What is an Event Loop?

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An Event loop 1) is a thread that waits for and dispatches events or messages that may contains:

  • data
  • or runnable code

An event loop is also known as:

  • message dispatcher,
  • message loop,
  • message pump,
  • or run loop

When the event loop in a event-driven system is the entry point, it's referred to:

  • the main loop
  • or main event loop.

because it runs on the main thread.

They are responsible for:

  • executing the code,
  • collecting and processing events,
  • and executing queued sub-tasks

Each event shall be processed in a reasonable amount of time to not block the event loop. This means that thread-blocking operations shall not be performed while executed on the event loop, exactly like processing events in a graphical user interface (e.g., freezing a Java / Swing interface by doing a slow network request).

An events can be :

It is almost non-blocking (of kernel threads) allowing it to handle a lot of concurrency (e.g. handle many connections, or messages) using a very small number of kernel threads, which allows it to scale very well.


Event loops are used to implement:


This is how asynchrony is implemented:


The event loop got its name from it's minimal implementation:

while (queue.waitForMessage()) {

where queue

The event loop works by:

  • making a request to some internal or external event provider (that generally blocks the request until an event has arrived),
  • then calls the relevant event handler (“dispatches the event”).

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