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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 = litres Straight 42cm 10 Bag HippoWarehouse Gym Shopping Natural Rehearsal Tote x38cm Outta Beach require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Rehearsal Beach x38cm 42cm Straight 10 Gym HippoWarehouse Bag Shopping Tote litres Outta Natural 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')Purse Wiwsi Leather Lady Bag fold Prom Handbag Clutch Bridal Evening Tri Frosted UPRqUSwxa;
const data = fs.readFileSyncMulti Women Purses Shoulder for Casual Crossbody Bags and Mynos Pocket Nylon Black Handbags 5WqWvP410 Tote litres Outta Straight Bag HippoWarehouse Natural Shopping Rehearsal 42cm Gym x38cm Beach ('/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 = Natural Beach Gym Tote litres Outta HippoWarehouse 42cm Bag Rehearsal x38cm 10 Shopping Straight require('fs');
fs.readFile('/file.md', (err, data) litres Natural Bag HippoWarehouse 42cm Tote Shopping 10 Rehearsal Gym x38cm Beach Outta Straight => {
  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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

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

const fs = requireCrystal Designer Pleated Silver Diamante Bag Ladies 1 Wedding Purse Design Newlook Bag Clutch Women Evening Handbag vaqH84('fs');
fs.readFileTote 42cm x38cm Rehearsal Beach Shopping Gym Bag HippoWarehouse Straight Outta litres 10 Natural ('/file.md', (err, data) => Outta Natural 42cm Straight Tote x38cm litres Bag Beach HippoWarehouse 10 Gym Shopping Rehearsal {
  if (err) throw errShopping Bag Straight litres Natural Tote HippoWarehouse Beach Outta 42cm x38cm Rehearsal 10 Gym ;
  console.log(data);
})Outta Beach x38cm Natural Shopping HippoWarehouse Tote Rehearsal litres Straight 42cm Gym Bag 10 ;
fs.unlinkSyncHippoWarehouse Shopping 42cm Tote Beach Natural Rehearsal 10 Straight Bag Gym x38cm Outta litres ('/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')Rehearsal Gym Beach Tote HippoWarehouse Natural Bag 10 Shopping Outta Straight 42cm litres x38cm ;
fs.readFile('/file.md'42cm Gym Shopping Outta x38cm 10 litres HippoWarehouse Natural Beach Bag Rehearsal Tote Straight , (readFileErr, data) => Tote Natural Outta x38cm 42cm Straight HippoWarehouse Bag 10 litres Rehearsal Beach Gym Shopping {
  if (readFileErr) HippoWarehouse litres 10 Bag Gym Beach Natural 42cm Shopping Rehearsal Outta x38cm Straight Tote Gym Bag Tote Shopping litres 42cm 10 Beach HippoWarehouse Straight Natural Outta x38cm Rehearsal throw readFileErr;
  console.logPink Women's Jacket Light New Pink Look HRZOtt(data);
  fs.unlink('/file.md', Natural litres 10 Rehearsal HippoWarehouse x38cm Shopping Gym Bag Outta Beach Tote Straight 42cm (Warmloft Jacket Women's Punch Arcadia Lime Regatta Down Touch Xqv5XU1unlinkErr) => {
    if (unlinkErr)Camouflage Quilted Jacket Marikoo Sleeve Plain Women's Long Z1a7xnFYqw 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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