Programming WCF Services, 2nd Edition

One-Way Operations

There are cases when an operation has no return value, and the client does not care about the success or failure of the invocation. To support this sort of fire-and-forget invocation, WCF offers one-way operations: once the client issues the call, WCF generates a request message, but no correlated reply message will ever return to the client. As a result, one-way operations cannot return values, and any exceptions thrown on the service side will not make their way to the client.

Ideally, when the client calls a one-way method, it should be blocked only for the briefest moment required to dispatch the call. However, in reality, one-way calls do not equate to asynchronous calls. When one-way calls reach the service, they may not be dispatched all at once but may instead be queued up on the service side to be dispatched one at a time, according to the service's configured concurrency mode behavior. (Chapter 8 will discuss concurrency management and one-way calls in depth.) How many messages the service is willing to queue up (be they one-way or request-reply operations) is a product of the configured channel and reliability mode. If the number of queued messages has exceeded the queue's capacity, the client will be blocked even if it's issued a one-way call. However, once the call is queued, the client will be unblocked and can continue executing, while the service processes the operation in the background.

It's also wrong to equate one-way calls with concurrent calls. ...