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Friday, 20 June 2008

Principles of SOA

Although there is no official standard for SOA, the community seems to agree that there are four guiding principles to achieve SOA.

  • Service boundaries are explicit.

Services expose business functionality by using well defined contracts, and these contracts fully describe a set of concrete operations and messages. Implementation details of the service are unknown, and the consumer of the service is agnostic to this, so the technology platform of the service is irrelevant to the client. What is relevant though is where the service is present so that the client can reach it to consume it.

Remoting, WCF and Web Services all support this principle. But what distinguishes WCF is its ability to answer some of the limitations in the other technologies. e.g for Remoting to be used, the underlying CLR types have to be shared between the client and the service, not so in WCF. Web Services with WSE allows a service to address this principle as well.

  • Services are autonomous.

As previously mentioned services expose business functionality, to achieve this they encapsulate the business functionality, what this means is that the service should encapsulate all tiers in the functionality from database access to business tier and the contract itself. At the end of the day the service should be replaceable and movable without affecting the consumer. As a rule of thumb to achieve this any external dependencies should be eliminated during the design process. Any component changes in  the service should change the version of the service as a logical unit. This is otherwise called atomicity.

  • Clients and Services share contracts, not code.

Given that we said service boundaries are explicit, the contract enforces the boundary between the client and service, and this leads us to conclude that a service once published cannot be changed and all future versions should be backward compatible. There is an argument that contracts may or may not be tied to a particular technology or platform. I am unable to comment on this but seems to be only fair is as long as the service can be consumed by a client and the client can remain agnostic to the technology used to implement the service, this principle is achieved.

  • Compatibility is based on policy.

Contracts define functionality offered by a service, while a policy defines the constraints imposed by the service on the consumer for attributes like security, communication protocol and reliability requirements. Prior to WCF , Web Services with WSE 3.0 helped developers achieve this, I have had the opportunity to do this on .Net 2.0 using WSE and i can say that this is pretty crucial to how your service behaves with the consumer :). At the end of the day the policy is also exposed in WSDL to the client , so that client knows the constraints imposed by the service.

As a architect/developer we may try to meet the principles listed above in our applications, but due to the influence of technology in implementation and deployment we may not be able to abide by these principles strictly, but that is probably alright. For e.g if we had three Services which encapsulate functionality for order processing, accounts and sales, we cant really have three databases for each service to work individually, we may have a reporting service which may need access to the three databases, in which case we may consolidate everything into a single database, and use schemas in the database to isolate functionality at the database level, then build a database access layer which servers the functionality required by the service. Relational data seems to be quite limiting in this respect, but would the Entity Framework allow us to address, May be may be not it is yet to be seen

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