"Publication - is the Auction Of the Mind of Man" Emily Dickinson
Friday, August 5, 2005

One of the benefits of service oriented systems is that they are loosely coupled.

David Orchard analyzes what loose coupling means from the perspective of the Web services stack. A human being can recognize that a field in a form is misplaced, software cannot. So for a particular message invocation, early binding is necessary. This is certainly true for standards. There needs to be a defined place for addresses and security tokens.

Orchard asks us to imagine Purchase Order system. A particular piece of information in a particular message must be bound to the appropriate programming types. If you need to know the name of the purchaser, you must early bind to the format of that name. Or to use fancy language, the service must understand its semantics. But it is only necessary for those programming types that the service needs to understand. Here is where building service interactions as messages rather than as remote procedure calls (RPC) is important.

If a service interaction is defined in terms of RPC, then if you change the semantics, you must change the service interface. As long as one type of the method call changes, the whole interface is broken. If you send messages (concretely XML messages), so long as the service can find the information it needs, the service is not bound to a particular message format. Other information can change, but the service does not care.

For example, if a service processing a message does not care about security, they can ignore the WS-Security SOAP headers. Those headers can change and the service can ignore all the security possibilities. The inventory service does not care if the credit information changes.

True, if XPath is used you are dependent on a certain structure to find information, but if you mark your documents with its version, or associated XML Schema, you could use the appropriate location path for the document. Or if you want to bind everything to type you can use the appropriate XML Schema instance to serialize the message to the appropriate programming types.

Loose coupling at the application level is about inserting levels of indirection to handle versioning (so what else is new?). But a message can do this because at the service interface the message is opaque. A RPC is not opaque.

At the application level loose coupling is how easy is to make a change that does not impact other parts of the system. With opaque messaging, a new version can be added without impacting other clients. If a service wants to reject a version it no longer supports, or does not yet support, it can do so without impacting other clients. In this restricted, but vitally important sense, semantic meaning in a Web service can be late bound.

Friday, August 5, 2005 11:23:35 AM (Eastern Daylight Time, UTC-04:00) | Comments [0] | All | Software Development#
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