All of the message enqueue and dequeue routines make use of context variables. Each context variable is used to keep track of a single "thread" of message enqueue or dequeue operations. By using multiple context variables, a program can manage and perform several simultaneous message enqueue and dequeue operations. While each enqueue context controls only a single message submission, each dequeue context can control an entire series of message dequeues (e.g., with a single dequeue context all message for a given channel can be processed and dequeued).