What is an Event Loop?

About

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 :

• receiving network buffer from packet
• to messages.

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.

Features

Event loops are used to implement:

• asynchronous programming models. The asynchronous functons are put in the queue.
• inter-process communication.

Example

This is how asynchrony is implemented:

• in Javascript (Javascript thread model)
• in other reactive framework such as Java - Vert.X Framework

Implementation

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

while (queue.waitForMessage()) {
  queue.processNextMessage()
}

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”).