The NodeJs Event Loop | Ravi Singh

The NodeJs Event Loop

The Node.js event loop is a critical component of the Node.js runtime that allows JavaScript code to be executed in a non-blocking, asynchronous manner. It is the mechanism that enables Node.js to handle multiple requests and perform multiple tasks at the same time, without having to wait for one task to finish before beginning the next.

The event loop is a single thread that runs indefinitely, looking for new events and tasks to perform. When a task is added to the event loop, it is queued in the task queue. The event loop then retrieves and executes tasks from the task queue.

One important aspect of the event loop is that it only runs a single task at a time. This means that even though the event loop is running multiple tasks, only one task is executing at any given moment. Once a task is finished executing, the event loop moves on to the next task in the queue.

The event loop features a separate queue for handling I/O events, such as network requests, file operations, and timers, in addition to the task queue. The I/O poll phase of the event loop is the method used to handle these events. The event loop enters the poll phase when it waits for fresh I/O events to happen when it is not carrying out any activities.

The related callback function is added by the event loop to the task queue whenever an I/O event is discovered and is then executed when the event loop gets to it.

The event loop also includes a separate mechanism for dealing with long-running tasks like heavy computations. These tasks are typically carried out in a separate thread known as the worker thread, and the event loop is notified when the task is finished. While the worker thread is busy, the event loop can continue to execute other tasks.

To prioritize tasks, Node.js employs a technique known as microtasks and macro tasks. Microtasks are small, quick tasks that are executed immediately after the current task is completed, whereas macro tasks are larger, more complex tasks that are executed during the event loop's next iteration.

This enables Node.js to prioritize certain tasks, such as user input or network communication, over less important tasks.

To summarise, the Node.js event loop is a powerful and efficient mechanism that allows Node.js to handle multiple tasks and requests at the same time while not interfering with the execution of other tasks. It is based on a single-threaded, non-blocking architecture that enables Node.js to scale to handle large numbers of concurrent connections.

The event loop's architecture, as well as its ability to handle I/O operations, timers, and long-running tasks in a separate thread, make it ideal for developing high-performance, high-concurrency network servers, and applications.