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Web Services Coordination (WS-Coordination)

 
Note   This specification is superseded by a new version.

Authors

Felipe Cabrera, Microsoft
George Copeland, Microsoft
Tom Freund, IBM
Johannes Klein, Microsoft
David Langworthy, Microsoft
David Orchard, BEA Systems
John Shewchuk, Microsoft
Tony Storey, IBM

Copyright© 2002 BEA Systems, International Business Machines Corporation, Microsoft Corporation. All rights reserved.

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Abstract

This specification (WS-Coordination) describes an extensible framework for providing protocols that coordinate the actions of distributed applications. Such coordination protocols are used to support a number of applications, including those that need to reach consistent agreement on the outcome of distributed transactions.

The framework defined in this specification enables an application service to create a context needed to propagate an activity to other services and to register for coordination protocols. The framework enables existing transaction processing, workflow, and other systems for coordination to hide their proprietary protocols and to operate in a heterogeneous environment.

Additionally this specification describes a definition of the structure of context and the requirements for propagating context between cooperating services.

Composable Architecture

By using the SOAP [SOAP] and WSDL [WSDL] extensibility model, SOAP-based and WSDL-based specifications are designed to be composed with each other to define a rich web services environment. As such, WS-Coordination by itself does not define all the features required for a complete solution.  WS-Coordination is a building block that is used in conjunction with other specifications and application-specific protocols to accommodate a wide variety of protocols related to the operation of distributed web services.

The web service protocols defined in this specification should be used when interoperability is needed across vendor implementations, trust domains, etc. Thus, the web service protocols defined in this specification can be combined with proprietary protocols within the same application.

Status

WS-Coordination and related specifications are provided for use as-is and for review and evaluation only. Microsoft, BEA and IBM will solicit your contributions and suggestions in the near future. Microsoft, BEA and IBM make no warrantees or representations regarding the specification in any manner whatsoever.

Acknowledgments

The following individuals have provided invaluable input into the design of the WS-Coordination specification:

Francisco Curbera, IBM
Don Ferguson, IBM
Frank Leymann, IBM
Jagan Peri, Microsoft
Satish Thatte, Microsoft
Sanjiva Weerawarana, IBM

We also wish to thank the technical writers and development reviewers who provided feedback to improve the readability of the specification.

Table of Contents

1. Introduction
   1.1 The Model
   1.2 Extensibility
   1.3 Notational Conventions
   1.4 Namespace
   1.5 XSD and WSDL Files
2. CoordinationContext
3. Coordination Service
   3.1 Activation Service
   3.1.1 CreateCoordinationContext Message
   3.1.2 CreateCoordinationContextResponse Message
   3.2 Registration Service
   3.2.1 Register Message
   3.2.2 RegistrationResponse Message
   3.3 Error Message
4. Security Considerations
5. Relationship to Other Web Services
6. Interoperability Considerations
Glossary
Appendix A. Port References
   A.1 XML Representation
   A.2 XML Example
   A.3 Security Considerations
Appendix B: Context
   B.1 XML Representation
   B.2 XML Example
   B.3 Security Considerations
References

1. Introduction

The current set of Web Service specifications [WSDL, SOAP] defines protocols for Web Service interoperability. Web Services increasingly tie together a large number of participants forming large distributed computational units – we refer to these computation units as activities.

The resulting activities are often complex in structure, with complex relationships between their participants. The execution of such activities often takes a long time to complete due to business latencies and user interactions.

This specification defines an extensible framework for coordinating activities using a coordinator and set of coordination protocols. This framework enables participants to reach consistent agreement on the outcome of distributed activities. The coordination protocols that can be defined in this framework can accommodate a wide variety of activities, including protocols for simple short-lived operations and protocols for complex long-lived business activities.

Note that the use of the coordination framework is not restricted to transaction processing systems; a wide variety of protocols can be defined for distributed applications.

1.1 The Model

This specification describes a framework for a coordination service (or coordinator) which consists of these component services:

  • An Activation service with an operation that enables an application to create a coordination instance or context.
  • A Registration service with an operation that enables an application to register for coordination protocols.
  • A coordination type specific set of coordination protocols.

This is illustrated below in Figure 1.

Applications use the Activation service to create the coordination context for an activity. Once a coordination context is acquired by an application, it is then sent by whatever appropriate means to another application.

The context contains the necessary information to register into the activity specifying the coordination behavior that the application will follow.

Additionally, an application that receives a coordination context may use the Registration service of the original application or may use one that is specified by an interposing, trusted, coordinator. In this manner an arbitrary collection of web services may coordinate their joint operation.

1.2 Extensibility

The specification provides for extensibility and flexibility along two dimensions. The framework allows for:

  • The publication of new coordination protocols.
  • The selection of a protocol from a coordination type and the definition of extension elements that can be added to protocols and message flows.

Extension elements can be used to exchange application-specific data on top of message flows already defined in this specification. This addresses the need to exchange such data as isolation level supported, signatures or other information related to business level coordination protocols. The data can be logged for auditing purposes, or evaluated to ensure that a decision meets certain business-specific constraints.

To understand the syntax used in this specification, you should be familiar with the WSDL specification, including its HTTP and SOAP binding styles. All WSDL port type definitions provided here assume the existence of corresponding SOAP and HTTP bindings.

Terms introduced in this specification are explained in the body of the specification and summarized in the glossary.

1.3 Notational Conventions

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119 [KEYWORDS].

Namespace URIs of the general form "some-URI" represents some application-dependent or context-dependent URI as defined in RFC2396 [URI].

This specification uses an informal syntax to describe the XML grammar of the XML fragments below:

  • The syntax appears as an XML instance, but the values indicate the data types instead of values.
  • Element names ending in "..." (such as <element.../> or <element...>) indicate that elements/attributes irrelevant to the context are being omitted.
  • Attributed names ending in "..." (such as name=...) indicate that the values are specified below.
  • Grammar in bold has not been introduced earlier in the document, or is of particular interest in an example.
  • <-- description --> is a placeholder for elements from some "other" namespace (like ##other in XSD).
  • Characters are appended to elements, attributes, and <!-- descriptions --> as follows: "?" (0 or 1), "*" (0 or more), "+" (1 or more). The characters "[" and "]" are used to indicate that contained items are to be treated as a group with respect to the "?", "*", or "+" characters. 
  • The XML namespace prefixes (defined below) are used to indicate the namespace of the element being defined.
  • Examples starting with <?xml contain enough information to conform to this specification; others examples are fragments and require additional information to be specified in order to conform.

XSD schemas and WSDL definitions are provided as a formal definition of grammars [xml-schema1] [WSDL].

1.4 Namespace

The XML namespace [XML-ns] URI that MUST be used by implementations of this specification is:

        http://schemas.xmlsoap.org/ws/2002/08/wscoor

The namespace prefix "wscoor" used in this specification is associated with this URI. 

The following namespaces are used in this document:

If an action URI is used then the action URI MUST consist of the coordination namespace URI concatenated with the '#' character and the operation name. For example:

        http://schemas.xmlsoap.org/ws/2002/08/wscoor#Register

1.5 XSD and WSDL Files

The following links hold the XML schema and the WSDL declarations defined in this document.

http://schemas.xmlsoap.org/ws/2002/08/wscoor/wscoor.xsd

http://schemas.xmlsoap.org/ws/2002/08/wscoor/wscoor.wsdl

2. CoordinationContext

The CoordinationContext is a Context type, as described in the Appendix B, that is used to pass Coordination information to parties involved in a coordination service.

CoordinationContext elements are placed within messages that are used by the parties. A CoordinationContext provides access to a coordination registration service, a coordination type, and relevant extensions.

The following is an example of a CoordinationContext supporting a transaction service:

<?xml version="1.0" encoding="utf-8"?>
<S:Envelope xmlns:S="http://www.w3.org/2001/12/soap-envelope"
    <S:Header>
        . . .
        <wscoor:CoordinationContext 
            xmlns:wsu="http://schemas.xmlsoap.org/ws/2002/07/utility" 
            xmlns:wscoor="http://schemas.xmlsoap.org/ws/2002/08/wscoor"
            xmlns:myApp="http://fabrikam123.com/myApp">
            <wsu:Expires>
                 2002-06-30T13:20:00.000-05:00
            </wsu:Expires>
            <wsu:Identifier>
                 http://Fabrikam123.com/SS/1234
            </wsu:Identifier>
            <wscoor:CoordinationType>
               http://schemas.xmlsoap.org/ws/2002/08/wstx
            </wscoor:CoordinationType>
            <wscoor:RegistrationService>
                <wsu:Address>
                 http://Business456.com/mycoordinationservice/registration
                </wsu:Address>
                <myApp:BetaMark> ... </myApp:BetaMark>
                <myApp:EBDCode> ... </myApp:EBDCode>
            </wscoor:RegistrationService>
            <myApp:IsolationLevel>
                  RepeatableRead
            </myApp:IsolationLevel>
        </wscoor:CoordinationContext>
        . . .
    </S:Header>

When an application propagates an activity using a coordination service, applications MUST include a Coordination context in the outgoing message.

3. Coordination Service

The Coordination service (or coordinator) is an aggregation of the following services:

  • Activation service: Defines a CreateCoordinationContext operation that allows a CoordinationContext to be created. The exact semantics are defined in the specification that defines the coordination type. The Coordination service MAY support the Activation service.
  • Registration service: Defines a Register operation that allows a web service to register to participate in a coordination protocol. The Coordination service MUST support the Registration service.
  • A set of coordination protocol services for each supported coordination type. These are defined in the specification that defines the coordination type.

Figure 2 illustrates how two application services (App1 and App2) with their own coordinators (CoordinatorA and CoordinatorB) interact as the activity propagates between them. The protocol Y and services Ya and Yb are specific to a coordination type, which are not defined in this specification.

  1. App1 sends a CreateCoordinationContext for coordination type Q, getting back a Context Ca that contains the activity identifier A1, the coordination type Q and a PortReference to CoordinatorA's Registration service RSa.
  2. App1 then sends an application message to App2 containing the Context Ca.
  3. App2 prefers CoordinatorB, so it uses CreateCoordinationContext with Ca as an input to interpose CoordinatorB. CoordinatorB creates its own CoordinationContext Cb that contains the same activity identifier and coordination type as Ca but with its own Registration service RSb.
  4. App2 determines the coordination protocols supported by the coordination type Q and then Registers for a coordination protocol Y at CoordinatorB, exchanging PortReferences for App2 and the protocol service Yb. This forms a logical connection between these PortReferences that the protocol Y can use.
  5. This registration causes CoordinatorB to forward the registration onto CoordinatorA's Registration service RSa, exchanging PortReferences for Yb and the protocol service Ya. This forms a logical connection between these PortReferences that the protocol Y can use.

3.1 Activation Service

The Activation service definition requires a port type on the coordinator side (for the request) and on the requester side (for the response).

The coordinator's Activation service is defined as:

<wsdl:portType name="ActivationCoordinatorPortType">
    <wsdl:operation name="CreateCoordinationContext">
        <wsdl:input message="wscoor:CreateCoordinationContext"/>
    </wsdl:operation>
</wsdl:portType>

The requester's Activation service is defined as:

<wsdl:portType name="ActivationRequesterPortType">
    <wsdl:operation name="CreateCoordinationContextResponse">
        <wsdl:input message="wscoor:CreateCoordinationContextResponse"/>
    </wsdl:operation>
    <wsdl:operation name="Error">
        <wsdl:input message="wscoor:Error"/>
    </wsdl:operation>
</wsdl:portType>

3.1.1 CreateCoordinationContext Message

This request is used to create a coordination context that supports a coordination type i.e. a service that provides a set of coordination protocols. This command is required when using a network accessible Activation service in heterogeneous environments that span vendor implementations. To fully understand the semantics of this operation it is necessary to read the specification where the coordination type is defined (e.g. WS-Transaction).

The CreateCoordinationContext message is defined as:

<CreateCoordinationContext ...>
    <ActivationService> ... </ActivationService>
    <RequesterReference> ... </RequesterReference>
    <CoordinationType> ... </CoordinationType>
    <wsu:Expires> ... </wsu:Expires>?
    <CurrentContext> ... </CurrentContext>?
    <!-- extensibility element -- >*
</CreateCoordinationContext>

/CreateCoordinationContext/ActivationService
This provides the activation service port reference.
/CreateCoordinationContext/RequesterReference
This provides the caller port reference – as described below this enables the response to be sent when using one way messaging.
/CreateCoordinationContext/CoordinationType
This provides the identifier for the desired coordination type for the activity (e.g., a URI to the Atomic Transaction coordination type).
/CreateCoordinationContext/wsu:Expires
Optional. The datetime for the returned CoordinationContext.
/CreateCoordinationContext/CurrentContext
Optional. The current CoordinationContext. This may be used for a variety of purposes including recovery and subordinate coordination environments.
/CreateCoordinationContext /{any}
Extensibility elements may be used to convey additional information.
/CreateCoordinationContext /@{any}
Extensibility attributes may be used to convey additional information.

The predefined Errorcodes for the CreateCoordinationContext request are:

  • wscoor:InvalidCreateParameters – The parameters passed to the CreateCoordinationContext were invalid.
Here is an example create message:
<CreateCoordinationContext>
    <ActivationService>
        <wsu:Address>
             http://Business456.com/tm/activation 
        </wsu:Address>
        <myapp:MyPrivateState>
             1234
        </myapp:MyPrivateState>
    </ActivationService>
    <RequesterReference>
        <wsu:Address>
             http://fabrikam123.com/app1
        </wsu:Address>
    </RequesterReference>
    <CoordinationType>
         http://schemas.xmlsoap.org/ws/2002/08/wstx
    </CoordinationType>
</CreateCoordinationContext>

3.1.2 CreateCoordinationContextResponse Message

This returns the CoordinationContext that was created.

The CreateCoordinationContextResponse message is defined as:

<CreateCoordinationContextResponse ...>
    <RequesterReference> ... </RequesterReference>
    <CoordinationContext> ... </CoordinationContext>
    <!-- extensibility element -- >*
</CreateCoordinationContextResponse>

/CreateCoordinationContext/RequesterReference
This provides the port reference of the caller that invoked CreateCoordinationContext
/CreateCoordinationContext/CoordinationContext
This is the created coordination context.
/CreateCoordinationContext /{any}
Extensibility elements may be used to convey additional information.
/CreateCoordinationContext /@{any}
Extensibility attributes may be used to convey additional information.
Here is an example create response message that does not contain additional information:
<CreateCoordinationContextResponse>
    <RequesterReference>
        <wsu:Address>
             http://fabrikam123.com/app1
        </wsu:Address>
    </RequesterReference>
    <CoordinationContext>
        <wsu:Identifier>
             http://Business456.com/tm/context1234
        </wsu:Identifier>
        <CoordinationType>
             http://schemas.xmlsoap.org/ws/2002/08/wstx
        </CoordinationType>
        <RegistrationService>
             <wsu:Address>
                  http://Business456.com/tm/registration
             <wsu:Address>
             <myapp:PrivateInstance>
                  1234
             </myapp:PrivateInstance>
        </RegistrationService>
    </CoordinationContext>
</CreateCoordinationContextResponse>

3.2 Registration Service

The Registration service definition requires a port type on the coordinator side (for the request) and on the requester side (for the response).

The coordinator's Registration service is defined as:

<wsdl:portType name="RegistrationCoordinatorPortType">
    <wsdl:operation name="Register">
        <wsdl:input message="wscoor:Register"/>
    </wsdl:operation>
</wsdl:portType>

The requester's Registration service is defined as:

<wsdl:portType name="RegistrationRequesterPortType">
    <wsdl:operation name="RegisterResponse">
        <wsdl:input message="wscoor:RegisterResponse"/>
    </wsdl:operation>
    <wsdl:operation name="Error">
        <wsdl:input message="wscoor:Error"/>
    </wsdl:operation>
</wsdl:portType>

Figure 3 illustrates the usage of PortReferences during registration. The target PortReference is indicated by bold. The coordinator provides the Registration PortReference in the CoordinationContext during the CreateCoordinationContext operation. The requesting service receives the Registration service PortReference (RegRef) in the CoordinationContext in an application message.

1.) The Register message targets this PortReference, also including the requesting service PortReference (ReqRef) and the participant protocol service PortReference as parameters.

2.) The RegisterResponse message targets the requesting service (ReqRef), also including the coordinator's protocol service PortReference.

3. & 4.) At this point both sides have the PortReferences of the other's protocol service, so the protocol messages can target the other side.

3.2.1 Register Message

The Register request is used to do the following:

  • Participant selection and registration in a particular Coordination protocol under the current coordination type supported by the Coordination Service.
  • Exchange port references. Each side of the coordination protocol (participant and coordinator) supplies a port reference.

Participants can register for multiple Coordination protocols by issuing multiple Register operations. WS-Coordination assumes that transport protocols provide for message-batching if required.

The Register operation is defined as:

< Register ...>
    <RegistrationService> ... </RegistrationService>
    <RequesterReference> ... </RequesterReference>
    <ProtocolIdentifier> ... </ProtocolIdentifier>
    <ParticipantProtocolService> ... </ParticipantProtocolService>
    <!-- extensibility element -- >*
</Register>

/Register/RegistrationService
This provides the registration port reference.
/Register/RequesterReference
The port reference that the application wants the registration service to return status information to.
/Register/ProtocolIdentifier
This URI provides the identifier of the coordination protocol selected for registration.
/Register/ParticipantProtocolService
The port reference that the registering participant wants the coordinator to use for the Coordination protocol.
/Register/{any}
Extensibility elements may be used to convey additional information.
/ Register/@{any}
Extensibility attributes may be used to convey additional information.
The predefined Errorcodes for the Register request are:
  • wscoor:AlreadyRegistered – The participant has already registered for this Coordination protocol under this activity identifier.
  • wscoor:InvalidState – The state of the coordinator no longer allows registration for this Coordination protocol.
  • wscoor:InvalidProtocol – The Coordination protocol is not supported.
  • wscoor:NoActivity – The activity does not exist.
The following is an example registration message:
<Register>
    <RegistrationService>
        <wsu:Address> 
             http://Business456.com/mycoordinationservice/registration
      </wsu:Address>
        <EBCXCode> H23974922Z </EBCXCode>
    </RegistrationService>
    <RequesterReference>
        <wsu:Address>http://Schedule456.com</wsu:Address>
    </RequesterReference>
    <ProtocolIdentifier>
        http://schemas.xmlsoap.org/ws/2002/08/wstx/2PC
    </ProtocolIdentifier>
    <ParticipantProtocolService>
        <wsu:Address> 
             http://Adventure456.com/participant2PCservice
      </wsu:Address>
        <BetaMark> AlphaBetaGamma </BetaMark>
    </ParticipantProtocolService>
</Register>

3.2.2 RegistrationResponse Message

Receipt of the RegisterResponse message indicates that registration has completed successfully. The RegisterResponse message is defined as:

<RegisterResponse>
    <RequesterReference> ... </RequesterReference>
    <CoordinatorProtocolService> ... </CoordinatorProtocolService>
    <!-- extensibility element -- >*
</RegisterResponse>

/Register/RequesterReference
The port reference that the application wants the registration service to return status information to. This should include enough information to correlate a request with a response.
/RegisterResponse/CoordinatorProtocolService
The port reference that the Coordination service wants the registered participant to use for the Coordination protocol.
/RegiserResponse/{any}
Extensibility elements may be used to convey additional information.
/ RegiserResponse /@{any}
Extensibility attributes may be used to convey additional information.

The following is an example of a RegisterResponse message where the MarkKey element is additional address information private to an implementation used to correlate the request:

<RegisterResponse>
     <RequesterReference>
          <wsu:Address>
               http://Schedule456.com
          </wsu:Address>
     </RequesterReference>
   <CoordinatorProtocolService>
        <wsu:Address>
        http://Business456.com/mycoordinationservice/coordinator
        </wsu:Address>   
        <myapp:MarkKey> %%F03CA2B%% </myapp:MarkKey>
   </CoordinatorProtocolService>
</RegisterResponse>

3.3 Error Message

Receipt of the Error message indicates that creation or registration request has failed.

The Error message is defined as:

<Error ...>
    <CoordinatorProtocolService> ... </CoordinatorControlService>?
    <Errorcode> ... </Errorcode>
    <!-- extensibility element -- >*
</Error>

/Error/CoordinatorProtocolService
The port reference that the Coordination service wants the registered participant to use for the Coordination protocol.
/Error/Errorcode
This required element is a QName that identifies the reason for the error.
/Error/{any}
Extensibility elements may be used to convey additional information.
/Error/@{any}
Extensibility attributes may be used to convey additional information.

4. Security Considerations

Because messages can be modified or forged, it is strongly RECOMMENDED that business process implementations use WS-Security to ensure messages have not been modified or forged while in transit or while residing at destinations. Similarly, invalid or expired messages could be re-used or message headers not specifically associated with the specific message could be referenced. Consequently, when using WS-Security, signatures MUST include the semantically significant headers and the message body (as well as any other relevant data) so that they cannot be independently separated and re-used.

In order to trust contexts, they SHOULD be signed using the mechanisms outlined in WS-Security. This allows readers of the context information to be certain that the contexts haven't been forged or altered in any way. It is strongly RECOMMENDED that contexts be signed. In addition to the mechanisms list above, contexts SHOULD include a message timestamp (as described in WS-Security).

It should also be noted that services implementing this are subject to various forms of denial-of-service attacks. Implementers should take this into account when building their services.

5. Relationship to Other Web Services

WS-Coordination depends on the <Expires> element defined in WS-Security and on 'Context' and 'PortReference' definitions contained in the appendices of this document.

6. Interoperability Considerations

In order for two parties to communicate, both parties will need to agree on the protocols provided. This specification facilitates this agreement and thus interoperability.

Glossary

The following definitions are used throughout this specification:

Activation service: This supports a CreateCoordinationContext operation that is used by participants to create a CoordinationContext.

CoordinationContext: Contains the activity identifier, its coordination type that represents the collection of behaviors supported by the activity and a Registration service port reference that participants can use to register for one or more of the protocols supported by that activity's coordination type.

Coordination protocol: The definition of the coordination behavior and the messages exchanged between the coordinator and a participant playing a specific role within a coordination type. WSDL definitions are provided, along with sequencing rules for the messages. The definition of coordination protocols are provided in additional specification (e.g., WS-Transaction).

Coordination type: A defined set of coordination behaviors, including how the service accepts context creations and coordination protocol registrations, and drives the coordination protocols associated with the activity.

Coordination service (or Coordinator): This service consists of an activation service, a registration service, and a set of coordination protocol services.

Participant: A service that is carrying out a computation within the activity. A participant receives the CoordinationContext and can use it to register for coordination protocols.

Registration service: This supports a Register operation that is used by participants to register for any of the coordination protocols supported by a coordination type, such as Atomic Transaction 2PC or Business Agreement NestedScope.

Web service: A Web service is a computational service, accessible via messages of definite, programming-language-neutral and platform-neutral format, and which has no special presumption that the results of the computation are used primarily for display by a user-agent.

Appendix A. Port References

Editor's note: The TX working group provides the following minimal definition of port reference for purposes of enabling interoperability. Specifically, the following is what we need in order to enable the WS-Transaction specification. Our expectation is that a more complete definition will be provided and this specification will be updated to support it.

A port reference conveys the capability to invoke operations against an individual port. Instance specific information may be added to reach a specific service instance in the target service, to scope a service interaction within a session, or to carry the values of context properties required for the invocation of the target.

A port reference contains at least the URI address of the port address.

A port reference can be extended with optional elements.

A.1 XML Representation

The following is a schema outline for <PortReference>:

<wsu:PortReference ...>
   <wsu:Address> ... </wsu:Address>
    <!-- extensibility element -- >*
</wsu:PortReference>

/PortReference/Address
Required. An absolute URI of a web services port.
/PortReference/{any}
This is an extensibility mechanism to allow additional elements to be added to the element. An implementation MAY ignore any element unless the element has a top level (child of <PortReference>) "wsu:MustUnderstand" attribute with a value of "true".
/PortReference/@{any}
This is an extensibility mechanism to allow additional attributes to be added to the element.

A.2 XML Example

The following is a simple example. The reference describes the port at address http://fabrikam123.com/accounting/auditPort with instance specific information of 1234.

<wsu:PortReference xmlns:acct="http://Adventure456.com/auditing">
    <wsu:Address>
         http://fabrikam123.com/asvc/auditPort
    </wsu:Address>
    <acct:MyPrivateInfo>
          1234 
    </acct:MyPrivateInfo>
</wsu:PortReference>

A.3 Security Considerations

In order to trust references, they SHOULD be signed using the mechanisms outlined in WS-Security.  This allows readers of the reference information to be certain that the references haven't been forged or altered in any way.  It is strongly RECOMMENDED that references be signed.

Appendix B: Context

The context is a container for sharing processing contexts between web service endpoints. This is used to allow different endpoints to correlate contexts between messages.

We define the type Context which specifies an optional expiration and an identifier. The identifier is a URI that is used to identify groups of related messages. It is expected that elements derived from this type will impose semantics about group membership. It should be noted that some derivations MAY omit the optional expiration element.

We further define a general purpose header <Context> based on this type.

B.1 XML Representation

The Context type is defined as:

<wsu:Context ...>
   <wsu:Identifier ...> ... </wsu:Identifier>
   <wsu:Expires> ... </wsu:Expires>?
    <!-- extensibility element -- >*
</wsu:Context>

/Context/Expires
This optional element indicates the expiration date of the context.
/Context/Identifier
This required element specifies a unique identifier for the context. This is a URI identifying a group of related messages.
/Context/{any}
This is an optional extensibility mechanism to allow additional elements to be specified.
/Context/@{any}
This is an extensibility mechanism to allow additional attributes to be added to the element.

B.2 XML Example

The following example illustrates the use of a context:

<wsu:Context>
   <wsu:Identifier> http://fabrikam123.com/SS/34194 </wsu:Identifier>
   <wsu:Expires> 2001-10-13T09:00:00Z </wsu:Expires>
   <myapp:MarkKey> 400 </myapp:MarkKey>
</wsu:Context>

B.3 Security Considerations

In order to trust contexts, they SHOULD be signed using the mechanisms outlined in WS-Security.  This allows readers of the context information to be certain that the contexts haven't been forged or altered in any way.  It is strongly RECOMMENDED that contexts be signed.

References

[KEYWORDS]
S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels," RFC 2119, Harvard University, March 1997
[SOAP]
W3C Note, "SOAP: Simple Object Access Protocol 1.1," 08 May 2000.
[URI]
T. Berners-Lee, R. Fielding, L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax," RFC 2396, MIT/LCS, U.C. Irvine, Xerox Corporation, August 1998.
[XML-ns]
W3C Recommendation, "Namespaces in XML," 14 January 1999.
[XML-Schema1]
W3C Recommendation, "XML Schema Part 1: Structures," 2 May 2001.
[XML-Schema2]
W3C Recommendation, "XML Schema Part 2: Datatypes," 2 May 2001.
[WSSec]
Web Services Security (WS-Security), IBM, Microsoft & VeriSign.
[WSTX]
Web Services Transactions, Microsoft, BEA & IBM.
[WSDL]
Web Services Description Language (WSDL) 1.1 "http://www.w3.org/TR/2001/NOTE-wsdl-20010315"
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