The Half-Sync/Half-Async pattern decouples synchronous I/O from asynchronous I/O in a system to simplify concurrent programming effort without degrading execution efficiency.

Use Half-Sync/Half-Async pattern when

• a system possesses following characteristics:
  • the system must perform tasks in response to external events that occur asynchronously, like hardware interrupts in OS
  • it is inefficient to dedicate separate thread of control to perform synchronous I/O for each external source of event
  • the higher level tasks in the system can be simplified significantly if I/O is performed synchronously.
• one or more tasks in a system must run in a single thread of control, while other tasks may benefit from multi-threading.

• BSD Unix networking subsystem
• Real Time CORBA
• Android AsyncTask framework

• Douglas C. Schmidt and Charles D. Cranor - Half Sync/Half Async
• Pattern Oriented Software Architecture Vol I-V