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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 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>
<h2>Content</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>
<h2>What is Web Services (WS) Policy?</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>
<h2>Client Side WS-Policy Support</h2>
<p>This release <strong>fully supports WS Policy at
client-side</strong>. 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>
<h3>How it works:</h3>
<h4>Phase 1: At PolicyEvaluator</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 <strong>PolicyExtension</strong> 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 (<strong>Multiple policy alternatives are not
supported</strong>). 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>
<pre>
  &lt;wsp:Policy&gt;         &lt;wsp:Policy&gt;       &lt;wsp:Policy&gt;        
    &lt;a:Foo/&gt;             &lt;a:Foo/&gt;           &lt;b:Foo/&gt;               
    &lt;b:Bar/&gt;      =&gt;                               &lt;/wsp:Policy&gt;       
                                   &lt;/wsp:Policy&gt;
  &lt;/wsp:Policy&gt;
</pre>
<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 &lt;username&gt;, &lt;passwd&gt; 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>
<h4>Phase 2: At AxisServiceBasedMultiLanguageClientEmitter</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>
<h4>Phase 3: Runtime</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>
<h2>Server Side WS-Policy Support</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>
<h2>Resources</h2>
<ul>
<li>Apache Neethi (WS Policy Implementation) official site-
<a href="http://ws.apache.org/commons/neethi/index.html">Home Page</a></li>
<li>Sanka Samaranayake, March 2006. <a href=
"http://wso2.org/library/23">Web services Policy - Why, What &amp; How</a></li>
<li><a href=
"https://github.com/apache/ws-neethi">WS-commons/policy GitHub</a></li>
<li><a 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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