Asynchronous I/O
Asynchronous I/O, or non-blocking I/O, is a form of input/output processing that permits other processing to continue before the transmission has finished.
What this means is, if a process wants to do aread()orwrite(), in a synchronous call, the process would have to wait until the hardware finishes the physical I/O so that it can be informed of the success/failure of the I/O operation.
On asynchronous mode, once the process issues a read/write I/O asynchronously, the system calls is returned immediately once the I/O has been passed down to the hardware or queued in the OS/VM. Thus the execution of the process isn't blocked (hence why it's called non-blocking I/O) since it doesn't need to wait for the result from the system call, it will receive the result later.
Asynchronous I/O (from Wikipedia)
Overview of Blocking vs Non-Blocking
This overview covers the difference between blocking and non-blockingcalls in Node.js.
This overview will refer to the event loop and libuv
libuv is a multi-platform support library with a focus on asynchronous I/O. It was primarily developed for use by Node.js, but it's also used by Luvit, Julia, pyuv, and others. In case you find errors in this documentation you can help by sending pull requests!
Readers are assumed to have a basic understanding of the JavaScript language and Node.js callback pattern.
"I/O" refers primarily to interaction with the system's disk and network supported by libuv.
Blocking
Blocking is when the execution of additional JavaScript in the Node.js process must wait until a non-JavaScript operation completes. This happens because the event loop is unable to continue running JavaScript while a blocking operation is occurring.
In Node.js, JavaScript that exhibits poor performance due to being CPU intensive rather than waiting on a non-JavaScript operation, such as I/O, isn't typically referred to as blocking. Synchronous methods in the Node.js standard library that use libuv are the most commonly used blocking operations. Native modules may also have blocking methods.
All of the I/O methods in the Node.js standard library provide asynchronous versions, which are non-blocking, and accept callback functions. Some methods also have blocking counterparts, which have names that end withSync.