Node.js Event Loop

Introduction to Node.js Event Loop

The Node.js event loop is a fundamental concept that enables Node.js to perform non-blocking I/O operations. It allows Node.js to handle multiple operations simultaneously by delegating tasks to the operating system whenever possible.

In essence, the event loop processes asynchronous operations and manages them across different phases, such as timers, I/O callbacks, and more.

Phases of the Event Loop

The event loop is divided into several phases, each with a specific purpose:

• Timers: Executes callbacks scheduled by setTimeout() or setInterval().
• I/O Callbacks: Handles I/O events and executes callbacks of completed I/O operations.
• Idle, Prepare: Internal use only, for executing callbacks during the idle phase.
• Poll: Retrieves new I/O events and executes related callbacks.
• Check: Executes callbacks scheduled by setImmediate().
• Close Callbacks: Executes callbacks on closing events like socket.on('close').

Timers vs Immediate Execution

There is often confusion between setTimeout() and setImmediate(). While both are used to schedule code execution, they function differently within the event loop phases.

• setTimeout() schedules an operation to execute after a minimum threshold of time has elapsed. It runs during the Timers phase.
• setImmediate() schedules an operation to execute immediately after the current poll phase completes, running during the Check phase.

Consider the following example to understand this difference:

In the example above, even though both functions are scheduled without delay, setImmediate() is likely to execute before setTimeout() because it runs after the poll phase.

Understanding the Poll Phase

The Poll phase is one of the crucial phases of the event loop where the system retrieves new I/O events and executes their callbacks. If the event loop enters the poll phase and no timers are scheduled, it will block and wait for callbacks to be added, effectively keeping the event loop alive.

This phase is essential for managing the incoming data and executing any I/O callbacks as soon as the I/O operation completes.

Example: Handling I/O with Event Loop

In this example, the readFile operation is asynchronous and will use the event loop to handle the file reading. The console.log('Reading file...') is executed first, and once the file reading is complete, the callback is executed, logging 'File read complete'.

Conclusion

Understanding the Node.js event loop is crucial for writing efficient and performant asynchronous code. By managing different phases and handling callbacks appropriately, Node.js can perform non-blocking operations, making it ideal for I/O-heavy applications.

In the next segment of this series, we will explore Async Patterns and how they can further optimize your Node.js applications.