Igor Wiedler has a recent post to his site about creating stateless services, specifically in the context of using a dependency injection container to manage the objects your application uses.

As more frameworks and libraries, particularly in the PHP world, move towards adopting the Dependency Injection pattern they are all faced with the problem of bootstrapping their application and constructing the object graph. In many cases this is solved by a Dependency Injection Container (DIC). Such a container manages the creation of all the things. The things it manages are services. Or are they?

He notes that, according to some of the principles of domain-driven design, "services" should be stateless - the results of calls to the service shouldn't alter it, it should only depend on the values passed in. He goes on to put this into the context of a DIC and gives an example of the "request service" (and how it violates the DDD principles of statelessness). He talks some about scopes (dependencies) and mutable services. He talks about methods to get around these issues with the "request" instance, ultimately coming to the conclusion that event listeners might be the way to go.