How many event queues are there in nodejs

Node.js is an event-driven asynchronous I/O framework, in which the event loop (Event Loop) is one of its core mechanisms. The event loop maintains multiple event queues to handle different types of events. So, how many types of event queues are there?

First, let us understand the basic process of the Node.js event loop. When Node.js starts, it creates a main thread and an event loop object. Node.js programs typically start by performing an I/O operation, such as reading a file or sending an HTTP request. These operations will be handled by the Libuv library, which will add events to the event queue of the event loop object.

In the Node.js event loop, there are six different types of event queues, namely:

1. Timers Queue (timer queue)
   maintained in this queue All timer events created by the setTimeout() and setInterval() functions are listed in chronological order.
2. I/O Queue (I/O event queue)
   The I/O event queue maintains callback functions for all asynchronous I/O requests, such as when data for file or network requests is ready, etc. . When the Libuv library detects that the I/O event is completed, the callback function of the event will be added to the I/O event queue.
3. Check Queue (detection event queue)
   In each round of the event loop, when all I/O events and timer events in the timer queue have been processed, the event loop will Arrival detection event queue, which is mainly used to process events added by the setImmediate() function.
4. Close Callback Queue (Close callback queue)
   This queue saves all callback functions added by socket.on('close', ...), and when it detects that the socket is closed, it will A callback function is added to this queue for asynchronous execution.
5. Microtask Queue (microtask queue)
   The microtask queue is executed in the current stage of the event loop process and is located between each stage of the event loop. All Promise callback functions and events added by the process.nextTick() function will be placed in this queue.
6. Node API Queue (Node API Queue)
   The Node API queue is controlled by the C module inside Node.js. Asynchronous events such as DNS requests or HTTP connections will be part of the queue.

In summary, the Node.js event loop maintains six different types of event queues, which are used to process timer events, I/O events, detection events, shutdown callbacks, and microtasks respectively. and Node API and other asynchronous events.

When developing Node.js applications, developers need to consider the impact of the event loop mechanism on program execution efficiency. Understanding how the event loop works and how certain events are queued in separate event queues waiting to be executed is key to writing efficient and reliable Node.js applications.

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