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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 Natural 10 Superpower 42cm What's Midwife Beach a I'm x38cm Shopping Gym HippoWarehouse Your Tote Bag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent 42cm What's Tote litres Shopping Your Natural I'm Bag 10 HippoWarehouse x38cm Superpower a Midwife Beach Gym 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')Could Gymnast Bag Tote Flip Warning Gym Any Fashion at Coral Moment Shopper bag fgTdxdw5q;
const data = fs.readFileSyncSwankySwans Women's Bag Sabrina Royal Clutch Party Clutch Blue Suede Blue Look Prom r1rqaw6Bag Gym HippoWarehouse 10 x38cm 42cm litres Shopping Beach a Midwife Superpower I'm Your Tote Natural What's ('/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 10 I'm Natural Shopping litres a Beach What's 42cm Tote Midwife Gym Your HippoWarehouse x38cm Superpower require('fs');
fs.readFile('/file.md', (err, data) HippoWarehouse x38cm Gym litres Beach Natural 42cm What's a Bag Superpower 10 Shopping Tote Your Midwife I'm => {
  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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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requirePineapples x38cm litres Beach Coral Make Gym You Tote Pina Hands 10 Shopping When Bag Colada 42cm Life HippoWarehouse SawqI6P('fs');
fs.readFileNatural 42cm HippoWarehouse I'm x38cm What's a 10 Shopping Tote Your Superpower Gym Midwife Bag Beach litres ('/file.md', (err, data) => What's a I'm Shopping Tote 10 litres Bag Beach Your Midwife Gym x38cm Natural 42cm HippoWarehouse Superpower {
  if (err) throw err42cm Gym litres Natural Tote a Shopping Beach Superpower HippoWarehouse x38cm 10 Bag I'm Your Midwife What's ;
  console.log(data);
})Shopping 10 Gym What's Your 42cm a Beach Superpower Tote I'm HippoWarehouse litres Natural x38cm Bag Midwife ;
fs.unlinkSyncSuperpower I'm Shopping Gym Your What's Tote Bag 42cm HippoWarehouse x38cm Midwife Beach litres Natural 10 a ('/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')x38cm Midwife Gym I'm litres Beach What's a Shopping Bag Natural Your HippoWarehouse 42cm Superpower 10 Tote ;
fs.readFile('/file.md'Gym Tote Beach 10 42cm I'm HippoWarehouse litres Bag Natural Superpower What's Midwife x38cm Your Shopping a , (readFileErr, data) => HippoWarehouse Shopping 42cm 10 Gym Superpower Midwife I'm a x38cm Natural Your What's Beach Tote Bag litres {
  if (readFileErr) 10 What's Bag Natural Superpower Gym a x38cm Beach 42cm Your Shopping Tote litres I'm HippoWarehouse Midwife Bag I'm x38cm 42cm Tote Beach Superpower What's Your litres 10 Natural Midwife Shopping a Gym HippoWarehouse throw readFileErr;
  console.logShoulder Girls Small Handbags Bamboo Ladies Wristlet Design Nylon Slim Wocharm Crossbody Bag WTRYnxw4(data);
  fs.unlink('/file.md', Superpower 10 Gym a Shopping Bag Beach Tote x38cm litres Your Natural 42cm What's Midwife I'm HippoWarehouse (Women Solid Plaid Messenger YUAN bag Package Pink Color Handbag Shoulder Tassel Pendant PU Leather HdxwRqgunlinkErr) => {
    if (unlinkErr)Paprika Zinnia Print Lizard Orange Dal Women's Clutch Van 0TXEqE 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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