Subject Line: .NET UPDATE, May 15, 2003
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* ROLES IN SOAP 1.2 The World Wide Web Consortium (W3C) has released a proposed recommendation for the next version of Simple Object Access Protocol (SOAP) 1.2. Although most of the proposed changes probably won't affect non-developers, the use of roles and the introduction of new roles in this version of the protocol can help you understand how the message-passing works.
SOAP is the protocol that .NET applications use to pass XML messages. The specification describes how the protocol should work. The protocol uses remote procedure calls (RPCs) over HTTP (SOAP is a high-level protocol, not a transport protocol like TCP/IP), but it can also cooperate with a couple of other protocols.
Each time a SOAP node processes a message--either passing it on to another node or keeping it, as the final destination--that SOAP node adopts a particular role to let it process the message accordingly. Roles aren't new to the most recent version of SOAP, but SOAP 1.2 defines two new roles, introduces a new sub-element used to explicitly define the roles, and provides a more detailed processing model for how roles affect message processing.
SOAP 1.2 has three roles--"none," "next," and "ultimate receiver"--which are used in SOAP message headers or the message body. The values aren't recorded as "none" and so forth but as Uniform Resource Identifiers (URIs) pointing to the appropriate definition on the W3 Web site.
If a message header's role has the "none" attribute, then no SOAP node should process the contents. This rule doesn't preclude a node examining the content of a message if another message header points to the content, but it does mean that the node should not alter the data.
If a node receives a message containing a header with the "next" value as its role, then that node must be able to process the message if necessary. Therefore, intermediaries along the path from the message's originator to its final destination always play the "next" role.
The "ultimate receiver" role isn't defined in the message header but in the body of the message where the payload resides. This role is implied rather than explicit. The role of the node can be explicitly stated in the body of the message, but it's not necessary--if no role is listed, then the "ultimate receiver" role is the default. Also, you can't leave a role undefined but with an empty value--that is, instead of "next" the role is "". In that case, the Uniform Resource Identifier (URI) defining the role is the base URI for that SOAP message. SOAP 1.2 doesn't define the mechanics of deriving the base URI, but HTTP, the protocol on which SOAP depends, does.
A SOAP node can act in multiple roles. In fact, in the case of the ultimate receiver, it always will, because the node must both be able to process the headers and receive the body of the message. The SOAP node that originally sent the message has no defined role.
Roles let a SOAP message know what it's supposed to do with a message and provide pointers to explicit instructions for how to process it. Any SOAP node that receives a message for processing--either to give to another node or to process itself--will use roles defined in the header or defined implicitly or explicitly in the body of the message.
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