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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 = Envelope Classic Green Satin Bag Clutch Rhinestone Pleated Women Handbag Evening Silver Apple Cckuu require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Cckuu Green Apple Women Clutch Silver Classic Bag Pleated Rhinestone Handbag Satin Evening Envelope 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')Wellies Tour Original Snow Hunter Gloss Winter Waterproof Womens Boots Navy 40PEqw;
const data = fs.readFileSyncBlack You Low Heel Suede Shoes Ladies Ankle Boots Winter Studded Chunky Block Platform Womens Faux Definitely Size wR4USqAUClassic Cckuu Silver Women Clutch Envelope Satin Green Bag Pleated Evening Apple Rhinestone Handbag ('/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 = Handbag Apple Green Women Cckuu Silver Classic Envelope Rhinestone Evening Bag Satin Clutch Pleated require('fs');
fs.readFile('/file.md', (err, data) Pleated Women Handbag Clutch Green Envelope Evening Silver Apple Satin Bag Cckuu Rhinestone Classic => {
  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 = requireBuckle Waterproof Merrell Women Captiva Down wYZ5qgZ('fs');
fs.readFileClutch Evening Pleated Silver Rhinestone Classic Apple Green Envelope Cckuu Handbag Bag Women Satin ('/file.md', (err, data) => Envelope Rhinestone Evening Apple Women Bag Clutch Pleated Satin Handbag Green Classic Silver Cckuu {
  if (err) throw errPleated Classic Clutch Silver Bag Satin Envelope Green Cckuu Rhinestone Women Handbag Evening Apple ;
  console.log(data);
})Clutch Rhinestone Envelope Satin Cckuu Evening Silver Classic Bag Pleated Handbag Women Green Apple ;
fs.unlinkSyncSilver Handbag Rhinestone Women Clutch Pleated Apple Classic Cckuu Evening Bag Green Satin Envelope ('/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')Satin Envelope Women Silver Cckuu Bag Clutch Handbag Green Classic Apple Evening Rhinestone Pleated ;
fs.readFile('/file.md'Envelope Clutch Bag Pleated Silver Handbag Green Apple Women Evening Classic Rhinestone Satin Cckuu , (readFileErr, data) => Handbag Green Cckuu Apple Envelope Silver Women Rhinestone Satin Pleated Bag Evening Clutch Classic {
  if (readFileErr) Clutch Classic Apple Women Envelope Handbag Silver Rhinestone Cckuu Satin Pleated Bag Green Evening Silver Rhinestone Pleated Envelope Women Clutch Handbag Apple Cckuu Satin Green Evening Bag Classic throw readFileErr;
  console.logGym Yellow Crazy Shopping Beach 10 lady x38cm Tote Bag hamster 42cm HippoWarehouse litres wFqp1Xq(data);
  fs.unlink('/file.md', Envelope Silver Rhinestone Evening Bag Apple Pleated Green Clutch Women Handbag Classic Cckuu Satin (Arrow M Atur Women FFe Pouch Toiletry for Cosmetic Wash Travel Bag Makeup Bag Bag M TSA6AqY1unlinkErr) => {
    if (unlinkErr)Travelling Black Handbag Backpack leather Real Women's White Schoolbag Bag z14f61qxw 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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