Through an asynchronous callback enhancement, a thread makes a non-blocking request (e.g., send, receive, I/O) to a message passing interface library, and a callback routine is associated with the request as an asynchronous callback to the thread. The callback is queued for execution in the requesting thread and so has a deterministic execution context. Callback queuing may occur in response to another thread detecting that the request is complete. Further control over callback execution is provided by state transitions which determine whether the thread is open to processing (e.g., executing) an asynchronous callback. Callback association is done by a broad or by narrow association routines. An application which has processes organized in ranks, each including a communication thread with associated callback(s), and multiple worker threads. Interruptible wait enhancements may also be present.