This is a concurrency pattern. The pattern separates asynchronous I/O from the synchronous one. Main thread doesn't block on incoming client requests and long-running operations are offloaded to a dedicated synchronous layer. Processing results are delivered by the means of callbacks.
Some resources:

• http://www.cs.wustl.edu/~schmidt/PDF/PLoP-95.pdf
• http://zezutom.blogspot.in/2014/08/concurrency-patterns-half-sync-half.html
• http://www.dre.vanderbilt.edu/~schmidt/cs282/PDFs/6-Concurrency-and-Synchronization-part10.pdf