You have a resource that is shared by multiple threads. You need to provide exclusive access to this resource when a thread is writing to it. However, you do not want the overhead of providing exclusive access to this resource when multiple threads are only reading from it. You want to allow one thread to access a shared resource only if it is writing to it, but you also want to allow multiple threads to read from this resource. While multiple threads can read from a resource, a write operation cannot occur while any thread is reading from this resource.