The process of waiting for the queue to synchronously receive a message is named the event loop. The event loop moves in increments, named ‘tick,’ and every time it ‘ticks’ it checks whether the call stack is empty. If it is empty, it executes the top function in the event queue after adding it to the call stack. It starts ticking again once it is finished processing this function.
The library which gives the event loop to Node.js is known as Libuv. Node.js is a platform that's event-based. Meaning, all that happens in Node is a reaction to an event. If a transaction going through Node traverses a cascade of callbacks. This is all handled by a library, now abstracted away from the developer, which provides a mechanism known as an event loop, called libuv.
By default, with four threads to offload asynchronous work to, Libuv creates a thread pool. For many input-output tasks (e.g., AIO on Linux), today’s operating systems already provide asynchronous interfaces. Libuv will use those asynchronous interfaces, whenever possible, to avoid usage of the thread pool. To third party subsystems like databases, the same applies. Here rather than utilizing a thread pool, the authors of the driver will use the asynchronous interface. Basically, the thread pool will be used for asynchronous I/O, only if there is no other way.
The Event Loop essentially works on phases. It goes through a new phase, for each tick of the loop. It maintains a FIFO queue of callbacks for every phase, and in that phase, it executes as many callbacks as it can (on every tick, there is a limit to how many callbacks can be called). The event loop moves to the next phase when the callback limit is reached, or the queue has been exhausted, and so on. Since new events are queued by the kernel after being processed in the poll phase and any of these operations could schedule more operations, while polling events are being processed, poll events can be queued. As a result, the poll phase can be allowed by long-running callbacks to run far longer than a timer's threshold.
I hope that some of the concepts explained here will be useful to you in the future.
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