Event bus

An event bus is a communication channel that transports events between the components of an event-driven system. It It is a kind of message broker organized around the publish-subscribe (pubsub) pattern. Producers publish events to the bus, and the bus delivers each event to every component that has subscribed to it.

The bus sits between producers and consumers so that neither side needs to know about the other. Producers emit events and continue their work without waiting for a response. Consumers register their interest in particular event types and react to events as they arrive. This asynchronous decoupling is the main reason event buses are used to make distributed software more modular and scalable.

A defining characteristic of an event bus is fan-out. Each published event is delivered to all of its subscribers, not to a single chosen consumer. This distinguishes an event bus from a message queue, which routes each message to exactly one consumer and removes it once it has been acknowledged. Message queues are well suited to distributing discrete units of work across a pool of workers, whereas an event bus is well suited to broadcasting the same occurrence to many independent reactions.

An event bus is also distinct from a stream processing system. Stream processing systems such as Kafka are optimized for very high throughput and retain messages for replay over a period of time. An event bus, by contrast, is generally concerned with dispatching events to live subscribers. Events are not retained once they have been delivered.

Implementations

The term "event bus" spans both in-process and distributed implementations.

In-process event buses live within a single application. Components register handlers for particular event types, and the bus dispatches each published event to the matching handlers. Dispatch may be synchronous, with handlers invoked on the publishing thread, or asynchronous, with events queued and delivered on a separate thread. Examples include Guava’s EventBus, Spring’s ApplicationEventMulticaster, and the event bus in Vert.x.

Distributed event buses connect separate services across a network. In this form the event bus is usually realized on top of a message broker or an event streaming platform, with the pubsub semantics provided by the underlying transport. The logical role of the bus is the same in both cases. It decouples producers from consumers and fans events out to subscribers.

See also

References