DEV Community πŸ‘©β€πŸ’»πŸ‘¨β€πŸ’»

Cover image for Node.js animated: Event Loop Phases
Fabrizio Lallo for Node Doctors

Posted on • Updated on

Node.js animated: Event Loop Phases

In the previous article, Node.js animated: Event Loop, you explored how Libuv and the event loop enable asynchronous task handling in Node.js' single-threaded environment. We simplified the event loop as a mechanism that pushes callbacks from aΒ single event queueΒ to an empty call stack. In reality, the event loop is composed of multiple phases, each responsible for specific asynchronous tasks.

event_loop_phases

The event loop phases

A phase is a FIFO queue of callbacks to execute. When the event loop enters a given phase, it executes callbacks until the queue is exhausted or the maximum number of callbacks is run and moves to the next stage.

The event loop hasΒ 6Β phases, and they run in the following order:

  • Timers: executes callbacks scheduled by setTimeout() and setInterval().
  • Pending callbacks: executes I/O callbacks deferred to the next loop iteration.
  • Idle-prepare: only used internally.
  • Poll: retrieve new I/O events and execute I/O-related callbacks
  • Check: executes callbacks scheduled by setImmediate().
  • Close callbacks: some close callbacks, e.g., socket.on('close', …).

event_loop_phases_handled

The event loop doesn’t constantly keep spinning. If there are no active I/O handlers and the event loop doesn’t need to process any callbacks, the Node.js program automatically exits. On the other hand, the event loop stops on the poll phase to capture incoming requests whenever you create a web server and the phases are empty.

Timers and check phase

You are ready to apply your newly acquired knowledge. Still, you can be disappointed when running a setTimeout after a setImmediate, or vice versa, because the result is not deterministic.

setTimeout schedules a callback afterΒ at leastΒ the defined milliseconds have elapsed. The CPU could be busy and Libuv is unable to set the task as complete before the event loop processes the timer phase. Consequently, the setImmediate callback could be run in advance, although the timer is the first phase.

In this example, the CPU is not busy, and the setTimeout callback is immediately added to its dedicated queue before setImmedate.

In this second example, the event loop starts processing the timer phase, but the timer has not expired yet, so setImmediate callback is run in advance.

Exploring the poll phase

You are now wondering how I can test the phases processing order. The following code snippet will help you prove the order of the phases.

poll_phase

Once Node.js starts the program, the event loop processes all the queues and blocks on the poll phase waiting for incoming requests. When someone makes a GET HTTP request to the server, the event loop goes through these steps:

  1. The event loop pushes the request handler on the call stack.
  2. A setImmediate callback is added in theΒ checkΒ phase.
  3. A setTimeout callback is scheduled in theΒ timersΒ phase.
  4. The event loop moves to the check phase. The setImmediate callback is popped off from the queue and pushed on the call stack for execution.
  5. The event loop does not move any callbacks from empty queues.
  6. The setTimeout callback is popped off from the timer queue and pushed on the call stack for processing.

The code is available at:

Animated Node.js-02-event-loop-phases repository

Conclusion

Now, you understand the event loop and its phases. You can also tell the difference between setTimeout and setImmediate and expect a non-deterministic behavior when running them in sequence. In addition, you know that event loop blocks on the poll phase waiting for new I/O requests on a Node.js web server.

But where are promises handled in Node.js?

You will explore it in more detail in the following articles.

Resources

If you liked the article, follow us on Twitter @fabrizio.lallo and @AndrewHu368

Top comments (2)

Collapse
 
rebeccapeltz profile image
Rebeccca Peltz

What tool(s) do you use to create animations?

Collapse
 
fabriziolallo profile image
Fabrizio Lallo

Keynote :)

Become a Moderator Can you help us make DEV a better place?

Fill out this survey and help us by becoming a tag moderator here at DEV.