---
tags:
  - backend
  - node-js
---

# The Event Loop

Node.js provides a single-threaded asynchronous architecture which is achieved
via means of the Event Loop.

## Multi-threaded synchronous architectures

In the context of backend, a thread as an instance of a request-response
transaction.

For example a request is made from the client for a resource contained in a
database. The back-end language is an intermediary between the client machine
and the server. It receives the request and returns the resource as a response.

Many backend frameworks are synchronous but multithreaded. This means that a
thread can only process one request-response cycle at a time. The thread cannot
initiate a new cycle until it has finished with its current cycle.

If there was only one thread, this would be inefficient and unworkable.
Therefore the framework will be multi-threaded: multiple request-response cycles
can be executed at once by different threads.

![sync-thread.svg](sync-thread.svg)

To accomodate the ability to increase the scale of synchronous applications you
need to be able to spawn more threads commensurate to increased demand. This
increases the resource consumption of the framework (more cores, more memory
etc). Moreover it is possible to reach a point where all threads are active and
no more can be spawned. In this case there will simply be delays in the return
of data.

## Node as a single-threaded asynchronous architecture

In contrast, Node only has a single thread but it works asynchronously, not
synchronously. Thus it has a **single-threaded asynchronous architecture**. This
means whilst there is only a single thread it can juggle responses by
dispatching them asynchronously. When a request is made it sends it off and
continues with its execution and handling new requests. Once these resolve, the
data is returned to the main thread.

![async.svg](async.svg)

## The Event Loop

Node implements its single-threaded asynchronous architecture through the Event
Loop.

This is the mechanism by which Node keeps track of incoming requests and their
fulfillment status: whether the data has been returned successfully or if there
has been error.

Node is continually monitoring the Event Loop in the background.

A running Node application is a single running process. Like everything that
happens within the OS, a process is managed by the
[kernel](The_Kernel.md) that dispatches operations to the CPU
in a clock cycle. A thread is a sequence of code that resides within the process
and utilises its memory pool (the amount of memory assigned by the kernel to the
Node process). The Event Loop runs on CPU ticks: a tick is a single run of the
Event Loop.

### Phases of the Event Loop

The Event Loop comprises six phases. The Event Loop starts at the moment Node
begins to execute your `index.js` file or any other application entry point.
These six phases create one cycle, or loop, equal to one **tick**. A Node.js
process exits when there is no more pending work in the Event Loop, or when
`process.exit()` is called manually. A program only runs for as long as there
are tasks queued in the Event Loop, or present on the
[call stack](Call_stack.md).

![](/img/node-event-loop.svg)

The phases are as follows:

1. **Timers**
   - These are functions that execute callbacks after a set period of time. As
     in standard JavaScript there are two global timer functions: `setTimeout`
     and `setInterval`. Interestingly these are not core parts of the JavaScript
     language, they are something that are made available to JS by the
     particular browser. As Node does not run in the browser, Node has to
     provide this functionality. It does so through the core `timers` module.
   - At the beginning of this phase the Event Loop updates its own time. Then it
     checks a queue, or pool, of timers. This queue consists of all timers that
     are currently set. The Event Loop takes the timer with the shortest wait
     time and compares it with the Event Loop's current time. If the wait time
     has elapsed, then the timer's callback is queued to be called once the
     [call stack](Call_stack.md) is empty.
2. **I/O Callbacks**
   - Once timers have been checked and scheduled, Node jumps to I/O operations.
   - Node implements a non-blocking input/output interface. This is to say,
     writing and reading to disk (files in the Node application directory) is
     implemented asynchronously. The asynchronous I/O request is recorded into
     the queue and then the call stack continues.
3. **Idle / waiting / preparation**
   - This phase is internal to Node and is not accessible to the programmer.
   - It is primarily used for gathering informtion, and planning what needs to
     be executed during the next tick of the Event Loop
4. **I/O polling**
   - This is the phase at which the main block of code is read and executed by
     Node.
   - During this phase the Event Loop is managing the I/O workload, calling the
     functions in the queue until the queue is empty, and calculating how long
     it should wait until moving to the next phase. All callbacks in this phase
     are called synchronously (although they return asynchronously) in the order
     that they were added to the queue, from oldest to newest.
   - This is the phase that can potentially block our application if any of
     these callbacks are slow or do not return asynchronously.
5. **`setImmediate` callbacks**
   - This phase runs as soon as the poll phase becomes idle. If `setImmediate()`
     is scheduled within the I/O cycle it will always be executed before other
     timers regardless of how many timers are present.
   - This is your opportunity to grant precedence to certain threads within the
     Node process
6. **Close events**
   - This phase occurs when the Event Loop is wrapping up one cycle and is ready
     to move to the next one.
   - It is an opportunity for clean-up and to guard against memory leaks.
   - This phase can be targetted via the `process.exit()` function or the close
     event of a web-socket.

## Event _loop_ and event _queue_

The terms _event loop_ and _event queue_ are often used interchangeably in the
literature but in fact they are distinct.

The Event Loop is the Node runtime's method of execution, the queue is the stack
of tasks that are lined up and executed by the loop. We can think of the queue
as being the input and the loop as what acts on the input. The queue obviously
emerges from the program we write but it is scheduled, organised and sequenced
by the loop.

https://blog.appsignal.com/2022/07/20/an-introduction-to-multithreading-in-nodejs.html
https://school.geekwall.in/p/Bk2xFs1DV
