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Sitota Alemu
Sitota Alemu

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Understanding Asynchronous I/O Operations in Node.js

Understanding how Node.js handles asynchronous I/O operations is vital for anyone interested in building applications with Node.js. Often, this concept is raised in interviews. So, in this post, I’ll be sharing an overview of how Node.js handles asynchronous tasks.
Before reading this post, it’s assumed that you have a basic understanding of event loops and other fundamental Node.js concepts.

Overview of Asynchronous I/O Process
Asynchronous Function ➔ Call Stack ➔ Delegating I/O to
System APIs ➔ Callback Queue ➔ Event Loop ➔ Call Stack

When asynchronous code is encountered (e.g., fetching data from a database, reading files, etc.), the call stack, event loop, and Node.js’s underlying APIs (like Libuv ) work together to handle it. The process involves delegating the asynchronous task to the appropriate background thread or system API. Let’s explore this in detail.

Here's a Breakdown
1. Asynchronous Function Call When an asynchronous function (e.g., a database query) is called, it is pushed onto the call stack like any other function.
2. Delegation to Background APIs If the function involves I/O (e.g., querying a database, reading a file, or making a network request), Node.js delegates this task to the appropriate system-level API or thread in the background (e.g., Libuv thread pool or database driver). The function is immediately popped off the call stack once the task is delegated, allowing the stack to continue processing other code.
3. Background Processing The actual I/O operation (e.g., querying the database) happens in the background. This is managed outside the JavaScript runtime by the underlying Libuv library, which handles threads and system resources.
4. Callback Queuing Once the asynchronous task completes (e.g., the database query finishes), the result is passed to the callback associated with the operation. The callback is placed into an appropriate queue (e.g., the I/O callback queue) for the event loop to process.
5. Event Loop Execution The event loop continuously checks if the call stack is empty. If it is, it takes the next task from the queue (i.e., the callback) and pushes it onto the call stack for execution.

In a nutshell, an asynchronous I/O operation can be summarized in the following steps:
1. Asynchronous Function Call:
◦ When an asynchronous function (e.g., a database query) is called, it is pushed onto the call stack like any other function.
2. Delegation to Background APIs:
◦ Since the function involves I/O (e.g., querying a database, reading a file, or making a network request), Node.js delegates this task to the appropriate system-level API or thread in the background (e.g., Libuv thread pool or database driver).
◦ The function is immediately popped off the call stack once the task is delegated, allowing the stack to continue processing other code.
3. Background Processing:
◦ The actual I/O operation happens in the background, managed by Libuv or other underlying libraries.
4. Callback Queuing:
◦ Once the asynchronous task completes, the callback is placed into the appropriate queue for the event loop to process.
5. Event Loop Execution:
◦ The event loop checks if the call stack is empty. If it is, it picks the next callback from the queue and pushes it onto the call stack for execution.

Thanks for reading! I’ll be writing about the event loop and its phases in an upcoming post. If you have any thoughts, questions, or feedback, don’t hesitate to leave them in the comments. I’d love to hear from you!

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