Programming WCF Services, 2nd Edition

Instance Management and Concurrency

Service-instance thread safety is closely related to the service instancing mode. A per-call service instance is thread-safe by definition, because each call gets its own dedicated instance. That instance is accessible only by its assigned worker thread, and because no other threads will be accessing it, it has no need for synchronization. However, a per-call service is typically state-aware. The state store can be an in-memory resource such as static dictionary, and it can be subject to multithreaded access because the service can sustain concurrent calls, whether from the same client or from multiple clients. Consequently, you must synchronize access to the state store.

A per-session service always requires concurrency management and synchronization, because the client may use the same proxy and yet dispatch calls to the service on multiple client-side threads. A singleton service is even more susceptible to concurrent access, and must support synchronized access. The singleton has some in-memory state that all clients implicitly share. On top of the possibility of the client dispatching calls on multiple threads, as with a per-session service, a singleton may simply have multiple clients in different execution contexts, each using its own thread to call the service. All of these calls will enter the singleton on different threads from the I/O completion thread pool—hence the need for synchronization.