Monday, 3 August 2015

SOA/MSA design - Isolating contract dependencies

Contract is a crucial part of any services anatomy. In a SOA/MSA approach, contracts exposes the services behaviour that consumers will rely on. This behaviour is usually exposed as series of interfaces and input/output structures and implemented by internal components. The last is usually hidden from external consumers. The previous described structure may look like this:

On this example, modules are splitted, there is only one dependency flow (internal modules doesn't know external ones), each module has Its own set of dependencies, consumers rely on abstractions (the contract), so far so good.
When consuming services through REST or SOAP, consumers will need to serialise and deserialise the structures service exposes. It means they will need to hold a copy of these structures locally in order to get this job done. Now lets say you want to avoid this structure replication through all consumers inside your company. Considering services are built following the structure illustrated before, the contract module could be splitted apart and then shared with the consumers. In this scenario, you may get ride of the structure replication issue, but will running into another. When issuing the contract module as is to consumers, they will now depend on the same libraries that contract depends, otherwise consumers wont be able to use them. What about services moving to a different library-A version that isn't compatible with previous ones? A scenario like this:

If all consumers don't upgrade to the same library-A version than service is they will be broken. More consumers you have on this model, more synchronisation between them will be needed to deploy. More services you have worst It get. The expected agility when choosing this architecture approach will be harder to achieve.
One possible solution for this problem is, accept that consumers will hold local copies and just deal with It. Accepting they will have some extra work is still better than the coupling scenario described before.
You can also design your service keeping the contract module with less dependencies as possible. It may looks like this:

This design guideline can increase services flexibility when sharing contract as library. Consumers would need be affected only contract behaviour changes.even here but there are techniques to mitigate these effects. It is also an example that being succeed on going for SOA/MSA approaches depends also on good design and architectures choices.

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