What is the Event Loop?
The event loop is the heart of Node.js’s asynchronous architecture. It is a mechanism that allows Node.js to perform non-blocking I/O operations, even though JavaScript is single-threaded. The event loop continuously checks the event queue and processes the events, allowing Node.js to handle multiple tasks efficiently.
How the Event Loop Works
The event loop operates in cycles known as “ticks.” Each tick represents a single pass through the phases of the event loop. During each tick, the event loop processes events in the phases.
Event Loop Phases
The Node.js event loop consists of six main phases:
Timers Phase
Pending Callbacks Phase
Idle, Prepare Phase
Poll Phase
Check Phase
Close Callbacks Phase
Understanding Each Event loop Phase
1. Event loop Timers Phase
What happens:
This phase executes callbacks scheduled by setTimeout() and setInterval().
Details:
Timers callbacks are executed once their scheduled time has passed. However, the actual execution time might be delayed if the previous phases take a long time to complete.
2. Event loop Pending Callbacks Phase
What happens:
Executes I/O callbacks deferred to the next loop iteration.
Details:
This phase handles callbacks for some system operations like TCP errors. These callbacks are not part of the timers phase because they are not scheduled using setTimeout or setInterval.
3. Event loop Idle, Prepare Phase
What happens:
Internal use only.
Details:
This phase is used internally by Node.js to prepare for the upcoming poll phase.
4. Event loop Poll Phase
What happens:
Retrieves new I/O events; executes I/O related callbacks (almost all with the exception of close callbacks, timers, and setImmediate()); will block here when appropriate.
Details:
This is the most important phase. Here, the event loop will pick up new events from the event queue and execute their callbacks. If there are no events to handle, it will block and wait for I/O events.
5. Event loop Check Phase
What happens:
Executes setImmediate() callbacks.
Details:
Callbacks scheduled with setImmediate() are executed here. This is similar to setTimeout() but it guarantees the callback will be executed immediately after the poll phase completes.
6. Event loop Close Callbacks Phase
What happens:
Executes close callbacks (e.g., socket.on(‘close’, …)).
Details:
This phase handles closing of all requests that need to be cleaned up. For example, closing of HTTP server or file descriptor.
Conclusion
Understanding the Node.js event loop and its phases is crucial for writing efficient and non-blocking applications. Each phase has its specific role, and knowing how they interact helps in optimizing the performance and debugging asynchronous code.
