||This article relies largely or entirely upon a single source. (July 2012)|
The reactor design pattern is an event handling pattern for handling service requests delivered concurrently to a service handler by one or more inputs. The service handler then demultiplexes the incoming requests and dispatches them synchronously to the associated request handlers.
- Resources: Any resource that can provide input to or output from the system.
- Synchronous Event Demultiplexer: Uses an event loop to block on all resources. When it is possible to start a synchronous operation on a resource without blocking, the demultiplexer sends the resource to the dispatcher.
- Dispatcher: Handles registering and unregistering of request handlers. Dispatches resources from the demultiplexer to the associated request handler.
- Request Handler: An application defined request handler and its associated resource.
All reactor systems are single threaded by definition, but can exist in a multithreaded environment.
The reactor pattern completely separates application specific code from the reactor implementation, which means that application components can be divided into modular, reusable parts. Also, due to the synchronous calling of request handlers, the reactor pattern allows for simple coarse-grain concurrency while not adding the complexity of multiple threads to the system.
The reactor pattern can be more difficult to debug than a procedural pattern due to the inverted flow of control. Also, by only calling request handlers synchronously, the reactor pattern limits maximum concurrency, especially on Symmetric multiprocessing hardware. The scalability of the reactor pattern is limited not only by calling request handlers synchronously, but also by the demultiplexer. 
- Apache MINA
- Apache Cocoon (for XML processing)
- JBoss Netty
- Disruptor (Java Framework)
See also 
- Proactor pattern (a pattern that also demultiplexes and dispatches events, but asynchronously)
- Application server
- C10k problem
- Schmidt, Douglas et al. Pattern-Oriented Software Architecture Volume 2: Patterns for Concurrent and Networked Objects. Volume 2. Wiley, 2000.
- Schmidt, Douglas C., An Object Behavioral Pattern for Demultiplexing and Dispatching Handles for Synchronous Events
- Kegel, Dan, The C10K problem, retrieved 2007-07-28