Synchronizing access to the service instance using
ConcurrencyMode.Single or an explicit synchronization lock only manages
concurrent access to the service instance state itself. It does not provide safe access to
the underlying resources the service may be using. These resources must also be thread-safe.
For example, consider the application shown in Figure 8-2.
Figure 8-2. Applications must synchronize access to resources
Even though the service instances are thread-safe, the two instances try to concurrently access the same resource (such as a static variable, a helper static class, or a file), and therefore the resource itself must have synchronized access. This is true regardless of the service instancing mode. Even a per-call service could run into the situation shown in Figure 8-2.
The naive solution to providing thread-safe access to resources is to provide each resource with its own lock, potentially encapsulating that lock in the resource itself, and ask the resource to lock the lock when it's accessed and unlock the lock when the service is done with the resource. The problem with this approach is that it is deadlock-prone. Consider the situation depicted in Figure 8-3.
Figure 8-3. Deadlock over resources access
If the figure, Instance A ...