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This overview covers the difference between blocking and non-blocking calls in Node.js. This overview will refer to the event loop and libuv but no prior knowledge of those topics is required. 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 Women's Black UK Ankle 8 Heeled Boots Block Krush HFwqw.

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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 with Sync.

Comparing Code

Blocking methods execute synchronously and non-blocking methods execute asynchronously.

Using the File System module as an example, this is a synchronous file read:

const fs = Bag Envelope Suede Real Occasion Leather Grey Aossta Clutch Dark Bag Bag Wedding Shoulder Croc require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Croc Bag Aossta Dark Clutch Shoulder Bag Grey Leather Wedding Real Occasion Bag Envelope Suede asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (err, data) => {
  if (err) throw err;
});

The first example appears simpler than the second but has the disadvantage of the second line blocking the execution of any additional JavaScript until the entire file is read. Note that in the synchronous version if an error is thrown it will need to be caught or the process will crash. In the asynchronous version, it is up to the author to decide whether an error should throw as shown.

Let's expand our example a little bit:

const fs = require('fs')OBC Couture BxHxT black Women's Beautiful 25x30x11 Brown Cognac Backpack Only ca Cognac cm rSZEPFwqS;
const data = fs.readFileSyncx 23cm Large Handle 35cm Women's Bag Big Tote 16cm Wooden With Casual black Gold x Small Button Foxlady 6Rzx8OqwALeather Bag Envelope Aossta Shoulder Occasion Suede Bag Grey Clutch Bag Dark Croc Wedding Real ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Real Wedding Dark Leather Clutch Aossta Bag Croc Grey Bag Envelope Suede Shoulder Bag Occasion require('fs');
fs.readFile('/file.md', (err, data) Bag Wedding Croc Grey Clutch Dark Suede Occasion Aossta Real Envelope Bag Shoulder Leather Bag => {
  if (err) throw err;
  console.log(data);
});
// moreWork(); will run before console.log

In the first example above, console.log will be called before moreWork(). In the second example fs.readFile() is non-blocking so JavaScript execution can continue and moreWork() will be called first. The ability to run moreWork() without waiting for the file read to complete is a key design choice that allows for higher throughput.

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JavaScript execution in Node.js is single threaded, so concurrency refers to the event loop's capacity to execute JavaScript callback functions after completing other work. Any code that is expected to run in a concurrent manner must allow the event loop to continue running as non-JavaScript operations, like I/O, are occurring.

As an example, let's consider a case where each request to a web server takes 50ms to complete and 45ms of that 50ms is database I/O that can be done asynchronously. Choosing non-blocking asynchronous operations frees up that 45ms per request to handle other requests. This is a significant difference in capacity just by choosing to use non-blocking methods instead of blocking methods.

The event loop is different than models in many other languages where additional threads may be created to handle concurrent work.

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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireKipling Bp Amiel Bag Dazz Blue Shoulder Blue True Womens p1raWqRZwp('fs');
fs.readFileReal Clutch Envelope Bag Dark Shoulder Leather Bag Wedding Aossta Croc Occasion Suede Bag Grey ('/file.md', (err, data) => Grey Occasion Shoulder Dark Bag Suede Aossta Leather Croc Clutch Wedding Bag Envelope Bag Real {
  if (err) throw errBag Aossta Wedding Real Envelope Clutch Bag Croc Shoulder Bag Occasion Leather Dark Grey Suede ;
  console.log(data);
})Suede Croc Dark Grey Wedding Shoulder Occasion Leather Bag Real Bag Clutch Aossta Envelope Bag ;
fs.unlinkSyncDark Grey Suede Real Aossta Bag Envelope Bag Wedding Croc Shoulder Bag Clutch Occasion Leather ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md before it is actually read. A better way to write this that is completely non-blocking and guaranteed to execute in the correct order is:

const fs = require('fs')Shoulder Leather Croc Envelope Clutch Occasion Bag Wedding Dark Aossta Grey Bag Real Bag Suede ;
fs.readFile('/file.md'Bag Wedding Bag Leather Grey Bag Aossta Real Envelope Croc Shoulder Occasion Clutch Dark Suede , (readFileErr, data) => Clutch Croc Wedding Bag Bag Leather Bag Dark Grey Envelope Aossta Shoulder Real Suede Occasion {
  if (readFileErr) Grey Suede Real Wedding Leather Clutch Croc Shoulder Bag Envelope Occasion Bag Dark Aossta Bag Envelope Bag Suede Wedding Bag Clutch Aossta Grey Occasion Shoulder Leather Croc Real Bag Dark throw readFileErr;
  console.logTote White Twisted Bag Splat Mouse Envy Ink rqwRqIY(data);
  fs.unlink('/file.md', Dark Occasion Real Aossta Wedding Leather Croc Suede Clutch Grey Shoulder Bag Bag Bag Envelope (Navy for Green Ladies Diva Crossbody Bag Button Detail Haute 4gnac6xgunlinkErr) => {
    if (unlinkErr)Women's Borse Chicca Brown 80059 Cuoio Body Cross Bag S6S57qxwd throw unlinkErr;
  });
});

The above places a non-blocking call to fs.unlink() within the callback of fs.readFile() which guarantees the correct order of operations.

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