Node.js Tutorial - 42 - Event Loop
Summary
TLDRThis video explains the event loop in Node.js, highlighting its role in asynchronous code execution. The speaker reviews key concepts such as JavaScript's synchronous, single-threaded nature and the importance of libuv in handling asynchronous tasks. The video provides an in-depth walkthrough of synchronous and asynchronous code execution, followed by a visual representation of the event loop. It covers how different queues (timer, I/O, check, close, and microtask) are prioritized and executed, helping viewers understand Node.js's internal processes for managing async code efficiently.
Takeaways
- 😀 JavaScript is a synchronous, blocking, single-threaded language, but asynchronous programming is enabled through external libraries like libuv.
- 💡 The V8 engine, which executes JavaScript, consists of a memory heap for variable storage and a call stack for function execution.
- 🛠️ When an async function is called, it's offloaded to libuv, which handles it using the system's async mechanisms or a thread pool, keeping the main thread free.
- 📜 In synchronous code execution, functions are pushed onto the call stack and popped off in a last-in-first-out order, producing a sequential output.
- ⏳ In asynchronous code, the async function is offloaded, and JavaScript continues executing other tasks while waiting for the async operation to finish.
- 🔄 The Node.js event loop is a key mechanism that manages the execution of async callbacks once the call stack is empty.
- 📝 There are multiple queues in the event loop, including the timer queue (setTimeout, setInterval), IO queue (async operations like file reads), check queue (setImmediate), close queue, and two microtask queues (nextTick, promise).
- ⏱️ The event loop processes these queues in a specific priority order, with microtasks being executed first, followed by timer callbacks, IO callbacks, and others.
- 🔀 Even if two async tasks complete at the same time (e.g., setTimeout and FS.readFile), timer callbacks are prioritized over IO callbacks.
- 📊 The event loop continues to run as long as there are callbacks to process, and exits when there are none left, ensuring efficient asynchronous execution in Node.js.
Q & A
What is the primary role of the event loop in Node.js?
-The event loop in Node.js orchestrates the execution of synchronous and asynchronous code. It ensures that tasks such as callbacks from async operations are handled appropriately and in a specific order after synchronous tasks have completed.
How does JavaScript handle asynchronous operations in Node.js?
-JavaScript offloads asynchronous operations to a library called libuv in Node.js. The async task is then executed either using native OS mechanisms or through a thread pool, ensuring the main thread is not blocked.
What happens to synchronous code in Node.js?
-Synchronous code in Node.js is executed in a straightforward, last-in-first-out manner on the call stack. Functions are pushed onto the stack and popped off once executed, with global scope execution starting first.
What is the role of the call stack and the heap in the V8 engine?
-The call stack is responsible for tracking the execution of functions in a last-in, first-out manner. The heap is where memory is allocated for variables and functions during the execution of code.
How does the event loop prioritize execution of different callback functions?
-The event loop executes callbacks in a specific order: first, callbacks in the microtask queues (next tick queue and promise queue), followed by callbacks in the timer queue (for setTimeout and setInterval), then the I/O queue, check queue, and close queue. Microtask queues are rechecked after every other callback queue.
What is the difference between the timer queue and the I/O queue in Node.js?
-The timer queue handles callbacks associated with setTimeout and setInterval, while the I/O queue handles callbacks related to asynchronous I/O operations, such as reading files with fs.readFile.
What are microtask queues, and why are they important?
-Microtask queues consist of the next tick queue (for process.nextTick callbacks) and the promise queue (for native promises). They are crucial because callbacks in these queues are given the highest priority and are executed before any other async tasks.
What happens if two async tasks, such as setTimeout and fs.readFile, complete simultaneously?
-If setTimeout and fs.readFile complete at the same time, the callback from the timer queue (setTimeout) is executed first, followed by the callback from the I/O queue (fs.readFile). This priority order is part of how the event loop manages task execution.
When does Node.js execute callbacks for asynchronous tasks?
-Node.js executes callbacks for asynchronous tasks only when the call stack is empty, ensuring that no synchronous code execution is interrupted.
What are some node-specific functions mentioned in the script?
-Node-specific functions mentioned in the script include setImmediate (which places callbacks in the check queue) and process.nextTick (which places callbacks in the next tick queue, a part of the microtask queues).
Outlines
🔄 Understanding Async Code Execution in Node.js
This section introduces the concept of asynchronous (async) code execution in Node.js, emphasizing that JavaScript is inherently synchronous, single-threaded, and blocking. It explains how async programming is made possible with the help of libraries like Libuv. The paragraph also details how code executes within the Node.js runtime, including the role of the V8 engine in memory allocation (Heap) and execution (Call Stack), and how Libuv offloads async tasks to the operating system’s native mechanisms or thread pool to ensure the main thread remains unblocked. Through a simple synchronous code example, the step-by-step execution of functions is illustrated.
📝 Comparing Synchronous and Asynchronous Code Execution
This part dives deeper into how Node.js handles async operations. By contrasting a synchronous code snippet with an asynchronous example (using `fs.readFile`), it explains how async tasks are handed over to Libuv for processing and how callbacks are executed after the main code completes. The explanation visualizes how the call stack manages both sync and async code and shows how the console outputs 'first', 'third', and 'second' in an asynchronous operation. It concludes with questions regarding the precise execution order of async callbacks, hinting at the complexity of callback execution timing and priorities.
🔁 The Event Loop: Coordinating Sync and Async Code
This section introduces the concept of the event loop, which is central to how Node.js handles asynchronous callbacks. It explains that the event loop coordinates the execution of both sync and async code, running through various queues that hold callbacks of different types (e.g., timer callbacks, I/O callbacks). The visual representation of the event loop is described in detail, covering six key queues: Timer, I/O, Check, Close, and two microtask queues (Next Tick Queue and Promise Queue). The microtask queues, specific to Node.js, are highlighted as distinct from the others, playing a crucial role in callback execution order.
Mindmap
Keywords
💡Event Loop
💡Call Stack
💡Async Programming
💡libuv
💡Timers Queue
💡Microtask Queue
💡I/O Queue
💡setImmediate
💡Synchronous Code
💡Callback Functions
Highlights
JavaScript is a synchronous, blocking, single-threaded language, making async programming essential for non-blocking operations.
Node.js achieves asynchronous programming using the V8 engine and libuv, with the latter handling async tasks and ensuring non-blocking of the main thread.
The V8 engine manages memory and a call stack, executing synchronous code in a last-in, first-out manner.
Libuv is responsible for executing async operations by offloading tasks to the operating system's native async mechanisms or using its thread pool.
Synchronous code execution follows a clear pattern where functions are pushed to and popped off the call stack, with no interruptions or delays.
Asynchronous operations like 'fs.readFile' are offloaded to libuv, allowing the call stack to proceed with other tasks while libuv processes the async task.
The event loop in Node.js is a core design pattern that coordinates the execution of synchronous and asynchronous code, ensuring efficient handling of tasks.
There are six different queues in the event loop: Timer queue, IO queue, Check queue, Close queue, and two microtask queues (Next Tick and Promise).
The priority order for callbacks within the event loop is crucial, with synchronous code always taking precedence over async operations.
Microtask queues, including Next Tick and Promise queues, have the highest priority, and are executed first when the call stack is empty.
Timers (setTimeout and setInterval) have priority over IO callbacks in the event loop, even if both are ready to be executed at the same time.
Set Immediate is specific to Node.js and adds tasks to the Check queue, executed after IO callbacks but before the Close queue.
The event loop continues running as long as there are tasks to process. If all callbacks are executed and no code remains, the loop exits.
The event loop's orchestration of tasks ensures non-blocking performance, answering questions like when async callbacks are executed.
Understanding the event loop helps clarify the execution priority of different async methods and how they interact with the call stack in Node.js.
Transcripts
welcome back in this video Let's
understand about the all-important event
Loop in node.js
let me Begin by reiterating a few points
about async code execution
first
JavaScript is a synchronous blocking
single threaded language
second
to make async programming possible we
need the help of Liberty I hope this is
clear to you
using these two points let me now paint
a picture in your mind as to how code
typically executes in the node runtime
on the left we have the V8 engine which
executes JavaScript code
it comprises of a memory he and a call
stack
whenever you declare variables or
functions memory is allocated on the
Heap
whenever you execute code functions are
pushed into the call stack and when the
function returns it is popped off the
call stack
straightforward last in first out
implementation of the stack data
structure
on the right we have libue
whenever you execute an async method it
is offloaded to the UV
Liberty will then run the task using
native async mechanisms of the operating
system and if that is not possible it
will utilize its thread pool to run the
task ensuring the main thread is not
blocked
let's now walk through two simple code
Snippets and understand how the V8
engine and libue are used by node
first let's take a look at synchronous
code execution
on the left we have a simple code
snippet
three console log statements that log
first second and third one after the
other
let's now walk through the code as if
the runtime is executing it
the main thread of execution always
starts in the global scope
so the global function if you can call
it that is pushed onto the stack
then on line 1 we have a console log
statement
the function is pushed onto the stack
and for the sake of understanding the
timeline let's assume this happens at
one millisecond
first is logged to the console
then the function is popped off the
stack
execution comes to line two
let's say at 2 milliseconds
log function is again pushed onto the
stack
second is locked to the console and the
function is popped off the stack
finally execution is on line three and
at three milliseconds function is pushed
onto the stack
third is locked to the console
and the function is popped off the stack
there is no more code to execute and
Global is also popped off
this is pretty much how synchronous code
execution can be visualized with the
node runtime
next let's take a look at async nescode
execution
on the left we have another code snippet
three log statements like before but
this time the second log statement is
within a callback function passed to FS
dot read file
let us once again walk through the code
as if the runtime is executing it
the main thread of execution always
starts in the global scope so the global
function is pushed onto the stack
execution comes to line one at one
millisecond console.log is pushed onto
the stack
first is logged in the console
and the function is popped off the stack
execution now moves on to line two
add 2 milliseconds the read file method
gets pushed onto the stack
in the earlier lecture I mentioned that
read file is an async operation that is
offloaded to libue
so what happens now is that the Callback
function is handed over to libue
JavaScript then simply pops off the
read5 method from the call stack because
its job is done as far as execution of
line 2 is concerned
in the background the beauty starts to
read the file contents on a separate
thread
at 3 milliseconds JavaScript proceeds to
line 5. it pushes the log function onto
the stack
third gets locked to the console
and the function is popped off the stack
now there is no more user written code
in the global scope to execute so the
call stack is empty
at about 4 milliseconds let's say that
the file read task is completed in the
thread pool
the associated callback function
is now pushed onto the call stack
within the Callback function we have the
log statement
that is pushed onto the call stack
second is locked to the console and the
function is popped off
as there are no more statements to
execute in the Callback function
that is popped off as well
no more code to run so the global
function as we call it is also popped
off the stack
the console output is going to read
first third and then second
this is how the node runtime executes an
asynchronous code snippet that uses FS
dot read file
similar execution holds good for other
async methods as well
I hope it is clear to you as to how code
is executed in the node runtime
now if you've understood this far
let me ask you a few questions
whenever an async task completes in liby
at what point does node decide to run
the associated callback function on the
call stack
does it wait for the call stack to be
empty or does it interrupt the normal
flow of execution to run the Callback
function
what about async methods like set
timeout and set interval which also
delay the execution of callback function
F2 async tasks such as set timeout and
Fs dot read file complete at the same
time how does node decide which callback
function to run first on the call stack
does 1 get priority over the other
at the moment we simply don't know
just when we thought we understood how
code is executed behind the scenes in
the node runtime it seems to have become
more complex
well let me tell you all these questions
can be answered by understanding about
the core part of liby which is the event
Loop
now what is the event Loop well
technically it is just a c program
but you can think of event loop as a
design pattern that orchestrates or
coordinates the execution of synchronous
and asynchronous code in node.js
and the way we are going to understand
how the event Loop works is a two-step
process
in this video we will take a look at a
visual representation of the event Loop
that will give you a brief overview of
the different parts that come together
in the event Loop
and then over the next few videos we
will conduct various experiments with
code to better understand the visual
representation
all right here is how we can visualize
the event Loop
I would like to pause and thank deepal
jsekara who has written an article where
I first came across a similar
representation
it has been very useful for me and now
my job is to make sure you easily
understand this as well
now the event Loop is a loop that is
alive as long as your node.js
application is up and running
in every iteration of the loop we come
across six different cues
each queue holds one or more callback
functions that need to be eventually
executed on the call stack
and of course the type of callback
functions are different for each queue
first we have the timer queue this
contains callbacks associated with set
timeout and set interval
second we have the ioq
this contains callbacks associated with
all the async methods that we have seen
so far
example methods associated with the fs
and HTTP modules
third we have the check queue now this
contains the callbacks associated with a
function called set immediate
this function is specific to node and
it's not something you would come across
when writing JavaScript for the browser
fourth we have the close queue this
contains callbacks associated with the
close event of an async task
finally we have a micro task Queue at
the center
this is actually two separate cues
the first queue is called Next tick
queue and contains callbacks associated
with a function called process dot next
take which is again specific to node.js
the second cue is the promise queue
which contains callbacks that are
associated with the native promise in
JavaScript
and one very important point to note is
that timer IO check and close queues are
all part of libuby
the two microtask use however are not
part of libuee
hopefully the color difference conveys
that
nevertheless there's still part of the
node runtime and play an important role
in the order of execution of callbacks
speaking of which let's understand that
next
the arrowheads are already a giveaway
but it is very easy to get confused
so let me explain the priority order of
the queues
first you should know that all user
written synchronous JavaScript code
takes priority over asynchronous code
that the runtime would like to execute
which means only after the call stack is
empty the event Loop comes into picture
within the event Loop though the
sequence of execution follows certain
rules and I will warn you there are
quite a few rules you have to wrap your
head around
let's go over them one at a time
step one
any callbacks in the micro task queues
are executed
first tasks in the next queue and only
then tasks in the promise queue
Step 2 all callbacks within the timer
queue are executed
step 3 callbacks in the micro task
queues if present are executed after the
execution of every callback in the timer
queue
again first tasks in the next queue and
then tasks in the promise queue
step 4
all callbacks within the ioq are
executed
step 5 callbacks in the micro task
queues if present are executed
next to queue followed by promise queue
step 6 all callbacks in the check queue
are executed
step 7 callbacks in the micro task
queues if present are executed after the
execution of every callback in the check
queue
again first tasks in the next IQ and
then tasks in the promise queue
step 8 all callbacks in the close queue
are executed
step 9
for one final time in the same Loop the
micro task queues are executed next a
queue followed by promise queue
at this point if there are more
callbacks to be processed the loop is
kept alive for one more run and the same
steps are repeated
on the other hand
if all callbacks are executed and there
is no more code to process the event
Loop exits
this is the role libuey's event Loop
plays in the execution of async code in
node.js
I'm also hopeful that we now have
answers to the questions we had a few
minutes ago
for our first question the answer is
that callback functions are executed
only when the call stack is empty the
normal flow of execution will not be
interrupted to run a callback function
for our second question we now know that
set timeout and set interval callbacks
are given first priority
for the third question We Now understand
that timer callbacks are executed before
I O callbacks even if both are ready at
the exact same time
apart from this we also learned about a
few other cues that have their own
priority
but this visual representation here is
what I want you to imprint in your mind
as it is how node executes async code
Under The Herd
if it is now clear as to what the event
Loop is over the next few videos let's
conduct a few different experiments to
understand and verify the order of
execution in the event Loop
thank you for watching please do
consider subscribing to the channel and
I'll see you in the next one
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