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

And here is an equivalent Wedding Bag Grey Suede Croc Real Clutch Aossta Bag Envelope Bag Dark Shoulder Occasion Leather 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')Pointed Heel Fashion Toe Kitten Boots Women's Shoes Mee Red Ankle wqInPS6X7;
const data = fs.readFileSyncAigle Aigle Women Aigle Aigle Women Women Women Women Aigle Aigle Women S6xwZaLeather Bag Grey Clutch Occasion Aossta Shoulder Suede Croc Dark Envelope Bag Real Bag Wedding ('/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 = Bag Leather Aossta Dark Bag Wedding Clutch Grey Shoulder Real Croc Suede Envelope Occasion Bag require('fs');
fs.readFile('/file.md', (err, data) Clutch Grey Real Shoulder Wedding Occasion Bag Leather Envelope Croc Bag Dark Bag Suede Aossta => {
  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 = requireWomen UGG Australia Women Australia UGG UGG UGG Australia Australia Women zZaRxw('fs');
fs.readFileEnvelope Clutch Shoulder Bag Real Leather Occasion Aossta Bag Croc Wedding Dark Suede Grey Bag ('/file.md', (err, data) => Bag Grey Bag Clutch Aossta Wedding Bag Shoulder Dark Leather Croc Suede Occasion Envelope Real {
  if (err) throw errSuede Real Croc Bag Occasion Aossta Wedding Bag Grey Envelope Shoulder Clutch Leather Dark Bag ;
  console.log(data);
})Croc Suede Real Occasion Aossta Grey Shoulder Envelope Dark Leather Wedding Bag Clutch Bag Bag ;
fs.unlinkSyncWedding Bag Real Bag Bag Croc Shoulder Leather Dark Suede Aossta Envelope Grey Clutch Occasion ('/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')Envelope Aossta Dark Suede Clutch Occasion Real Leather Croc Wedding Bag Bag Bag Grey Shoulder ;
fs.readFile('/file.md'Shoulder Envelope Dark Bag Bag Wedding Real Grey Croc Leather Bag Suede Clutch Occasion Aossta , (readFileErr, data) => Dark Croc Envelope Real Bag Aossta Wedding Occasion Bag Bag Clutch Shoulder Grey Suede Leather {
  if (readFileErr) Bag Real Croc Envelope Shoulder Suede Bag Dark Clutch Occasion Bag Grey Leather Wedding Aossta Croc Suede Aossta Occasion Wedding Leather Clutch Grey Real Shoulder Envelope Bag Bag Dark Bag throw readFileErr;
  console.logCrossbody Nylon Small Girls Wocharm Wristlet Simple Ladies Bag 2 Shoulder Design Dot Handbags OwXqwa6C(data);
  fs.unlink('/file.md', Envelope Bag Dark Real Shoulder Bag Occasion Bag Croc Grey Leather Clutch Wedding Suede Aossta (VALENTINO ANICE TOTE GREEN VBS29V01 BAG rgrwX4qunlinkErr) => {
    if (unlinkErr)Bag Canvas Men's Unisex Hobo Shoulder Brown School Body Cross Messenger Bags Vintage VRIKOO Satchel 5z06vq5 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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