State in services architecture

I recently came across a post from Harry Pierson from Microsoft questioning the received wisdom that Services should be stateless. Harry makes a valid point that any meaningful business process has state and goes on to deduce that this implies that services should have state too.

I think Harry is missing the crucial point here by implicitly conflating business process and service state. In a good SOA ( and by good I mean one which delivers on the key promise of loose coupling), business processes indeed have state - they will be meaningless otherwise. However, there is a difference between process and service state.

In my view process state should not creep into services as this makes it difficult to replace service implementations over time and can lead to tight coupling in architectures. Ideally process state should be held in an intermediate metadata layer that forms part of the SOA infrastructure ( insert your favourite 3 letter flavour-of-the-month acronym in this sentence at this point - ESB being a prime candidate).

Service state is a different issue though. I think good SOA architectures should ensure that service state is kept inside the boundary and only exposed through contracts. This way, the service can truly deliver on the promise of being an independent blob of functionality that is independent of the context in which it is used. If this isn't the case in a specific instance, I would question the value in creating an SOA architecture.

To illustrate the point imagine a service that manages customers in any organisation. A CustomerManagement service in what I would see as good design would support an operation to return the status of a customer and would allow non specific states of the entity it manages to be tracked. For example, it may provide methods to change the state of a customer entity from enabled to disabled and have methods to return the current status. Note that such interactions cause the underlying entity to change state across all the processes that may be using the service. However, this still leads to loosely coupled architectures as the complete maintenance of this state lies inside the service so any change to state is only available to other servces through defined contracts.

Now consider an AddCustomer business process. In an organisation this may require a 3 day process that uses 3 manual steps including calls to the CustomerManagement Service above. Obviously keeping the state of this process inside a service makes it difficult to modify the process in the future - that is why the whole suite of BPM type tools ( including Microsoft's own Biztalk) exists. Typically one would model this process not in a service but as as some sort of orchestration using multiple services. The SOA infrastructure would manage the state of the process while the servces themselves would manage business state of the associated entities as indicated above.

This has gone on a bit but I think it is a crucial point which I summarise here in the form of some architecture principles :

- Services should only have state that is exposed through well defined contracts and makes sense in an Enterprise wide, cross-process sense ( like customer status).
- Service state should never creep outside the service boundary.
- Process state should ideally be maintained in infrastructure software - indeed it may form the justification for the purchase of such SOA infrastructure in an enterprise.
- Keeping process state in infrastructure and service state inside the boundary is key to creating loosely coupled, flexible architectures whilst supporting dynamic business processes.