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| <h1 align="center">Web Services Policy Support In Apache Axis2</h1> |
| <p>This document gives you an introduction to the role of Web |
| services policy in Apache Axis2.</p> |
| <p>Send your feedback to: <a class="externalLink" href="mailto:java-dev@axis.apache.org?subject=[Axis2]">java-dev@axis.apache.org</a>. |
| (Subscription details are available on the <a href="../mail-lists.html">Axis2 site</a>.) |
| Kindly prefix every email subject with [Axis2].</p> |
| <div class="section"><h2>Content<a name="Content"></a></h2> |
| <ul> |
| <li><a href="#what">What is Web Services (WS) Policy?</a></li> |
| <li><a href="#client">Client Side WS-Policy Support</a></li> |
| <li><a href="#server">Server Side WS-Policy Support</a></li> |
| <li><a href="#resources">Resources</a></li> |
| </ul> |
| <a name="what" id="what"></a> |
| <div class="section"><h2>What is Web Services (WS) Policy?<a name="What_is_Web_Services_WS_Policy"></a></h2> |
| <p>To consume non trivial web services you must fully understand |
| its XML contract (WSDL) along with any other additional |
| requirements, capabilities, or preferences that translate to the |
| configuration of the service and essentially becomes the policies |
| of the service.</p> |
| <p>WS Policy framework provides a way to express the policies of a |
| service in a machine-readable way. A Web services infrastructure |
| can be enhanced to understand and enforce policies at runtime. For |
| instance, a service author might write a policy requiring a digital |
| signature and encryption, while service consumers can use the |
| policy information to reason out whether they can adhere to this |
| policy information to use the service.</p> |
| <p>Furthermore, Web service infrastructure can be enhanced to |
| enforce those requirements without requiring the service author to |
| write even a single line of code.</p> |
| <a name="client" id="client"></a> |
| <div class="section"><h2>Client Side WS-Policy Support<a name="Client_Side_WS-Policy_Support"></a></h2> |
| <p>This release <b>fully supports WS Policy at |
| client-side</b>. It means that when you codegen a stub against |
| a WSLD which contains policies, the stub will contain the |
| capability to engage the required modules with the appropriate |
| configurations, plus it will generate additional methods in the |
| stub where the user can set certain properties. For instance, if |
| there is a security policy attached to a binding, the generated |
| stub will engage the security module for that service with the |
| appropriate security configurations with some addtional methods |
| that the user can use to set properties in the generated stub.</p> |
| <div class="section"><h3>How it works:<a name="How_it_works:"></a></h3> |
| <div class="section"><h4>Phase 1: At PolicyEvaluator<a name="Phase_1:_At_PolicyEvaluator"></a></h4> |
| <p>The Codegen engine runs a few of its registered extensions |
| before it generates the stub. When the PolicyEvalutor (which is a |
| registered Codegen extension) is initialized, it populates a |
| registry of QNames of supported policy assertions to |
| PolicyExtensions.</p> |
| <p>For instance, module Foo might have a mapping of assertion |
| {http://test.com/foo, foo} which means any assertion that has this |
| name will be processed by this module. The Foo module might |
| implement the ModulePolicyExtension interface through which the |
| PolicyExtension object can be obtained.</p> |
| <p>A <b>PolicyExtension</b> is the access point for a |
| module to add any additional methods to the stub. For instance a |
| Reliable Messaging module can add startSequence() and endSequence() |
| methods to the stub, that the user must call to start and end an RM |
| sequence.</p> |
| <p>Then at the engagement of the PolicyEvaluator, the effective |
| policy of each message of every operation is calculated based on |
| policy information declared in the WSDL document. Here we assume |
| that the effective policy of an operation contains a single |
| alternative (<b>Multiple policy alternatives are not |
| supported</b>). Then we split that policy as follows into few |
| other policies such that, each policy will contain assertions that |
| can be processed by a single module.</p> |
| <div><pre> |
| <wsp:Policy> <wsp:Policy> <wsp:Policy> |
| <a:Foo/> <a:Foo/> <b:Foo/> |
| <b:Bar/> => </wsp:Policy> |
| </wsp:Policy> |
| </wsp:Policy> |
| </pre></div> |
| <p>Then each policy is given the appropriate PolicyExtension with |
| an org.w3c.Element type object to which the module can append any |
| other elements/attributes it wishes. Those attributes/elements |
| should resolve to meaningful stub functions through the Custom |
| PolicyExtensionTemplate.xsl at a latter point of time.</p> |
| <p>For instance, depending on the policy, the Security module can |
| append <username>, <passwd> elements to the given |
| element as children, which are later resolved into setUsername(..), |
| setPasswd(..), functions of the stub. This way a module can include |
| additional methods to the stub that can be used to get specific |
| propreties from the user. These methods store any user input in the |
| ServiceClient properties |
| (ServiceClient.getOptions().putProperty(...)) which can later be |
| accessed by the module.</p> |
| </div><div class="section"><h4>Phase 2: At AxisServiceBasedMultiLanguageClientEmitter<a name="Phase_2:_At_AxisServiceBasedMultiLanguageClientEmitter"></a></h4> |
| <p>Further, policies (based on the WSDL) at appropriate levels |
| (service level, operation level) are stored as policy strings in |
| the stub. If there are a few policies at a given level, they are |
| merged together and represented as a single policy in the stub. Few |
| more generic methods are also added to the stub which are used to |
| evaluate and process the policies at runtime.</p> |
| </div><div class="section"><h4>Phase 3: Runtime<a name="Phase_3:_Runtime"></a></h4> |
| <p>When a new stub object is created, the policy strings in the |
| stub are converted into policy objects and are set in the |
| AxisDescription hierarchy that is used in the stub. In other words, |
| any policy information available in the WSDL will be preserved in |
| the AxisService object that is used in the stub.</p> |
| <p>Then based on its policy, each AxisDescription is engaged to a |
| set of modules. Modules can do a prior calculation of |
| configurations if needed at the engagement.</p> |
| <p>When the stub method is invoked, those modules which are engaged |
| to that AxisDescription, access the policy for that operation via |
| the AxisDescription object. It can get the other required |
| information from the MessageContext, which is stored by stub |
| methods that the module has added to the stub earlier, through the |
| ModulePolicyExtension implementation. The modules are required to |
| load their configurations according to the effective policy, which |
| is set at AxisDescription, and the properties they get via |
| MessageContext.</p> |
| <a name="server" id="server"></a> |
| </div></div><div class="section"><h2>Server Side WS-Policy Support<a name="Server_Side_WS-Policy_Support"></a></h2> |
| <p>In this current release, the Apache Axis2 framework uses the |
| WS-Commons/Neethi framework to manipulate policy documents. All its |
| description builders store the policy information included in |
| description documents (services.xml, axis2.xml, .. etc) in the |
| appropriate description classes. This information is available at |
| both deployment and run time via these description classes.</p> |
| <p>When generating WSDL dynamically for each service, policy |
| information in the description classes is included. For instance, |
| if you declare a policy in axis2.xml, then that policy is reflected |
| in the service elements of the WSDL of every service. If a policy |
| is declared in a services.xml, it is shown in the service element |
| of WSDL for that particular service.</p> |
| <p>Further, when a service is deployed, an arbitary policy |
| alternative is selected and set for each AxisOperation and |
| AxisMessages of the AxisService. If the selected Policy alternative |
| cannot be supported by any modules that are capable of processing |
| the selective alternative, then the service is considered as a |
| faulty service. Else, the set of modules is engaged at appropriate |
| levels to support the requirments and capabilities that are defined |
| in the Policies associated with the AxisDescription.</p> |
| <p>It is evident that there is some work left to make Apache Axis2 |
| a fully fledged ws-policy supported Web service infrastructure. |
| However, it is encouraging to note that we've taken the first steps |
| towards this goal. We appreciate any suggestions, patches, etc., |
| you send us in this regard. Keep on contributing!</p> |
| <a name="resources" id="resources"></a> |
| <div class="section"><h2>Resources<a name="Resources"></a></h2> |
| <ul> |
| <li>Apache Neethi (WS Policy Implementation) official site- |
| <a class="externalLink" href="http://ws.apache.org/commons/neethi/index.html">Home Page</a></li> |
| <li>Sanka Samaranayake, March 2006. <a class="externalLink" href="http://wso2.org/library/23">Web services Policy - Why, What & How</a></li> |
| <li><a class="externalLink" href="http://svn.apache.org/viewcvs.cgi/webservices/commons/trunk/modules/neethi/">WS-commons/policy SVN</a></li> |
| <li><a class="externalLink" href="http://specs.xmlsoap.org/ws/2004/09/policy/ws-policy.pdf">Web Services Policy Framework (WS-Policy)</a></li> |
| </ul> |
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