_depth">
\r
1483 <title>Registering OpenSRF methods in depth</title>
\r
1484 <simpara>Let’s explore the call to <literal>__PACKAGE__->register_method()</literal>; most of the members
\r
1485 of the hash are optional, and for the sake of brevity we omitted them in the
\r
1486 previous example. As we have seen in the results of the <link linkend="opensrfIntrospection">introspection call</link>, a
\r
1487 verbose registration method call is recommended to better enable the internal
\r
1488 documentation. Here is the complete set of members that you should pass to
\r
1489 <literal>__PACKAGE__->register_method()</literal>:</simpara>
\r
1493 The <literal>method</literal> member specifies the name of the procedure in this module that is being registered as an OpenSRF method.
\r
1498 The <literal>api_name</literal> member specifies the invocable name of the OpenSRF method; by convention, the OpenSRF service name is used as the prefix.
\r
1503 The optional <literal>api_level</literal> member can be used for versioning the methods to allow the use of a deprecated API, but in practical use is always 1.
\r
1508 The optional <literal>argc</literal> member specifies the minimal number of arguments that the method expects.
\r
1513 The optional <literal>stream</literal> member, if set to any value, specifies that the method supports returning multiple values from a single call to subsequent requests. OpenSRF automatically creates a corresponding method with ".atomic" appended to its name that returns the complete set of results in a single request. Streaming methods are useful if you are returning hundreds of records and want to act on the results as they return.
\r
1518 The optional <literal>signature</literal> member is a hash that describes the method’s purpose, arguments, and return value.
\r
1523 The <literal>desc</literal> member of the <literal>signature</literal> hash describes the method’s purpose.
\r
1528 The <literal>params</literal> member of the <literal>signature</literal> hash is an array of hashes in which each array element describes the corresponding method argument in order.
\r
1533 The <literal>name</literal> member of the argument hash specifies the name of the argument.
\r
1538 The <literal>desc</literal> member of the argument hash describes the argument’s purpose.
\r
1543 The <literal>type</literal> member of the argument hash specifies the data type of the argument: for example, string, integer, boolean, number, array, or hash.
\r
1550 The <literal>return</literal> member of the <literal>signature</literal> hash is a hash that describes the return value of the method.
\r
1555 The <literal>desc</literal> member of the <literal>return</literal> hash describes the return value.
\r
1560 The <literal>type</literal> member of the <literal>return</literal> hash specifies the data type of the return value: for example, string, integer, boolean, number, array, or hash.
\r
1570 <section id="_evergreen_specific_opensrf_services">
\r
1571 <title>Evergreen-specific OpenSRF services</title>
\r
1572 <simpara>Evergreen is currently the primary showcase for the use of OpenSRF as an
\r
1573 application architecture. Evergreen 1.6.1 includes the following
\r
1574 set of OpenSRF services:</simpara>
\r
1578 The <literal>open-ils.actor</literal> service supports common tasks for working with user
\r
1579 accounts and libraries.
\r
1584 The <literal>open-ils.auth</literal> service supports authentication of Evergreen users.
\r
1589 The <literal>open-ils.booking</literal> service supports the management of reservations
\r
1590 for bookable items.
\r
1595 The <literal>open-ils.cat</literal> service supports common cataloging tasks, such as
\r
1596 creating, modifying, and merging bibliographic and authority records.
\r
1601 The <literal>open-ils.circ</literal> service supports circulation tasks such as checking
\r
1602 out items and calculating due dates.
\r
1607 The <literal>open-ils.collections</literal> service supports tasks that assist collections
\r
1608 agencies in contacting users with outstanding fines above a certain
\r
1614 The <literal>open-ils.cstore</literal> private service supports unrestricted access to
\r
1615 Evergreen fieldmapper objects.
\r
1620 The <literal>open-ils.ingest</literal> private service supports tasks for importing
\r
1621 data such as bibliographic and authority records.
\r
1626 The <literal>open-ils.pcrud</literal> service supports permission-based access to Evergreen
\r
1627 fieldmapper objects.
\r
1632 The <literal>open-ils.penalty</literal> penalty service supports the calculation of
\r
1633 penalties for users, such as being blocked from further borrowing, for
\r
1634 conditions such as having too many items checked out or too many unpaid
\r
1640 The <literal>open-ils.reporter</literal> service supports the creation and scheduling of
\r
1646 The <literal>open-ils.reporter-store</literal> private service supports access to Evergreen
\r
1647 fieldmapper objects for the reporting service.
\r
1652 The <literal>open-ils.search</literal> service supports searching across bibliographic
\r
1653 records, authority records, serial records, Z39.50 sources, and ZIP codes.
\r
1658 The <literal>open-ils.storage</literal> private service supports a deprecated method of
\r
1659 providing access to Evergreen fieldmapper objects. Implemented in Perl,
\r
1660 this service has largely been replaced by the much faster C-based
\r
1661 <literal>open-ils.cstore</literal> service.
\r
1666 The <literal>open-ils.supercat</literal> service supports transforms of MARC records into
\r
1667 other formats, such as MODS, as well as providing Atom and RSS feeds and
\r
1673 The <literal>open-ils.trigger</literal> private service supports event-based triggers for
\r
1674 actions such as overdue and holds available notification emails.
\r
1679 The <literal>open-ils.vandelay</literal> service supports the import and export of batches of
\r
1680 bibliographic and authority records.
\r
1684 <simpara>Of some interest is that the <literal>open-ils.reporter-store</literal> and <literal>open-ils.cstore</literal>
\r
1685 services have identical implementations. Surfacing them as separate services
\r
1686 enables a deployer of Evergreen to ensure that the reporting service does not
\r
1687 interfere with the performance-critical <literal>open-ils.cstore</literal> service. One can also
\r
1688 direct the reporting service to a read-only database replica to, again, avoid
\r
1689 interference with <literal>open-ils.cstore</literal> which must write to the master database.</simpara>
\r
1690 <simpara>There are only a few significant services that are not built on OpenSRF in
\r
1691 Evergreen 1.6.0, such as the SIP and Z39.50 servers. These services implement
\r
1692 different protocols and build on existing daemon architectures (Simple2ZOOM
\r
1693 for Z39.50), but still rely on the other OpenSRF services to provide access
\r
1694 to the Evergreen data. The non-OpenSRF services are reasonably self-contained
\r
1695 and can be deployed on different servers to deliver the same sort of deployment
\r
1696 flexibility as OpenSRF services, but have the disadvantage of not being
\r
1697 integrated into the same configuration and control infrastructure as the
\r
1698 OpenSRF services.</simpara>
\r
1700 <section id="OpenSRF_attribution">
\r
1701 <simpara>This chapter was taken from Dan Scott's <emphasis>Easing gently into OpenSRF</emphasis> article, June, 2010.</simpara>
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