The Node.js event loop is a fundamental part of the runtime that allows Node.js to perform non-blocking I/O operations efficiently. It works by continuously monitoring the event queue and executing any events that are ready to be processed.
¶ Appendix: About Non-Blocking I/O Operation
In Node.js, a non-blocking I/O operation is an operation that does not block the execution of the program while it is waiting for the operation to complete. This means that the program can continue to run and perform other tasks while the I/O operation is being performed in the background.
Node.js is built on top of the Chrome V8 JavaScript engine, which is designed to run JavaScript on the server side. One of the key features of Node.js is its use of non-blocking I/O, which allows it to efficiently handle many concurrent connections and perform tasks asynchronously.
¶ Appendix: About V8 JavaScript Engine
Node.js is built on top of the V8 JavaScript engine, which is developed and maintained by Google. V8 is a high-performance open-source JavaScript engine that is designed to execute JavaScript code quickly and efficiently. It is written in C++ and is used to power the Google Chrome web browser, as well as several other projects.
V8 is designed to execute modern JavaScript code, including code written in ECMAScript versions 6 and later. It includes features such as support for classes, arrow functions, and Promises, as well as just-in-time (JIT) compilation, which allows it to execute code faster by compiling it to native machine code at runtime.
V8 is an important component of the Node.js runtime, as it is responsible for executing the JavaScript code that runs in a Node.js application. It is tightly integrated with the Node.js runtime and provides many of the features that make Node.js well-suited for building scalable network applications.
Here is a simplified illustration of how the V8 engine works:
- The V8 engine receives JavaScript code as input.
- The code is parsed and transformed into an intermediate representation (IR) that is easier for the V8 engine to understand.
- The IR is optimized for faster execution by applying various optimization techniques such as constant folding and inlining.
- The optimized IR is compiled into native machine code using a JIT compiler.
- The native machine code is executed by the CPU.
The event loop has several phases, including:
setTimeout()
and setInterval()
.setImmediate()
.socket.on('close', ...)
.In each iteration of the event loop, the event loop processes events in the order specified by the phases described above. When an event is processed, the associated callback function is executed, and the event is removed from the queue.
The event loop is an important concept in Node.js because it allows the runtime to perform many tasks concurrently, instead of blocking and waiting for each task to complete before moving on to the next one. This makes it possible to build high-performance, scalable applications with Node.js.
This phase executes callback functions that have been scheduled using setTimeout()
and setInterval()
. When a timer is scheduled, it is added to the timer queue, and the event loop will execute its callback when the specified time has elapsed. If multiple timers are scheduled to expire at the same time, their callbacks will be executed in the order in which they were added to the queue.
Q. Does the timer in the event loop guarantee an exact execution time?
No, the event loop's
tick
in Node.js does not guarantee an exact execution time, just like the timer functionssetTimeout()
andsetInterval()
.The
tick
in the Node.js event loop is a unit of time that represents the smallest possible delay between two successive iterations of the event loop. It is typically much shorter than the time units used by the timer functions, such as milliseconds or seconds.However, the actual execution time of a callback function scheduled using the event loop's
tick
may vary due to factors such as system load and the presence of other tasks in the event queue. Therefore, it is not possible to guarantee an exact execution time for tasks scheduled using the event loop'stick
.In general, the error will be relatively small for most applications, and may not be noticeable in most cases. However, if you need to execute a task at a specific time or with a specific level of accuracy, it is important to be aware that the timer functions in the event loop and the event loop's
tick
function may not be suitable for your needs.In these cases, you may need to use a different approach, such as using a hardware timer or implementing your own timing mechanism. It is also important to carefully consider the requirements of your application and the level of accuracy that is needed, and to choose an appropriate approach based on those requirements.
This phase executes callbacks that have completed a blocking operation, such as reading from a file or making an HTTP request. These callbacks are added to the "pending callback" queue by the Node.js runtime when the blocking operation is initiated, and they are executed in the order in which they were added to the queue.
These are internal phases of the event loop that are used to perform various tasks such as updating the list of watchers for file system events, and cleaning up internal data structures.
Q. What happens in the "Idle, prepare" phase of the event loop?
In the "Idle, prepare" phase of the Node.js event loop, the event loop checks for any tasks that need to be scheduled for the next iteration of the loop.
The event loop in Node.js consists of a series of phases, each of which processes a specific type of task. The "Idle, prepare" phase is the first phase in the event loop, and it is followed by the "Poll" phase, the "Check" phase, and the "Close callbacks" phase.
During the "Idle, prepare" phase, the event loop performs the following tasks:
- It checks for any tasks that have been scheduled using the
process.nextTick()
function, and adds them to the event queue to be executed on the next iteration of the event loop.- It checks for any
Promise
objects that have been rejected and have no error handler, and schedules their rejection handling to be performed on the next iteration of the event loop.- It checks for any timers that have expired (such as those set using
setTimeout()
orsetInterval()
), and adds their callback functions to the event queue to be executed on the next iteration of the event loop.Once these tasks have been completed, the event loop moves on to the next phase, the "Poll" phase, where it waits for new events to arrive or for previously scheduled tasks to be ready for execution.
It waits for new events to arrive or for previously scheduled tasks to be ready for execution. The event loop uses an internal mechanism, such as poll()
, epoll()
, or kqueue()
, to monitor the file descriptor sources that have been registered with it, and to receive notifications when events occur.
If a new event or task is available, the event loop retrieves it from the event queue and adds it to the "Check" phase queue to be processed on the next iteration of the event loop.
If the event queue is empty and there are no more events or tasks to process, the event loop goes into a "waiting" state, where it waits for new events or tasks to arrive.
Once the event loop has finished processing all available events and tasks in the "Poll" phase, it moves on to the next phase, the "Check" phase, where it processes any tasks that have been scheduled for the current iteration of the event loop.
This phase executes callbacks that have been scheduled using setImmediate()
. These callbacks are added to the "check" queue and are executed in the order in which they were added to the queue.
This phase executes close callbacks, such as those scheduled using socket.on('close', ...)
. These callbacks are executed when a socket or other resource is closed, and they are typically used to clean up resources or perform other tasks related to closing the resource.
setTimeout(function() {
console.log('Timeout callback');
}, 1000);
console.log('Start');
// Output: "Start", followed by "Timeout callback" after 1 second
In this example, we schedule a callback function using setTimeout()
to be executed after 1 second (1000 milliseconds). The event loop will enter the "timers" phase after 1 second has elapsed, and it will execute the callback function.
const fs = require('fs');
fs.readFile('file.txt', function(err, data) {
console.log('File read callback');
});
console.log('Start');
// Output: "Start", followed by "File read callback" when the file has been read
In this example, we initiate a blocking I/O operation by reading a file using the fs module. When the file read operation is completed, the callback function is added to the "pending callbacks" queue, and it will be executed by the event loop in the next iteration.
setImmediate(function() {
console.log('Immediate callback');
});
console.log('Start');
// Output: "Start", followed by "Immediate callback"
In this example, we schedule a callback function using setImmediate() to be executed as soon as possible. The event loop will enter the "check" phase on the next iteration and execute the callback function.
const fs = require('fs');
// Pending callbacks
const pendingCallbacks = [];
// Timers
const timers = [];
// SetImmediate callbacks
const immediateCallbacks = [];
// A map of file descriptor to event handlers
const fdHandlers = new Map();
// A queue of file descriptor events to process
const fdEvents = [];
// Add a callback to the pending callbacks queue
function addPendingCallback(callback) {
pendingCallbacks.push(callback);
}
// Add a timer to the timer queue
function addTimer(callback, timeout) {
const expiration = Date.now() + timeout;
timers.push({ callback, expiration });
timers.sort((a, b) => a.expiration - b.expiration);
}
// Add a SetImmediate callback to the queue
function addImmediateCallback(callback) {
immediateCallbacks.push(callback);
}
// Add an event handler for a file descriptor
function addFdHandler(fd, handler) {
fdHandlers.set(fd, handler);
}
// Add a file descriptor event to the queue
function addFdEvent(fd, eventType) {
fdEvents.push({ fd, eventType });
}
// The main event loop
function eventLoop() {
// Execute any pending callbacks
while (pendingCallbacks.length > 0) {
const callback = pendingCallbacks.shift();
callback();
}
// Execute any timers that have expired
const now = Date.now();
while (timers.length > 0 && timers[0].expiration <= now) {
const timer = timers.shift();
timer.callback();
}
// Execute any SetImmediate callbacks
while (immediateCallbacks.length > 0) {
const callback = immediateCallbacks.shift();
callback();
}
// Process any file descriptor events
while (fdEvents.length > 0) {
const { fd, eventType } = fdEvents.shift();
const handler = fdHandlers.get(fd);
if (handler) {
handler(fd, eventType);
}
}
// Wait for new events to arrive
waitForEvents();
}
// Wait for new events to arrive
function waitForEvents() {
// Use a blocking system call like epoll() or select() to wait for new events
// When events are available, add them to the appropriate queue (pending callbacks, timers, etc.)
// and schedule the event loop to run again using process.nextTick()
}
// Run the event loop
function run() {
process.nextTick(eventLoop);
}
run();
// Example usage
fs.readFile('file.txt', function(err, data) {
addPendingCallback(function() {
console.log('File read callback');
});
});
addTimer(function() {
console.log('Timeout callback');
}, 1000);
setImmediate(function() {
console.log('Immediate callback');
});
The code defines several arrays and maps to store various types of events:
pendingCallbacks
: An array of callbacks that have completed a blocking operation and are ready to be executed.timers
: An array of timers that have been scheduled using setTimeout()
or setInterval()
.immediateCallbacks
: An array of callbacks that have been scheduled using setImmediate() and are ready to be executed.fdHandlers
: A map of file descriptor to event handlers.fdEvents
: An array of file descriptor events that are ready to be processed.The code also defines several functions to add events to these arrays and maps:
addPendingCallback(callback)
: Adds a callback to the pendingCallbacks
array.addTimer(callback, timeout)
: Adds a timer to the timers
array.addImmediateCallback(callback)
: Adds a callback to the immediateCallbacks
array.addFdHandler(fd, handler)
: Adds an event handler for a file descriptor to the fdHandlers
map.addFdEvent(fd, eventType)
: Adds a file descriptor event to the fdEvents
array.The main event loop is defined in the eventLoop() function. This function processes events in the following order:
pendingCallbacks
array are executed in the order in which they were added.timers
array that have expired are executed in the order in which they were added.immediateCallbacks
array are executed in the order in which they were added.fdEvents
array are processed by executing the corresponding event handler from the fdHandlers
map.Finally, the waitForEvents()
function is called to wait for new events to arrive. This function could be implemented using a variety of techniques such as using setTimeout()
or using a blocking system call like epoll()
on Linux.
¶ Appendix: About epoll() on Linux
epoll
is a Linux kernel system call that provides a scalable I/O event notification mechanism. It is used to monitor a set of file descriptors for events such as data availability or connection readiness, and it can be used to implement efficient I/O multiplexing in networked applications.
epoll
operates in one of two modes: edge-triggered or level-triggered. In edge-triggered mode, an event is generated only when the state of a file descriptor changes. In level-triggered mode, an event is generated whenever the file descriptor is in a particular state, such as when data is available to be read.
epoll
is often used in conjunction with theselect()
system call, which allows a program to monitor multiple file descriptors for events.epoll
provides a more efficient way to monitor large numbers of file descriptors and is especially useful for high-concurrency servers.
libuv
, the cross-platform library used by Node.js, provides a wrapper aroundepoll
and other platform-specific APIs to provide a consistent interface for asynchronous I/O operations. This allows Node.js applications to take advantage of the scalability and efficiency ofepoll
on Linux without having to directly use theepoll
system call.MacOS/BSD has a
kqueue()
that does the same thing asepoll
, and Windows hasIOCP
(Input/Output Completion Ports).
¶ About libuv
libuv is a cross-platform library that provides support for asynchronous I/O and other utility functions. It is used as the underlying platform layer for Node.js and is responsible for implementing the event loop and other asynchronous mechanisms in the Node.js runtime.
libuv is written in C and provides a consistent API for various types of asynchronous operations such as file I/O, network operations, timers, and process management. It abstracts away the differences between various platform-specific APIs, such as epoll on Linux and kqueue on macOS, making it easier to write cross-platform code that works on multiple operating systems.
libuv is an important component of the Node.js runtime and is responsible for providing many of the features that make Node.js well-suited for building scalable network applications.