Monday, January 05, 2009

Steve Jones on the Business Service Bus (BSB)

I've read the post about Business Service Buses (BSB) by Steve Jones and the associated GoogleDoc with interest. Steve describes how a enterprise-wide BSB mediates multiple lowever-level and more domain-specific buses (DSB). I've seen such a model (I call it "hierarchical ESB topology") used in practice by at least one client.

pro arguments:
  • divide and conquer philosophy: hide and package structural complexity, only expose services to the upper level that are required there -- hide domain-internal services
  • the bus infrastructure effectively models the organizational structure
  • distributed governance -- i.e., each business unit can manage and control governance according to its particular requirements (also see the IBM article Choose an ESB topology to fit your business model)
  • extra complexity ("middleware for your middleware")
  • heterogeneous technology mix - leads to duplicate license costs, operational & maintenance costs

An alternative?
An alternative to the hierarchical ESB topology that Steve proposes would be the "intermediary layer" pattern: Only the business-unit or domain-specific services are encapsulated by an ESB or other type of intermediation layer (legacy wrappers or adapters, domain-specific intermediation services, XML appliances, etc.). This intermediation layer is necessary so that a standard SOAP + WS* protocol can be exposed to the upper level.

Instead of using the BSB one would then rely on the SOAP stack plus the advanced WS-standards to govern the reliability, security, availability, location, routing and other non-functional aspects of the message exchange. The endpoints exposed by the different intermediary layers would also need to agree on messaging patterns and on data formats (see canonical schema (158)).

I've modified Steve's diagram and replaced the BS with "SOAP and WS*" so that it looks like this:

Now I'd be really curious: what are the pros and cons of this alternative model compared to the original hierarchical topology pattern?

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