Programming WCF Services, 2nd Edition

Instances and Concurrent Access

Using the same proxy, a single client can issue multiple concurrent calls to a service. The client can use multiple threads to invoke calls on the service, or it can issue one-way calls in rapid succession on the same thread. In both of these cases, whether the calls from the same client are processed concurrently is the product of the service's configured instancing mode, the service's concurrency mode, and the configured delivery mode (that is, the transport session). The following discussion applies equally to request-reply and one-way calls.

Per-Call Services

In the case of a per-call service, if there is no transport-level session, concurrent processing of calls is allowed. Calls are dispatched as they arrive, each to a new instance, and execute concurrently. This is the case regardless of the service concurrency mode. I consider this to be the correct behavior.

If the per-call service has a transport-level session, whether concurrent processing of calls is allowed is a product of the service concurrency mode. If the service is configured with ConcurrencyMode.Single, concurrent processing of the pending calls is not allowed, and the calls are dispatched one at a time. The reason is that with ConcurrencyMode.Single WCF tries to maintain the guarantee of the transport session that messages are processed strictly in the order in which they were received in that session by having exactly one outstanding instance per channel. You should avoid lengthy ...