Programming WCF Services, 3rd Edition

Durable Services

Consider the case of a long-running business process or workflow, comprised of multiple execution sequences, that lasts many days or even weeks.

Note

I use the term workflow to denote a business workflow in general, not one that is necessarily supported by or related to the product called Windows Workflow.

Such long-running processes may involve clients (or even end users) that connect to the application, perform a finite amount of work, transition the workflow to a new state, and then disconnect for an indeterminate amount of time before connecting again and continuing to execute the workflow. The clients may at any point also decide to terminate the workflow and start a new one, or the backend service supporting the workflow may end it. Obviously, there is little point in keeping proxies and services in memory waiting for the clients to call. Such an approach will not robustly withstand the test of time; at the very least, timeout issues will inevitably terminate the connection, and there is no easy way to allow machines on both sides to reboot or log off. The need to allow the clients and the services to have independent lifecycles is an important one in a long-running business process, because without it there is no way to enable the clients to connect, perform some work against the workflow, and disconnect. On the host side, over time you may even want to redirect calls between machines.

The solution for long-running services is to avoid keeping the service state ...