Node.js Event Loop Explained
Summary
TLDRThis video dives into the Node.js event loop, explaining its various stages and how they interact to handle asynchronous operations. It starts by detailing how the event loop runs continuously when a server is active, handling timers, callbacks, and other tasks. The video also covers the differences between microtasks and macrotasks, and their priorities in the loop. The creator emphasizes the importance of understanding these details for optimizing Node.js applications, and hints at future videos on improving performance and avoiding event loop blocking.
Takeaways
- 🔄 Node.js has an event loop that operates continuously, especially when running processes like a web server.
- ⏳ The first phase of the event loop is called 'timers', where functions like `setTimeout` and `setInterval` are checked and executed.
- 📋 Microtasks, such as `process.nextTick()` and promises, have higher precedence than macrotasks like timers and I/O operations.
- ⚙️ The V8 engine, which powers Node.js and browsers, separates tasks into microtasks and macrotasks, with microtasks running first.
- ⏱️ Inside the timers phase, microtasks and macrotasks run with their own order of precedence.
- ⏲️ The 'poll' phase of the event loop is crucial for handling incoming I/O operations such as network requests and file reads.
- 💻 Understanding the internal workings of the event loop helps in optimizing Node.js applications for performance.
- 🕒 The 'check' phase is specifically for handling `setImmediate`, which executes with higher precedence over other timers.
- ❌ The last phase, 'close callbacks', handles events like socket closures and cleanup operations.
- 📊 Optimizing Node.js performance involves reducing event loop lag and avoiding blocking operations, with strategies like using worker threads.
Q & A
What is the event loop in Node.js and when does it start?
-The event loop in Node.js starts as soon as you run a Node.js script (e.g., `node index.js`). It is responsible for handling asynchronous operations, and it keeps running as long as there are tasks, such as web servers, waiting for events like incoming network connections.
What happens in the 'timers' phase of the Node.js event loop?
-In the 'timers' phase, Node.js checks for and processes any scheduled functions that were set using `setTimeout` or `setInterval`. These are executed in the order they were set, based on their delay times.
What are microtasks and how do they differ from macrotasks in Node.js?
-Microtasks have a higher priority than macrotasks and are executed before macrotasks in the event loop. Examples of microtasks include promise callbacks, `process.nextTick`, and `queueMicrotask`. Macrotasks include `setTimeout`, `setInterval`, and I/O operations like network requests.
What is the priority order for tasks within the 'timers' phase?
-The priority order is: synchronous code, `process.nextTick` (next tick queue), microtasks (e.g., promise callbacks), and finally macrotasks (timers like `setTimeout` and `setInterval`).
What role do 'pending callbacks' play in the Node.js event loop?
-The 'pending callbacks' phase handles callbacks from I/O operations that were deferred in the previous cycle of the event loop. These callbacks are executed in this phase after all timers and microtasks are processed.
What is the 'poll' phase and why is it considered the most important part of the event loop?
-The 'poll' phase is where most of the asynchronous operations happen, such as incoming connections (HTTP, TCP) and file I/O. It checks for new events and handles them, making it critical for performance in a Node.js application.
What is the significance of 'setImmediate' in Node.js?
-`setImmediate` is a special function in Node.js that schedules code to run at the next iteration of the event loop. It has a higher precedence than normal timers (`setTimeout`) and is executed in the 'check' phase.
What is the 'close callbacks' phase in the Node.js event loop?
-The 'close callbacks' phase is the final step of the event loop, where it handles callbacks for closing events, such as when a TCP connection or file stream is closed.
Why is understanding the event loop important for Node.js developers?
-Understanding the event loop helps developers write more efficient and optimized code by knowing how asynchronous tasks are handled, how to prevent blocking the event loop, and when to use certain functions like `setImmediate`, `process.nextTick`, and microtask queues.
What is 'event loop lag' and how can it affect the performance of a Node.js application?
-Event loop lag occurs when long-running operations or synchronous code block the event loop, preventing it from handling other tasks. This can degrade the performance of a Node.js application, especially in cases involving high concurrency or heavy I/O.
Outlines
🌀 Understanding the Node.js Event Loop
The Node.js event loop is a fundamental yet often misunderstood aspect of Node.js, rarely covered in detail by videos or blogs. The event loop consists of multiple stages that occur rapidly while a Node.js application is running. This section introduces the concept, noting that as soon as a Node.js app is executed, the event loop begins. For small scripts, the loop may start and stop quickly, but for web servers, it persists, waiting for incoming events such as TCP connections. Understanding these details is crucial for leveraging Node.js effectively.
⏲️ Event Loop Stages and Timer Handling
This part delves deeper into the event loop, specifically focusing on the timer phase. It explains that Node.js checks for timers like `setTimeout` and `setInterval` during this phase. Through an example code with multiple timeouts and microtasks, the section introduces the concept of task queues—microtasks and macrotasks—highlighting their different priorities. Microtasks like promises and `process.nextTick()` take precedence over macrotasks like timeouts and intervals. The importance of understanding this hierarchy in developing efficient Node.js applications is emphasized.
🔄 Precedence and Task Queues in Node.js
Here, the script explains how task execution in Node.js is further subdivided into queues within each phase of the event loop. Tasks like `nextTick` are manually fired and take precedence even within `setTimeout`. This hierarchy is crucial for managing the flow of synchronous and asynchronous operations. The example illustrates how microtasks, promises, and other operations are prioritized, ensuring that developers understand how to manage task execution efficiently within the event loop.
📋 Pending Callbacks and Polling Phase
The focus shifts to the 'pending callbacks' phase and the 'poll' phase, which is where most asynchronous operations, such as network connections and file system reads, are handled. The poll phase plays a key role in ensuring the performance of a Node.js app. There’s a brief mention of optimizations, particularly how to avoid blocking the event loop and ensuring smooth operation. The video promises a follow-up on optimizing Node.js applications for performance by effectively managing these phases.
✅ The Check Phase and SetImmediate
This section introduces the 'check' phase, which is responsible for handling `setImmediate`, a function similar to `setTimeout` but with a higher precedence in the event loop. The script explains how `setImmediate` can either execute immediately during the poll phase or be deferred, depending on the system’s decisions. The final stage, 'close callbacks', is briefly mentioned, noting its role in managing events like socket closures.
👋 Wrapping Up and Future Topics
In the final paragraph, the speaker wraps up the video, reiterating that the discussion was an overview of the Node.js event loop. While optimizations and detailed explanations were not covered, the speaker promises a future video on how to optimize performance, particularly avoiding event loop lag. The video ends with a call to action for viewers to subscribe and await the next tutorial.
Mindmap
Keywords
💡Event Loop
💡Timers
💡Microtasks
💡Macrotasks
💡Pending Callbacks
💡Poll
💡setImmediate
💡process.nextTick()
💡Event Loop Lag
💡Close Callbacks
Highlights
Node.js Event Loop consists of multiple steps, occurring within milliseconds during the execution of a Node.js application.
When a Node.js app starts, the event loop initiates, running continuously unless the app terminates or a server is actively waiting for TCP connections.
The first step of the Event Loop is 'timers,' which handles functions like `setTimeout` and `setInterval`.
Microtasks and macrotasks have different levels of precedence in Node.js. Microtasks (like Promise callbacks and process.nextTick) are higher priority than macrotasks (like setTimeout, I/O operations, and event handlers).
The `nextTick` queue is manually triggered, and its tasks are executed first within a timeout, even before other microtasks and macrotasks.
Promise callbacks are considered microtasks, so they have a higher precedence and are executed before macrotasks like setTimeout or setInterval.
The 'poll' phase in the Event Loop is critical as it handles most I/O operations, including incoming HTTP or TCP connections, file reading, and more.
Pending callbacks are deferred tasks that were not executed in the previous cycle of the Event Loop and will be processed in the next round.
In the 'idle and prepare' phase, Node.js performs bookkeeping tasks, but it does not impact application performance or behavior.
SetImmediate has a higher precedence than setTimeout or setInterval and is executed during a specific step in the Event Loop.
The 'check' phase is where setImmediate is executed, giving it precedence over standard timers.
Close callbacks are triggered when a socket or any other connection is closed, allowing developers to clean up resources and manage disconnection events.
Understanding the internals of the Event Loop helps developers optimize Node.js applications and avoid blocking the Event Loop, ensuring high performance.
Event Loop lag is a critical issue to avoid, as it can delay execution of asynchronous tasks, degrading the performance of the Node.js application.
The speaker plans to release further content about optimizing Node.js performance, focusing on using worker threads and ensuring non-blocking operations.
Transcripts
we are missing some very important
knowledge about the event Loop of
node.js because I feel like this is not
really covered in many videos and blog
articles but nodejs event Loop actually
consists of many steps that are going
through that are happening within every
millisecond when you are running a no JS
app so let's talk about this and what
are the implications and what you need
to be aware of so first of all nodejs
does have an event Lo okay if you go to
the official noj website you will see
that there's a big article on how the
event Loop Works actually and the all
the steps that we're going to talk about
now so let's go back to the Blackboard
and this round figure that you see here
that goes clockwise is basically an
event Loop that starts as soon as you
run a node script okay you run node
index.js your app starts and as soon as
your app starts or the process starts
the loop starts as well well now the
thing is if you only have a very few
lines of code within your index.js file
let's say you're just doing a console
log or something like that the app is
going to start the event Lo is going to
start as well and then it's going to
close it's going to exit because your
process stopped there's nothing that we
need to listen to on the other hand if
you're starting a web server as you know
when you start the web server you do
this thing server. listen and under the
hood it basically waits for TCP
connections and the event Loop does not
stop it just keeps going and it's
basically going through these all steps
waiting for some events to happen and
this is the crucial part of
understanding how the event Loop works
okay so let's call this one the event
Loop e l just so that we know what we're
referring to so as soon as the event
Loop starts we are going to go all of
these over all of these steps like many
times within one second okay so what is
happening in the very first step called
timers well it turns out that nodejs is
checking for timers that are available
okay so if you have done a a timeout set
set timeout or set interval they're are
going to be handled within this uh part
here in in set timeouts or in the timer
section okay let's look at this piece of
code and understand what
has precedence when we are in this step
timers okay so we're going to see that
we have a console log or first of all we
have three timeouts we have a console
loog set timeout one then we have a
resolve and reject and then we have some
function called q microtask and then we
have nexti what are these because if you
go for a normal no JS tutorial video
you're not going to know anything about
this this is like very lowlevel node.js
stuff but let me explain I think if you
are using node.js as your tool to
accomplish your business needs the
better you know the underlying
Technologies of your tool or how it
works under the hood the better equipped
you are okay so it turns out let me
switch to another page uh nodejs or
basically the engine V8 engine that
nodejs is based upon it's it applies the
same for the browsers um they have
microt tasks and macro tasks meaning
microt tasks are more important so they
have higher priority than M microtasks
microtasks include promise callbacks
mutation observe API uh process.
nexttick which is a method that we can
fire manually within nodejs Q microtask
function that we basically sew and
weight Expressions on the other hand
microtasks or macro tasks consist of set
timeouts set intervals D manipulation IO
operations for example reading a file
Network requests and event handlers now
what does it mean for us so in this
timeout we're going the president is
going to be like the following so we
enter the timeout okay we're going to
log the first thingy because conso loog
is actually synchronous so we're going
to stumble upon this and it's going to
be locked out next what what's going to
happen are we going to L out the
promises or are we going to look out the
Q microtask or the next tick this is a
very interesting question and the answer
is actually here so we have these three
bubbles here which are also kind of a
loop within the loop all right and all
of these three bubbles actually are
living within every step that we have
here so they're inside the timers
they're also inside the pending call
bags they're also inside the pole so
they are running under every step and
they also have a precedence okay I know
this is confusing but first of all we
have these steps and then we have a
precedence within each queue so the next
TI queue is the one that we are firing
manually okay here for example we do
next tick and then we do console log so
that's going to fire first meaning
within our set timeout we're going to
get next tick one first the next thing
that's going to happen is we're going to
look for microtasks microtasks are the
ones that we already discussed for
example promises so promises are going
to have a higher precedence and then we
are going to fire the Q microtask which
also basically related to a microtask
and then we're going to look for the
macro tasks okay in this case we don't
have any macro tasks in this piece of
code but you can imagine that it will
fire next as soon as these all three are
done within our first timeout and
remember that we're still in this
section here we're going to jump to the
second time out so basically event Loop
starts we go to the timers section and
the timers also have its own precedence
so it's going to make sure that
everything has run here has around here
and then has around here and then we
jump to the next basically function
within the timers and as soon as these
are done now the event Lo can proceed to
pending callbacks what is pending
callbacks basically every time we're
going to come to this the poll section
is pretty much the most important one
it's the one that executes most of the
stuff including the synchronous stuff
okay sometimes poll does not poll stage
does not execute things and it defers
them it defers them to the next round
trip of the event Loop and sometimes
basically from the previous round things
end up here pending callbacks for
example a timeout can end up there for
one or other reason so so as you know
pending callbacks are going to as I
mentioned also have this precedence of
three different types of cues and it's
going to execute them one by one as soon
as the pending callbacks have been
executed we're going to jump to the next
section called idle and prepare there's
nothing to cover there because it is
it's native to V8 or nodejs and it's
basically doing some bookkeeping under
the hood to make sure that we're intact
so there's no impact for us when it
comes to the optimizations that we can
make okay so the next point will be poll
as I said this is the most important one
why because this is where the incoming
connections for example uh HTTP or TCP
connections that are being established
socket connections and so on file
reading all the synchronous stuff is
going to happen here and this is exactly
where we need to make sure that our
basically our app is performant wor not
I'm going to release another video as
soon as possible probably most after
this one in order to learn how to
actually make your node.js app
performance do you do you need new
worker threats do you need how do you
make everything asynchronous and most of
all
how basically how not to block the event
Loop and there's a thing called uh event
Loop lag basically when your operations
are executed with a small like it's
really hard to control that and I'm
going to cover that in the next video as
well so stay tuned And subscribe if you
haven't yet now we covered the polling
okay so we're going to execute mple
tasks here and then we also have a thing
called check okay what is a check well
we're basically checking for one and
only thing think called set set
immediate yeah you might not heard heard
about that we have set inter set timeout
set interval and we also have a set
intermediate here it works the same way
as a set timeout or sent interval but
the difference is that it has a presence
because it's happening within the event
Loop so basically if you have a set
inter immediate in your code it's going
to be executed right here on the other
hand set timeout can either be executed
in a timers depending on what what your
operating system decides during the poll
event or it can be deferred to the next
round okay so set immediate basically
has a higher precedence than other
timers and then we also have the last
step called close callbacks basically
these are going to listen for uh close
events for example when you're closing
the socket connection or when you are
manually closing the call bags or uh
removing the call bags this is it what I
wanted to tell you guys in this video
this is was this was just an rough
overview of the internals of the event
Loop of nodejs and as I said all the
optimizations optimization topics are
going to be covered in the next video so
smash like subscribe and I will see you
in the next one goodbye
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