A fundamental idea of Node.js is the event loop. It enables you to get knowledge about Node's non-blocking I/O and asynchronous processes. It explains the processes that lead to Node's accomplishments, dominance, and acceptance as a contemporary framework. This lesson is great for Node.js programmers who wish to fully control every stage of an application's life cycle while also getting a better knowledge of what is going on underneath any given application.
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Before understanding Event Loop, it is important to understand Threads. When we start a programme, we generate an example of it, and we have internal components known as threads attached to that instance. A thread can be thought of as a group of tasks that our CPU must carry out on our behalf. The OS scheduler, a component of our operating system, is in charge of managing the scheduling mechanism, which is utilised to decide which thread to run at any given time.
A thread is created automatically each time a Node programme is executed. The only thread in which the entirety of our codebase will be run is this one. The so-called event loop is created inside of it. This loop's function is to schedule the tasks that our single thread should be working on at any given moment.
Please take note that even when we execute our programme immediately, the event loop doesn't appear right away. In actuality, it doesn't start until the entire application has finished running.
The following are some of the features of the Event Loop:
Let us try to understand the Node.js Event Loop working via an example:
In the aforementioned example, the task is eliminated from the call stack once the first console log statement is sent to it, logging "XYZ" on the console and pushing the task to the call stack. The job is then delivered to the operating system, and the setTimeout is added to the queue, and also the timer for the task is set. After that, this job is eliminated from the stack. The task is then popped off the stack when the third console log statement is sent to the call stack and "DEF" is printed to the console.
Whenever the timer defined by the setTimeout function (in this example, 1000 ms) expires, the callback is sent to the event queue. The event loop takes the task at the head of the event queue & feeds it to the call stack whenever the call stack is empty. The instruction is carried out by the callback function of the setTimeout function, which also removes the task from the stack and logs the message "ABC" on the console.
Node.js commences running the event loop after initializing the event loop, parsing the input script that may include async API calls, and scheduling timers. The initial input script for the previous example contained console.log() statements as well as a setTimeout() function to set a timer.
A unique library module named libuv is used by Node.js to carry out async activities. The libuv thread pool is a unique thread pool that is managed by this library and the back logic of Node. Four threads from this pool are utilised to delegate tasks that are too complex for the event loop. Examples of these operations are I/O activities, opening and closing connections, and setTimeouts.
A callback function is invoked whenever the thread pool wraps up a task, and it deals with errors (if any) or performs other actions. The event queue receives a callback function like this one. The event moves through the event queue and delivers the callback to the call stack when the call stack is empty.
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Let us now learn about the different phases of the event loop in more detail.
In essence, the event loop performs two tasks during this stage:
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It is clear that the event loop regulates all activities within a Node application. This indicates that if we are able to extract measurements from it, they ought to give us useful details about the overall performance and health of an application.
Any monitoring tool needs to provide its own metrics since there is no API for getting runtime data from the event loop. This is what we came up with:
Metrics are useless, of course, until you understand how to utilize them to identify potential problems to solve. Here are some recommendations for what to do when it seems like the event loop is losing steam.
In this blog, we learned about what Node.js and event loop is and how the node.js event loop work along with the help of an example. We also discussed the phases of the event loop like timers, pending callbacks, idle, prepare, poll, check and close callbacks. We further learned about monitoring and tuning the event loop. If you want to use Node.js effectively, learn how to boost its performance, or just find a new, compelling reason to take on new technology, you must first understand how the event loop works.
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When the function stack is empty, an event loop is anything that moves items from the queue to the function execution stack.
By using the cluster APIs or the previously described child processes, node.js can have several event loops, but by default only uses one on the main thread.
The Event Loop uses just one thread. It serves as the Node JS Platform Processing Model's main building block. Loop also examines every Client Request put in the Event Queue.
The event loop waits for the Call Stack to be cleared prior to pushing callbacks from the Task Queue to the Call Stack. Once the stack has been emptied, the event loop is started, and checks the Task Queue for any callbacks that might be available.