2 SOAP Message ExchangeProcessing Model

SOAP messages are fundamentally one-way transmissions from a SOAP sender to a SOAP receiver; however, SOAP messages are often combined to implement patterns such as request/response.

SOAP implementations can be optimized to exploit the unique characteristics of particular network systems. For example, the HTTP binding described in [1](SOAP in HTTP) provides a request/response message exchange pattern that may be leveraged by SOAP applications.provides for SOAP response messages to be delivered as HTTP responses, using the same connection as the inbound request.

SOAP provides a distributed processing model that assumes that a SOAP message originates at an initial SOAP sender and is sent to an ultimate SOAP receiver via zero or more SOAP intermediaries. This section defines the SOAP distributed processing model. Section 5 defines a framework for describing how additional features such as routing, reliability and security fit into the distributed processing model.

2.1 SOAP Nodes

A SOAP node can be the initial SOAP sender, the ultimate SOAP receiver, or a SOAP intermediary, in which case it is both a SOAP sender and a SOAP receiver. SOAP does not provide a routing mechanism, however SOAP does recognise that a SOAP sender originates a SOAP message which is sent to an ultimate SOAP receiver, via zero or more SOAP intermediaries.

A SOAP node receiving a SOAP message MUST perform processing according to the SOAP processing model as described in this section and, if appropriate, generate SOAP faults, SOAP responses and send additional SOAP messages, as provided by the remainder of this specification.

2.2 SOAP Actors Roles and SOAP Nodes

In processing a SOAP message, a SOAP node is said to act in the one or more SOAP roles of one or more SOAP actors, each of which is identified by a URI known as the SOAP actor role name. Each SOAP node MUST act in the role of the special SOAP actor role named "http://www.w3.org/2001/12/soap-envelope/actorrole/next", and can additionally assume the roles of zero or more other SOAP actorsroles. SOAP nodes MUST NOT act in the role of the special SOAP role named "http://www.w3.org/2001/12/soap-envelope/role/none". A SOAP node can establish itself as the ultimate SOAP receiver by acting in the (additional) role of the anonymous SOAP actor. The roles assumed MUST be invariant during the processing of an individual SOAP message; because this specification deals only with the processing of individual SOAP messages, no statement is made regarding the possibility that a given piece of software might or might not act in varying roles when processing more than one SOAP message.

SOAP nodes MUST NOT act in the role of the special SOAP actor named "http://www.w3.org/2001/12/soap-envelope/actor/none". Header blocks targeted to this special actor are carried with the message to the ultimate receiver, but are never formally "processed". Such blocks MAY carry data that is required for processing of other blocks.

While the purpose of a SOAP actor role name is to identify a SOAP node, there are no routing or message exchange semantics associated with the SOAP actor role name. For example, SOAP Actors roles MAY be named with a URI useable to route SOAP messages to an appropriate SOAP node. Conversely, it is also appropriate to use SOAP actor roles with names that are related more indirectly to message routing (e.g. "http://example.org/banking/anyAccountMgr") or which are unrelated to routing (e.g. a URI meant to identify "all cache management software"; such a header might be used, for example, to carry an indication to any concerned software that the containing SOAP message is idempotent, and can safely be cached and replayed.)

Except for "http://www.w3.org/2001/12/soap-envelope/actorrole/next", and "http://www.w3.org/2001/12/soap-envelope/actorrole/none" and the anonymous actor this specification does not prescribe the criteria by which a given node determines the (possible empty) set of roles in which it acts on a given message. For example, implementations can base this determination on factors including, but not limited to: hardcoded choices in the implementation, information provided by the transport binding (e.g. the URI to which the message was physically delivered), configuration information made by users during system installation, etc.

2.3 Targeting SOAP Header Blocks

SOAP header blocks carry optional actor role attribute information items (see 4.2.2 SOAP actor Attribute) that are used to target them to the appropriate SOAP node(s). SOAP header blocks with no such attribute information item are implicitly targeted at the anonymous SOAP actor, implying that they are to be processed by the ultimate SOAP receiver. This specification refers to the (implicit or explicit) value of the SOAP actor role attribute as the SOAP actor role for the corresponding SOAP header block.

A SOAP header block is said to be targeted to a SOAP node if the SOAP actor role (if present) on the header block matches (see [7]) a role played by the SOAP node, or in the case of a SOAP header block with no actor role attribute information item, if the SOAP node is acting in the role of the ultimate SOAP receiver.

Header blocks targeted to the special role "http://www.w3.org/2001/12/soap-envelope/role/none" are carried with the message to the ultimate receiver, but are never formally "processed". Such blocks MAY carry data that is required for processing of other blocks.

A SOAP Node that acts in the role of the anonymous actor with respect to a particular SOAP message becomes the The ultimate SOAP recipient of that a SOAP message. Such a SOAP node is additionally responsible for processing all parts of the message intended for the anonymous actor, including the message body, according to the rules described in this section. The SOAP message path for that message ends at the ultimate recipient. However, the ultimate recipient may delegate all or part of its responsibility to other entities. The ultimate recipient may happen to use distinct SOAP message exchanges to interact with such entities. Nevertheless, responsibility for the processing of parts of a SOAP message targetted at the anonymous actor rests with the ultimate recipient of that message[1]

2.4 Understanding SOAP Headers

It is likely that specifications for a wide variety of header functions will be developed over time, and that some SOAP nodes MAY include the software necessary to implement one or more such extensions. A SOAP header block is said to be understood by a SOAP node if the software at that SOAP node has been written to fully conform to and implement the semantics conveyed by the combination of local name and namespace name of the outer-most element information item of that SOAP header block.

SOAP header blocks carry optional mustUnderstand attribute information items (see 4.2.3 SOAP mustUnderstand Attribute). When the value of such an attribute information item is "true" or "1", the SOAP block is said to be mandatory.

For every mandatory SOAP header block targeted to a node, that node MUST either process the block according to the semantics conveyed by the combination of local name and namespace name of the outer-most element information item of that block; or not process the SOAP message at all, and instead generate a fault (see 2.6 Processing SOAP Messages and 4.4 SOAP Fault).

Mandatory blocks MUST be presumed to somehow modify the semantics of other headers or body elements. Tagging SOAP blocks as mandatory thus assures that such changes in semantics will not be silently (and, presumably, erroneously) ignored by a SOAP node to which the header block is targetted.

The mustUnderstand attribute information item is not intended as a mechanism for detecting errors in routing, misidentification of nodes, failure of a node to serve in its intended role(s), etc., any of which may result in a failure to even attempt processing, and the subsequent removal, of a given SOAP header block from a SOAP envelope. This specification therefore does not require any fault to be generated based on the presence or value of this attribute on a SOAP header block not targeted at the current processing node, for example when it is suspected that such a block has survived erronously due to a routing or targeting error at a preceeding intermediairy. In particular, it is not an error for a mandatory header block targeted to a role other than the ones assumed by the ultimate SOAP receiver to reach that node without having been processed.

2.5 Structure and Interpretation of SOAP Bodies

A SOAP body consists of zero or more namespace qualified element information items, which are the immediate children of the Body element information item. The ultimate SOAP receiver MUST correctly process all such body elements. However, Part 1 of this specification (this document) mandates no particular structure or interpretation of such elements, and provides no standard means for specifying the processing to be done.

When multiple body elements are present, such elements MAY represent a single unit of work to be performed, MAY represent multiple separate processing steps, possibly but not necessarily in order, MAY represent data or metadata, MAY convey a mixture of work units and data, etc. The ultimate SOAP recipient MAY use the local name(s) and namespace name(s), on any or all body elements, to determine the processing to be performed. Indeed, the SOAP RPC convention (see [1]Using SOAP for RPC) uses just such a method. Conversely, other information in the body and/or headers MAY be used to make such a determination.[2]

2.6 Processing SOAP Messages

This section sets out the rules by which SOAP messages are processed. Unless otherwise stated, processing must be semantically equivalent to performing the following steps separately, and in the order given. Note however that nothing in this specification should be taken to prevent the use of optimistic concurrency, roll back, or other techniques that might provide increased flexibility in processing order as long as all SOAP messages, SOAP faults and application-level side effects are equivalent to those that would be obtained by direct implementation of the following rules in the order shown below.

1.      Determine the set of roles in which the node is to act. The contents of the SOAP envelope, including header blocks and the body, MAY be inspected in making such determination.

2.      Identify all header blocks targeted at the node that are mandatory.

3.      If one or more of the header blocks identified in the preceding step are not understood by the node then generate a single SOAP MustUnderstand fault (see 4.4.6 MustUnderstand Faults). If such a fault is generated, any further processing MUST NOT be done. Faults relating to the existence or contents of the body MUST NOT be generated in this step.[3]

4.      Process all header blocks targeted at the node and, in the case of the ultimate SOAP recipient, the SOAP body. A SOAP node MUST process all SOAP header blocks targeted at it. A SOAP node MAY choose to ignore the processing implied by non-mandatory SOAP header blocks targeted at it.

5.      In the case of a SOAP intermediary, and where the message is to be forwarded further along the message path, remove all SOAP header blocks targeted at the node, and possibly insert new SOAP header blocks.

In all cases where a SOAP header block is processed, the SOAP node must understand the SOAP header block and must do such processing in a manner fully conformant with the specification for that block. The ultimate recipient MUST process the SOAP body, in a manner consistent with 2.5 Structure and Interpretation of SOAP Bodies.

If processing is unsuccessful, exactly one fault MUST be generated by the node. Header-related faults other than mustUnderstand faults (see 4.4 SOAP Fault) MUST be SOAP Sender or DataEncodingUnknown faults (see 4.4.5 SOAP Fault Codes) and MUST conform to the specification for the corresponding SOAP header block. Faults relating to the body MUST be SOAP Sender or DataEncodingUnknown faults (see 4.4.5 SOAP Fault Codes).[4]

SOAP nodes can make reference to any information in the SOAP envelope when processing a SOAP block. For example, a caching function can cache the entire SOAP message, if desired.

The processing of particular SOAP header block MAY control or determine the order of processing for other SOAP header blocks and/or the SOAP body. For example, one could create a SOAP header block to force processing of other SOAP header blocks in lexical order. In the absence of such a controlling block, the order of header and body processing is at the discretion of the SOAP node; header blocks MAY be processed in arbitrary order, and such processing MAY precede, be interleaved with, or MAY follow processing of the body. For example, a "begin transaction" header block would typically precede, a "commit transaction" would likely follow, and a "logging" function might run concurrently with body processing.

If the SOAP node is a SOAP intermediary, the SOAP message pattern and results of processing (e.g. no fault generated) MAY require that the SOAP message be sent further along the SOAP message path. Such relayed SOAP messages MUST contain all SOAP header blocks and the SOAP body from the original SOAP message, in the original order, except that SOAP header blocks targeted at the SOAP intermediary MUST be removed (such SOAP blocks are removed regardless of whether they were processed or ignored). Additional SOAP header blocks MAY be inserted at any point in the SOAP message, and such inserted SOAP header blocks MAY be indistinguishable from one or more just removed (effectively leaving them in place, but emphasizing the need to reinterpret at each SOAP node along the SOAP message path.)


The above rules apply to processing at a single node. SOAP extensions features MAY be designed to ensure that mandatory (and other) headers are processed in an appropriate order, as the message moves along the message path towards the ultimate recipient. Specifically, such extensions might specify that a (Sender) fault is generated if some SOAP header blocks have inadvertently survied past some intended point in the message path. Such extensions features MAY depend on the presence or value of the mustUnderstand attribute information item in the surviving headers when determining whether an error has occurred.


[1] Implementation details.

[2] This was deleted because it seems to be primer stuff rather than spec material.

[3] This is implied by the context, but it might still be worth stating explicitly.

[4] Why limit the set of soap faults.