This is my stake in the ground for now. SOA in the market-place places total emphasis on 2 things. Web Services as a basis for communication and Re-use as a basis for convincing the boss to put some cash into your middleware bunker..no…play-pen…err…seat of learning.
In addition the militant splinter-groups of the new-wave of RESTafarians (of whom I am an empathising skeptic on this specific point about service specification) call for the death of WSDL and the reliance on (WSDL-lite) WADL in the case of the less extremist, but plain-old inference from sample instance documents in the case of the hard-core….
I am finding myself sailing down the no-mans land between these two polarised viewpoints, and see the need for specification in the more complex end of the interface spectrum, but similarly don’t see how specifications help when decoding the specification is harder than inferring from samples when interface contracts are relatively intuitive. So there we have a basic mental picture of my map of the universe.
Now I’m getting to the point.
I’m now convinced that SOA’s push for re-use established through WSDL everywhere, but equally the more recent RESTafarian voices relating to unspecification both have flaws when we are attempting to open up a generalised interface into a service endpoint capable of dealing with a range of entity variants (say product types for example).
My view here is that in the SOA landscape, the static and limited semantic capability of XMLSchema, and in the RESTian lanscape, the inability of humans to infer correctness without a large number of complex instance document snapshots, leads me to the conclusion that there is a vast, yawning, gaping, chasm of understanding in what constitutes an effective contract in locking down the permissible value permutations – aka the semantic contract.
I’ve seen MS-Word. I’ve seen MS-Excel. I’ve seen bleeding-eyes-on-5-hour-conference-calls-relating-to-who-means-what-when-we-say-customer, and best of all I’ve seen hardwired logic constructed in stove-pipes behind re-usable interfaces, aka lipstick on the pig.
I reckon the semantic contract – the contract locking down the permissible instances is far more important than the outer structural contract who’s value decays as the level of re-use and inherent complexity of the interface increases. In addition there are likely to be multiple iterations/increments of a semantic contract within the context of a structural contract as service functionality is incremented over successive iterations – adding product support incrementally to an ordering service of example. This leads to to the notion of the cable cross-section:
In the SOA context…WSDL drives tooling to abstract me from the structural contract. But the formation of the semantic contract as the expression of what the provider is willing to service via that re-usable and loose structural contract is the key to effective integration.
If we don’t pay this enough respect we’ll be using our system testing to mop-up the simple, avoidable instance-data related problems that could be easily avoided if we’d formalised the semantic contract earlier in the development lifecycle…
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