Web services are being successfully used for interoperable solutions across various industries. One of the key reasons for interest and investment in Web services is that they are well-suited to enable service-oriented systems. XML-based technologies such as SOAP, XML Schema and WSDL provide a broadly-adopted foundation on which to build interoperable Web services. The WS-Policy and WS-PolicyAttachment specifications extend this foundation and offer mechanisms to represent the capabilities and requirements of Web services as Policies.

Service metadata is an expression of the visible aspects of a Web service, and consists of a mixture of machine- and human-readable languages. Machine-readable languages enable tooling. For example, tools that consume service metadata can automatically generate client code to call the service. Service metadata can describe different parts of a Web service and thus enable different levels of tooling support.

First, service metadata can describe the format of the payloads that a Web service sends and receives. Tools can use this metadata to automatically generate and validate data sent to and from a Web service. The XML Schema language is frequently used to describe the message interchange format within the SOAP message construct, i.e. to represent SOAP Body children and SOAP Header blocks.

Second, service metadata can describe the ‘how’ and ‘where’ a Web service exchanges messages, i.e. how to represent the concrete message format, what headers are used, the transmission protocol, the message exchange pattern and the list of available endpoints. The Web Services Description Language is currently the most common language for describing the ‘how’ and ‘where’ a Web service exchanges messages. WSDL has extensibility points that can be used to expand on the metadata for a Web service.

Third, service metadata can describe the capabilities and requirements of a Web service, i.e. representing whether and how a message must be secured, whether and how a message must be delivered reliably, whether a message must flow a transaction, etc. Exposing this class of metadata about the capabilities and requirements of a Web service enables tools to generate code modules for engaging these behaviors. Tools can use this metadata to check the compatibility of requesters and providers. Web Services Policy can be used to represent the capabilities and requirements of a Web service.

Web Services Policy is a machine-readable language for representing the capabilities and requirements of a Web service. These are called ‘policies’. Web Services Policy offers mechanisms to represent consistent combinations of capabilities and requirements, to determine the compatibility of policies, to name and reference policies and to associate policies with Web service metadata constructs such as service, endpoint and operation. Web Services Policy is a simple language that has four elements - Policy, All, ExactlyOne and PolicyReference - and one attribute - wsp:Optional.

Let us start by considering a SOAP Message in the example below.

This message uses message addressing headers. The wsa:To and wsa:Action header blocks identify the destination and the semantics implied by this message respectively. (The prefix wsa is used here to denote the Web Services Addressing XML Namespace. lists all the namespaces and prefixes that are used in this document.)

Let us look at a fictitious scenario used in this document to illustrate the features of the policy language. Tony is a Web service developer. He is building a client application that retrieves real time stock quote information from Contoso, Ltd. Contoso supplies real time data using Web services. Tony has Contoso’s advertised WSDL description of these Web services. Contoso requires the use of addressing headers for messaging. Just the WSDL description is not sufficient for Tony to enable the interaction between his client and these Web services. WSDL constructs do not indicate requirements such as the use of addressing.

(The example companies, organizations, products, domain names, e-mail addresses, logos, people, places, and events depicted herein are fictitious. No association with any real company, organization, product, domain name, email address, logo, person, places, or events is intended or should be inferred.)

Providers have the option to convey requirements, such as the use of addressing, through word-of-mouth and documentation – as they always have. To interact successfully with this service, Tony may have to read any related documentation, call someone at Contoso to understand the service metadata, or look at sample SOAP messages and infer such requirements or behaviors.

Web Services Policy is a machine-readable language for representing these Web service capabilities and requirements as policies. Policy makes it possible for providers to represent such capabilities and requirements in a machine-readable form. For example, Contoso may augment the service WSDL description with a policy that requires the use of addressing. Tony can use a policy-aware client that understands this policy and engages addressing automatically.

How does Contoso use policy to represent the use of addressing? The example below illustrates a policy expression that requires the use of addressing.

The policy expression in the above example consists of a Policy main element and a child element wsap:UsingAddressing. Child elements of the Policy element are policy assertions. Contoso attaches the above policy expression to a WSDL binding description.

The wsap:UsingAddressing element is a policy assertion. (The prefix wsap is used here to denote the Web Services Addressing – WSDL Binding XML Namespace.) This assertion identifies the use of Web Services Addressing information headers. A policy-aware client can recognize this policy assertion, engage addressing automatically, and use headers such as wsa:To and wsa:Action in SOAP Envelopes.

It is important to understand the association between the SOAP message and policy expression in the above example. As you can see by careful examination of the message, there is no reference to any policy expression. Just as WSDL does not require a message to reference WSDL constructs (such as port, binding and portType), Web Services Policy does not require a message to reference a policy expression though the policy expression describes the message.

In addition to requiring the use of addressing, Contoso requires the use of transport-level security for protecting messages.

The SOAP message in the example above includes security timestamps that express creation and expiration times of this message. Contoso requires the use of security timestamps and transport-level security - such as HTTPS – for protecting messages. (The prefixes wss and wsu are used here to denote the Web Services Security and Utility namespaces.)

Similar to the use of addressing, Contoso indicates the use of transport-level security using a policy expression. The example below illustrates a policy expression that requires the use of addressing and transport-level security for securing messages.

The sp:TransportBinding element is a policy assertion. (The prefix sp is used here to denote the Web Services Security Policy XML Namespace.) This assertion identifies the use of transport-level security – such as HTTPS - for protecting messages. Policy-aware clients can recognize this policy assertion, engage transport-level security for protecting messages and include security timestamps in SOAP Envelopes.

Tony can use a policy-aware client that recognizes this policy expression and engages both addressing and transport-level security automatically.

For the moment, let us set aside the contents of the sp:TransportBinding policy assertion and consider its details in a later section.

Thus far, we explored how Contoso uses policy expressions and assertions for representing behaviors that must be engaged for a Web service interaction. What is a policy assertion? What role does it play? In brief, a policy assertion is a piece of service metadata, and it identifies a domain (such as messaging, security, reliability and transaction) specific behavior that is a requirement. Contoso uses a policy assertion to convey a condition under which they offer a Web service. A policy-aware client can recognize policy assertions and engage these behaviors automatically.

Providers, like Contoso, have the option to combine behaviors for an interaction from domains such as messaging, security, reliability and transactions. Using policy assertions, providers can represent these behaviors in a machine-readable form. Web service developers, like Tony, can use policy-aware clients that recognize these assertions and engage these behaviors automatically.

Who defines policy assertions? Where are they? Policy assertions are defined by Web services developers, product designers, protocol authors and users. Like XML Schema libraries, policy assertions are a growing collection. Several WS-* protocol specifications and applications define policy assertions:

Policy assertions can be combined in different ways to express consistent combinations of behaviors (capabilities and requirements). There are three policy operators for combining policy assertions: Policy, All and ExactlyOne (the Policy operator is a synonym for All).

Let us consider the All operator first. The policy expression in the example below requires the use of addressing and transport-level security. There are two policy assertions. These assertions are combined using the All operator. Combining policy assertions using the Policy or All operator means that all the behaviors represented by these assertions are required.

In addition to requiring the use of addressing, Contoso allows either the use of transport- or message-level security for protecting messages. Web Services Policy language can indicate this choice of behaviors in a machine-readable form. To indicate the use of message-level security for protecting messages, Contoso uses the sp:AsymmetricBinding policy assertion (see the example below).

The sp:AsymmetricBinding element is a policy assertion. (The prefix sp is used here to denote the Web Services Security Policy namespace.) This assertion identifies the use of message-level security – such as WS-Security 1.0 - for protecting messages. Policy-aware clients can recognize this policy assertion, engage message-level security for protecting messages and use headers such as wss:Security in SOAP Envelopes.

To allow the use of either transport- or message-level security, Contoso uses the ExactlyOne policy operator. Policy assertions combined using the ExactlyOne operator requires exactly one of the behaviors represented by the assertions. The policy expression in the example below requires the use of either transport- or message-level security for protecting messages.

Contoso requires the use of addressing and requires the use of either transport- or message-level security for protecting messages. They represent this combination using the All and ExactlyOne operators. Policy operators can be mixed to represent different combinations of behaviors (capabilities and requirements). The policy expression in the example below requires the use of addressing and one of transport- or message-level security for protecting messages.

Using this policy expression, Contoso gives the choice of mechanisms for protecting messages to clients (or requesters).

Through a customer survey program, Contoso learns that a significant number of their customers prefer to use the Optimized MIME Serialization (as defined in the MTOM specification) for sending and receiving messages. Contoso adds optional support for the Optimized MIME Serialization and expresses this optional behavior in a machine-readable form.

To indicate the use of optimization using the Optimized MIME Serialization, Contoso uses the mtom:OptimizedMimeSerialization policy assertion (see the example below).

The mtom:OptimizedMimeSerialization element is a policy assertion. (The prefix mtom is used here to denote the Optimized MIME Serialization Policy namespace.) This assertion identifies the use of MIME Multipart/Related serialization for messages. Policy-aware clients can recognize this policy assertion and engage Optimized MIME Serialization for messages. The semantics of this assertion are reflected in messages: they use an optimized wire format (MIME Multipart/Related serialization).

Like Contoso’s optional support for Optimized MIME Serialization, there are behaviors that may be engaged (in contrast to must be engaged) for a Web service interaction. A service provider will not fault if these behaviors are not engaged. Policy assertions can be marked optional to represent behaviors that may be engaged for an interaction. A policy assertion is marked as optional using the wsp:Optional attribute. Optional assertions represent the capabilities of the service provider as opposed to the requirements of the service provider.

In the example below, the Optimized MIME Serialization policy assertion is marked optional. This policy expression allows the use of optimization and requires the use of addressing and one of transport- or message-level security.

Contoso is able to meet their customer needs by adding optional support for the Optimized MIME Serialization. An optional policy assertion represents a behavior that may be engaged.

In the previous sections, we considered two security policy assertions. In this section, let us look at one of the security policy assertions in little more detail.

As you would expect, securing messages is a complex usage scenario. Contoso uses the sp:TransportBinding policy assertion to indicate the use of transport-level security for protecting messages. Just indicating the use of transport-level security for protecting messages is not sufficient. To successfully interact with Contoso’s Web services, Tony must know what transport token to use, what secure transport to use, what algorithm suite to use for performing cryptographic operations, etc. The sp:TransportBinding policy assertion can represent these dependent behaviors. In this section, let us look at how to capture these dependent behaviors in a machine-readable form.

A policy assertion – like the sp:TransportBinding - identifies a visible domain specific behavior that is a requirement. Given an assertion, there may be other dependent behaviors that need to be enumerated for a Web Service interaction. In the case of the sp:TransportBinding policy assertion, Contoso needs to identify the use of a transport token, a secure transport, an algorithm suite for performing cryptographic operations, etc. A nested policy expression can be used to enumerate such dependent behaviors.

What is a nested policy expression? A nested policy expression is a policy expression that is a child element of a policy assertion element. A nested policy expression further qualifies the behavior of its parent policy assertion.

In the example below, the child Policy element is a nested policy expression and further qualifies the behavior of the sp:TransportBinding policy assertion. The sp:TransportToken is a nested policy assertion of the sp:TransportBinding policy assertion. The sp:TransportToken assertion requires the use of a specific transport token and further qualifies the behavior of the sp:TransportBinding policy assertion (which already requires the use of transport-level security for protecting messages).

The sp:AlgorithmSuite is a nested policy assertion of the sp:TransportBinding policy assertion. The sp:AlgorithmSuite assertion requires the use of the algorithm suite identified by its nested policy assertion (sp:Basic256Rsa15 in the example above) and further qualifies the behavior of the sp:TransportBinding policy assertion.

Setting aside the details of using transport-level security, Web service developers, like Tony, can use a policy-aware client that recognizes this policy assertion and engages transport-level security and its dependent behaviors automatically. That is, the complexity of security usage is absorbed by a policy-aware client and hidden from these Web service developers.

Contoso has numerous Web service offerings that provide different kinds of real-time quotes and book information on securities such as GetRealQuote, GetRealQuotes and GetExtendedRealQuote. To accommodate the diversity of Contoso’s customers, Contoso supports multiple WSDL bindings for these Web services. Contoso provides consistent ways to interact with their services and wants to represent these capabilities and requirements consistently across all of their offerings without duplicating policy expressions multiple times. How? It is simple - a policy expression can be named and referenced for re-use.

A policy expression may be identified by an IRI and referenced for re-use as a standalone policy or within another policy expression. There are two mechanisms to identify a policy expression: the wsu:Id and Name attributes. A PolicyReference element can be used to reference a policy expression identified using either of these mechanisms.

In the example above, the wsu:Id attribute is used to identify a policy expression. The value of the wsu:Id attribute is an XML ID. The relative IRI for referencing this policy expression (within the same document) is #common. If the policy document IRI is http://real.contoso.com/policy.xml then the absolute IRI for referencing this policy expression is http://real.contoso.com/policy.xml#common. (The absolute IRI is formed by combining the document IRI, # and the value of the wsu:Id attribute.)

For re-use, a PolicyReference element can be used to reference a policy expression as a standalone policy or within another policy expression. The example below is a policy expression that re-uses the common policy expression above.

For referencing a policy expression within the same XML document, Contoso uses the wsu:Id attribute for identifying a policy expression and an IRI to this ID value for referencing this policy expression using a PolicyReference element.

The example below is a policy expression that re-uses the common policy expression within another policy expression. This policy expression requires the use of addressing, one of transport- or message-level security for protecting messages and allows the use of optimization.

The Name attribute is an alternate mechanism to identify a policy expression. The value of the Name attribute is an absolute IRI and is independent of the location of the XML document where the identified policy expression resides in. As such, referencing a policy expression using the Name attribute relies on additional out of band information. In the example below, the Name attribute identifies the policy expression. The IRI of this policy expression is http://real.contoso.com/policy/common.

The example below is a policy expression that re-uses the common policy expression above.

A majority of Contoso’s customers use WSDL for building their client applications. Contoso leverages this usage by attaching policy expressions to the WSDL binding descriptions.

In the example below, the SecureBinding WSDL binding description defines a binding for an interface that provides real-time quotes and book information on securities. (The prefixes wsdl and tns are used here to denote the Web Services Description language XML namespace and target namespace of this WSDL document.) To require the use of security for these offerings, Contoso attaches the secure policy expression in the previous section to this binding description. The WSDL binding element is a common policy attachment point. The secure policy expression attached to the SecureBinding WSDL binding description applies to any message exchange associated with any port that supports this binding description. This includes all the message exchanges described by operations in the RealTimeDataInterface.

In addition to providing real-time quotes and book information on securities, Contoso provides other kinds of data through Web services such as quotes delayed by 20 minutes and security symbols through Web services (for example GetDelayedQuote, GetDelayedQuotes, GetSymbol and GetSymbols). Contoso does not require the use of security for these services, but requires the use of addressing and allows the use of optimization.

In the example above, the OpenBinding WSDL binding description defines a binding for an interface that provides other kinds of data such as quotes delayed by 20 minutes and security symbols. To require the use of addressing and allow the use of optimization, Contoso attaches the common policy expression in the previous section to this binding description. As we have seen in the SecureBinding case, the common policy expression attached to the OpenBinding WSDL binding description applies to any message exchange associated with any port that supports this binding description. This includes all the message exchanges described by operations in the DelayedDataInterface.

As mentioned earlier, providers have the option to convey requirements, such as the use of addressing or security, through word-of-mouth and documentation – as they always have. The absence of policy expressions in a WSDL document does not indicate anything about the capabilities and requirements of a service. The service may have capabilities and requirements that can be expressed as policy expressions, such as the use of addressing, security and optimization. Or, the service may not have such capabilities and requirements. A policy aware client should not conclude anything (other than ‘no claims’) about the absence of policy expressions.

Service providers, like Contoso, can preserve and leverage their investments in WSDL and represent the capabilities and requirements of a Web service as policies. A WSDL document may specify varying behaviors across Web service endpoints. Web service developers, like Tony, can use a policy-aware client that recognizes these policy expressions in WSDL documents and engages behaviors automatically for each of these endpoints. Any complexity of varying behaviors across Web service endpoints is absorbed by a policy-aware client or tool and hidden from these Web service developers.

As you have seen, Web Services Policy is a simple language that has four elements - Policy, All, ExactlyOne and PolicyReference - and one attribute - wsp:Optional. In practice, service providers, like Contoso, use policy expressions to represent combinations of capabilities and requirements. Web service developers, like Tony, use policy-aware clients that understand policy expressions and engage the behaviors represented by providers automatically. A sizable amount of complexity is absorbed by policy-aware clients (or tools) and is invisible to these Web service developers.

Web Services Policy extends the foundation on which to build interoperable Web services, hides complexity from developers and automates Web service interactions.

A policy expression is the XML representation and interoperable form of a Web Services Policy. A policy expression consists of a Policy wrapper element and a variety of child and descendent elements. Child and descendent elements from the policy language are Policy, All, ExactlyOne and PolicyReference. Other child elements of Policy, All and ExactlyOne are policy assertions. (The Policy element plays two roles: wrapper element and operator.) Policy assertions can contain a nested policy expression. Policy assertions can also be marked optional to represent behaviors that may be engaged (capabilities) for an interaction. The optional marker is the wsp:Optional attribute which is placed on a policy assertion element.

Let us take a closer look at Contoso’s policy expression (see below) from the previous section.

The Policy element is the wrapper element. The All and ExactlyOne elements are the policy operators. All other child elements of the All and ExactlyOne elements are policy assertions from domains such as messaging, addressing, security, reliability and transactions.

Web Services Policy language defines two forms of policy expressions: compact and normal form. Up to this point, we have used the compact form. The compact form is less verbose than the normal form. The compact form is useful for authoring policy expressions. The normal form is an intuitive representation of the policy data model. We will look into the policy data model in the next section.

The normal form uses a subset of constructs used in the compact form and follows a simple outline for its XML representation:

The normal form consists of a Policy wrapper element and has one child ExactlyOne element. This ExactlyOne element has zero or more All child elements. Each of these All elements has zero or more policy assertions. The PolicyReference element and wsp:Optional attribute are not used in the normal form. And, a nested policy expression in the normal form has at most one policy alternative.

The normal form represents a policy as a collection of policy alternatives and a policy alternative as a collection of policy assertions in a straight-forward manner.

The example below is a policy expression in the normal form. This expression contains two policy alternatives: one that requires the use of transport-level security and the other that requires the use of message-level security for protecting messages.

A policy expression in the compact form can be converted to the normal form. Web Services Policy language describes the algorithm for this conversion.

Let us re-consider Contoso’s policy expression (see the example below). Contoso requires the use of addressing and either transport- or message-level security and allows the use of optimization. This policy expression is in the compact form and has four policy alternatives for requesters:

Let us look at the normal form for this policy expression. The example below is Contoso’s policy expression in the normal form. As you can see, the compact form is less verbose than the normal form. The normal form represents a policy as a collection of policy alternatives. Each of the All operators is a policy alternative. There are four policy alternatives in the normal form. These alternatives map to bullets (a) through (d) above.

The wsp:Optional attribute, nested policy expression and PolicyReference element are converted to their corresponding normal form. The wsp:Optional attribute converts to two alternatives, one with and the other without the assertion. A policy alternative containing an assertion with a nested policy expression that has multiple policy alternatives converts to multiple policy alternatives where the assertion contains a nested policy expression that has at most one policy alternative.

The PolicyReference element is replaced with its referenced policy expression. Just as other service metadata languages, Web Services Policy does not mandate any specific policy retrieval mechanism. Any combination of any retrieval mechanisms in any order may be used for referencing policy expressions. Example retrieval mechanisms are:

If the referenced policy expression is in the same XML document as the reference, then the policy expression should be identified using the wsu:Id (XML ID) attribute and referenced using an IRI reference to this XML ID value.

In the previous section, we considered the normal form for policy expressions. As we discussed, the normal form represents a policy as a collection of policy alternatives. In this section, let us look at the policy data model.

Contoso uses a policy to convey the conditions for an interaction. Policy-aware clients, like the one used by Tony (as explained earlier in ), view policy as an unordered collection of zero or more policy alternatives. A policy alternative is an unordered collection of zero or more policy assertions. A policy alternative represents a collection of behaviors or requirements or conditions for an interaction. In simple words, each policy alternative represents a set of conditions for an interaction. The diagram below describes the policy data model.

A provider, like Contoso, and a requester, like Tony’s policy-aware client, may represent their capabilities and requirements for an interaction as policies and want to limit their message exchanges to mutually compatible policies. Web Services Policy defines an intersection mechanism for selecting compatible policy alternatives when there are two or more policies.

The example below is a copy of Contoso’s policy expression (from ). As we saw before, Contoso offers four policy alternatives. Of them, one of the policy alternatives requires the use of addressing and transport-level security.

Tony’s organization requires the use of addressing and transport-level security for any interaction with Contoso’s Web services. Tony represents these behaviors using a policy expression illustrated in the example below in normal form. This policy expression contains one policy alternative that requires the use of addressing and transport-level security.

Tony lets his policy-aware client select a compatible policy alternative in Contoso’s policy. How does this client select a compatible policy alternative? It is simple – it uses the policy intersection. That is, Tony’s policy-aware client uses these two policy expressions (Tony’s and Contoso’s) and the policy intersection to select a compatible policy alternative for this interaction. Let us look at the details of policy intersection.

For two policy assertions to be compatible they must have the same QName. And, if either assertion has a nested policy, both assertions must have a nested policy and the nested policies must be compatible. For example, policy assertions (c2) and (t1) have the same QName, sp:TransportBinding. For this discussion, let us assume that these two assertions have compatible nested policies. These two assertions are compatible because they have the same QName and their nested policies are compatible.

Two policy alternatives are compatible if each policy assertion in one alternative is compatible with a policy assertion in the other and vice-versa. For example, policy assertions (c1) and (c2) in Contoso’s policy alternative are compatible with policy assertions (t2) and (t1) in Tony’s policy alternative. Contoso’s policy alternative (a) and Tony’s policy alternative are compatible because assertions in these two alternatives are compatible.

Two policies are compatible if a policy alternative in one is compatible with a policy alternative in the other. For example, Contoso’s policy alternative (a) is compatible with Tony’s policy alternative. Contoso’s policy and Tony’s policy are compatible because one of Contoso’s policy alternative is compatible with Tony’s policy alternative.

For this interaction, Tony’s policy-aware client can use policy alternative (a) to satisfy Contoso’s conditions or requirements.

Similarly, policy intersection can be used to check if providers expose endpoints that conform to a standard policy. For example, a major retailer might require all their supplier endpoints to be compatible with an agreed upon policy.

In , we looked into how Contoso attached their policy expressions to the WSDL binding element. In addition to the WSDL binding element, a policy expression can be attached to other WSDL elements such as service, port, operation and message. These elements are the WSDL policy attachment points in a WSDL document.

The WSDL attachment points are partitioned (as illustrated below) into four policy subjects: message, operation, endpoint and service. When attached, capabilities and requirements represented by a policy expression apply to a message exchange or message associated with (or described by) a policy subject.

Multiple policy expressions may be attached to WSDL constructs. Let us consider how Contoso could have used multiple policy expressions in a WSDL document. In the example below, there are two policy expressions #common2 and #secure2 attached to the SecureBinding WSDL binding and RealTimeDataPort WSDL port descriptions.

As we discussed before, the WSDL port, binding and portType elements collectively represent the endpoint policy subject. In the example above, the #common2 and #secure2 policy expressions attached to the SecureBinding WSDL binding and RealTimeDataPort WSDL port descriptions collectively apply to any message exchange associated with the RealTimeDataPort WSDL port.

As in the example above, multiple policy expressions may be attached to Web service constructs that collectively represent a single policy subject. When there are multiple policy expressions attached to the same policy subject then the effective policy or combination of these policy expressions apply to the associated policy subject.

The effective policy is the combination of two or more policy expressions attached to the same policy subject. The combination of two policy expressions, also known as the merged policy expression, is a new policy expression that combines these two policy expressions using the All policy operator.

The policy expression below is the combination of the two policy expressions attached to the SecureBinding WSDL binding and RealTimeDataPort WSDL port descriptions. The #common2 policy expression has two policy alternatives. The #secure2 policy expression has two policy alternatives. The combination of these two policies is equivalent to Contoso’s secure policy in and has four policy alternatives. In other words, the combination of two policies is the cross product of alternatives in these two policies.

Of course, the above policy expression can be normalized. There are four policy alternatives in the normal form. As we have seen in the policy data model, a policy is an unordered collection of policy alternatives. That is, the order of policy alternatives is insignificant. Therefore, the order of combining these policy expressions is insignificant.

Web Services Policy language is an extensible language by design. The Policy, ExactlyOne, All and PolicyReference elements are extensible. The Policy, ExactlyOne and All elements allow child element and attribute extensibility. The PolicyReference element allows attribute extensibility. Extensions must not use the policy language XML namespace name. A consuming processor processes known attributes and elements, ignores unknown attributes and treats unknown elements as policy assertions.

Web Services Policy language enables simple versioning practices that allow requesters to continue the use of any older policy alternatives in a backward compatible manner. This allows service providers, like Contoso, to deploy new behaviors using additional policy assertions without breaking compatibility with clients that rely on any older policy alternatives.

The example below represents a Contoso version 1 policy expression. This expression requires the use of addressing and transport-level security for protecting messages.

Over time, Contoso adds support for advanced behaviors: requiring the use of addressing and message-level security for protecting messages. They added this advanced support without breaking compatibility with requesters that rely on addressing and transport-level security. The example below is Contoso’s version 2 policy expression. In this version, Contoso’s adds a new policy alternative that requires the use of addressing and message-level security. The clients that rely on addressing and transport-level security may continue to interact with Contoso’s using the old policy alternative. Of course, these clients have the option to migrate from using old policy alternatives to new policy alternatives.

When Contoso added support for advanced behaviors, they spent time to plan for the continued support for existing clients, the smooth migration from using current to advanced behaviors, and the switch to use only the advanced behaviors in the near future (i.e. sun-setting current behaviors). In this versioning scenario, policy can be used to represent current and advanced behaviors in a non-disruptive manner: no immediate changes to existing clients are required and these clients can smoothly migrate to new functionality when they choose to. This level of versioning support in policy enables the same class of versioning best practices built into WSDL constructs such as service, port and binding.

Let us look at tooling for unknown policy assertions. As service providers, like Contoso, incrementally deploy advanced behaviors, some requesters may not recognize these new policy assertions. As discussed before, these requesters may continue to interact using old policy alternatives. New policy assertions will emerge to represent new behaviors and slowly become part of everyday interoperable interaction between requesters and providers. Today, most tools use a practical tolerant strategy to process new or unrecognized policy assertions. These tools consume such unrecognized assertions and designate these for user intervention. As you would recognize, there is nothing new in this practice. This is similar to how a proxy generator that generates code from WSDL creates code for all the known WSDL constructs and allows Web service developers to fill in code for custom or unknown constructs in the WSDL.

As you have seen, Web Services Policy is a simple language that has four elements -Policy, All, ExactlyOne and PolicyReference - and one attribute - wsp:Optional. Policy is a flexible language to represent consistent combinations of behaviors using policy operators: Policy, All and ExactlyOne. Policy is an expressive language and capable of representing behaviors from a variety of domains. Let us look for the key parts that unlock this potential.

Service providers combine behaviors for an interaction from domains such as messaging, security, reliability and transactions. To enable clients to engage these behaviors, services require some way to advertise these behaviors. Providers require machine readable metadata pieces that identify these behaviors. A policy assertion is a machine-readable metadata piece that requires the use of a behavior identified by the assertion. Web service developers can use policy-aware clients that recognize these assertions and engage their corresponding behaviors automatically.

Policy assertions are the key parts and play a central role to unlock the potential offered by the Web Services Policy language. Assertions are defined by product designers, protocol authors, protocol implementers and Web service developers.

Policy assertion authors identify behaviors required for Web services interactions and represent these behaviors as policy assertions. By designing policy assertions, assertion authors make a significant contribution to automate Web services interactions and enable advanced behaviors.

As we discussed, a policy assertion identifies a domain specific behavior or requirement or condition. A policy assertion has a QName that identifies its behavior or requirement or condition. A policy assertion may contain assertion parameters and a nested policy.

Let us look at the anatomy of a policy assertion from the security domain. The policy expression in the diagram below uses the sp:IssuedToken policy assertion. This assertion illustrates the use of assertion parameters and nested policy.

As we illustrated in the previous section, requiring the use of a security token issued by a third party is represented as a policy assertion. In simple words, a policy assertion identifies a domain specific behavior:

Given that interoperability and automation are the motivations, policy assertions that represent opt-in, shared and visible behaviors are useful pieces of metadata. Such assertions enable tooling and improve interoperability. The key to understanding when to design policy assertions is to have clarity on the characteristics of a behavior represented by a useful policy assertion: opt-in, shared and visible.

The policy language allows assertion authors to invent their own XML dialects to represent policy assertions. The policy language builds on natural XML nesting and leverages XML Schema validation. The policy language relies only on the QName of the policy assertion XML element. Everything else is left for the policy assertion authors to design. The policy language offers plenty of options to assertion authors such as independent assertions, dependent assertions, nested policies and assertion parameters.

The description of a policy assertion should identify a single domain specific behavior in an objective manner and answer the following questions:

As you would have expected, the policy assertion design is more than a technical design. Given that interoperability and automation are the motivations, policy assertion design is a business process to reach agreements with relevant stakeholders for interoperability and tooling. Setting aside the business aspects of the design, the rest of this section walks through a few tradeoffs or dimensions to consider and provides technical guidelines for designing policy assertions.

Thus far, we walked through the dimensions of a policy assertion and guidelines for authoring policy assertions. Let us look at what are the minimum requirements for describing policy assertions in specifications: