Monday, February 25, 2008

Web Services: RPC, REST and Messaging

[Originally posted on my old BEA Dev2Dev blog on February 25, 2008]
Choosing a model for interoperable communication in the enterprise

For the implementation of Web Services in the enterprise environment, I've seen many different technologies used. Recently, in my spare moments, I've reflected on this and have come to the conclusion that all these technologies tend to fit one of three models (or hybrids of these models).

I would summarise these three models as:
  1. Remote Procedure Calls (RPC). A client-server based remotable pattern where a subset of an existing system's local functions is exposed pretty much 'as-is' over the wire to client programs.
  2. Resource-oriented Create-Read-Update-Delete (CRUD). A client-server based resource-oriented pattern where the server-side provides a representation of a set of resources (often hierarchical) and exposes Create, Read, Update and Delete capabilities for these resources to client programs.
  3. Messaging (eg. as commonly seen with Message Oriented Middleware and B2B). Messages or documents are passed asynchronously between peer systems in either, but not always both, directions.
Sometimes its hard to distinguish between these models and where the boundaries lie. In fact, I don't think there are boundaries, only grey areas and all three models lie in the same spectrum. I've attempted to visualise this spectrum in the diagram below.

Depending on where your implementation lies in the spectrum, the different highlighted characteristics should manifest themselves. 
In the Web Services world, we may typically implement these three models using one of the following three approaches:
  1. Remote Procedure Calls: SOAP using a synchronous RPC programming approach and, typically, generated 'skeletons/stubs' and some sort of Object-to-XML marshalling technology
  2. Resource-oriented Create-Read-Update-Delete: REST or 'RESTful Web Services' or ROA, re-using World-Wide-Web based approaches and standards like HTTP and URIs
  3. Messaging: SOAP using an asynchronous Message/Document passing approach where invariably the documents are defined by schemas and, often, the use of message-level (rather than transport-level) security elements is required
The confusing thing is the fact that SOAP can happily and naturally satisfy two of these three models (ie. both RPC and Messaging), albeit that these two models are polar opposites which causes much confusion and is probably part of the reason why SOAP gets a bad name in some quarters.

The problem is further compounded with SOAP because the SOAP specification and accompanying collateral talks about two possible 'Styles' (ie. 'RPC' style and 'Document' style). However as I explained here, just because your SOAP Web Services are marked as 'Document' style in your WSDLs, it doesn't mean you are not doing remote procedure call based programming.

When faced with the REST zealot or the WS-* zealot, we probably need to bear this spectrum in mind. For the Web Services paradigm, there is not a 'one-size fits all' and specific requirements for a given situation should dictate which position in this spectrum best lends itself to satisfying the requirements. Also, the overlap between the models may be greater than shown in the diagram. For example, some would argue that REST can happily and more appropriately be used to fulfil what would otherwise be RPC oriented problems, in addition to solving Resource-oriented CRUD style problems.

Soundtrack for today: Purity by The God Machine

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