EventLoopGroups

EventLoopGroups refer to collections of event loop threads that collectively handle I/O and event processing for channels in asynchronous, non-blocking frameworks such as Netty. An event loop is a single thread dedicated to monitoring I/O readiness and dispatching events to channel handlers. A group of such event loops, managed as an EventLoopGroup, provides a scalable pool of threads that can execute many channels concurrently. The EventLoopGroup interface extends the broader EventExecutorGroup and offers mechanisms to register channels and to shut down gracefully. Each channel is assigned to an event loop within the group, and the group distributes new channels across its loops to balance load.

Netty implements several concrete EventLoopGroup types to adapt to different transports and platforms. The standard NioEventLoopGroup uses Java NIO selectors for portable, non-blocking I/O. Platform-specific options include EpollEventLoopGroup on Linux, which leverages the native epoll mechanism for improved performance, and KQueueEventLoopGroup on macOS and other BSDs, which uses the kqueue API. For legacy blocking I/O, OioEventLoopGroup is available, though it is less common in modern applications. The choice of transport affects performance, compatibility, and resource usage.

A typical usage pattern in server applications involves two distinct EventLoopGroups: a boss group to accept incoming connections and a worker group to handle I/O of established connections. The server bootstrap is configured with both groups, enabling separation of connection acceptance from data processing and allowing independent tuning of their thread counts. Lifecycle management is performed through graceful shutdowns that release resources and terminate event loops cleanly.

Overall, EventLoopGroups provide the foundation for scalable, event-driven I/O by coordinating multiple single-thread event loops across a configurable pool.