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Tenet 2: Services Are Autonomous

When speaking of services as “autonomous” entities, we are referring to the fact that a given WCF

service is (as much as possible) an island unto itself. An autonomous service should be independ-

ent with regard to version issues, deployment issues, and installation issues. To help promote this

tenet, we yet again fall back on a key aspect of interface-based programming. Once an interface is

in production, it should never be changed (or you will risk breaking existing clients). When you

need to extend the functionality of your WCF service, simply author new interfaces that model the

desired functionality.

Tenet 3: Services Communicate via Contract, Not Implementation

The third tenet is yet another byproduct of interface-based programming in that the implementa-

tion details of a WCF service (which language it was written in, how it gets its work accomplished,

etc.) are of no concern to the external caller. WCF clients interact with services solely through their

exposed public interfaces. Furthermore, if the members of a service interface expose custom com-

plex types, they need to be fully detailed as a data contract to ensure all callers can map the content

into a particular data structure. 

Tenet 4: Service Compatibility Is Based on Policy

Because CLR interfaces provide strongly typed contracts for all WCF clients (and may also be used

to generate a related WSDL document based on your choice of binding), it is important to point out

that interfaces/WSDL alone is not expressive enough to detail aspects of what the service is capable

of doing. Given this, SOA allows us to define “policies” that further qualify the semantics of the serv-

ice (e.g., the expected security requirements used to talk to the service). Using these policies, we are

able to basically separate the low-level syntactic description of our service (the exposed interfaces)

from the semantic details of how they work and how they need to be invoked.