Half-Sync/Half-Async

This article makes a brief introduction of the Half-Sync/Half-Async pattern from POSA2 as my reading note.

The Half-Sync/Half-Async architectural pattern decouples asynchronous and synchronous service processing in concurrent systems, to simplify programming without unduly reducing performance. The pattern introduces two intercommunicating layers, one for asynchronous and one for synchronous service processing.

The structure of the Half-Sync/Half-Async pattern follows the Layers pattern and includes four participants:

Synchronous service layer: The synchronous service layer performs high level processing services. Services in the synchronous layer run in separate threads or processes that can block while performing operations.

Asynchronous service layer: The asynchronous service layer performs  lower level processing services, which typically emanate from one or more external event sources. Services in the asynchronous layer cannot block while performing operations without unduly degrading the performance of other services.

Queueing layer: The queueing layer provides the mechanism for communicating between services in the synchronous and asynchronous layers. For example, messages containing data and control information are produced by asynchronous services, then buffered at  the queueing layer for subsequent retrieval by synchronous services, and vice versa. The queueing layer is responsible for notifying services in one layer when messages are passed to them from the other layer. The queueing layer therefore enables the asynchronous and synchronous layers to interact in a 'producer/consumer' manner, similar to the structure defined by the  Pipes and Filters pattern.

External event sources: External event sources generate events that  are received and processed by the asynchronous service layer. Common sources of external events for operating systems include network interfaces, disk controllers, and end-user terminals.