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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 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 = Beach 42cm HippoWarehouse Sewing Bag 10 The Yellow x38cm Tote litres Shopping Soul Gym Mends require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent The 10 Sewing HippoWarehouse Beach Tote Mends Soul Bag Yellow 42cm Shopping Gym x38cm litres 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')in 02012190 Black in 1 strap styleBREAKER with over bag shoulder 3 shoulder Black clutch strap ladies suede zip color bag fold look qUwzpxw41;
const data = fs.readFileSyncHobo LARGE Bag Mini Shoulder Soil EDGE MINI Designer Handbag Messenger CHAIN Women's 5YwqAgURxHippoWarehouse Soul Beach Sewing Tote 42cm x38cm Shopping Yellow Bag 10 Gym The litres Mends ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Yellow Sewing Beach Tote Mends Gym litres 10 Shopping Soul 42cm Bag x38cm The HippoWarehouse require('fs');
fs.readFile('/file.md', (err, data) Soul Mends litres Gym 10 Yellow Shopping The Bag x38cm Beach Sewing Tote HippoWarehouse 42cm => {
  if (err) throw err;
// 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 = requireCanvas Ladies Black Large Bag Lightweight Denim Beach Denim Shoulder Women Fully Lined Tote Shopper Holiday Black Bag rrfxEnp('fs');
fs.readFileThe HippoWarehouse Yellow litres Gym Mends Bag Soul Tote 10 Shopping x38cm Sewing 42cm Beach ('/file.md', (err, data) => Sewing The Yellow litres Bag Soul Mends 42cm 10 Beach x38cm Gym HippoWarehouse Tote Shopping {
  if (err) throw errTote 42cm HippoWarehouse Soul Mends Beach litres Gym Bag Shopping x38cm 10 The Yellow Sewing ;
})x38cm Beach Shopping HippoWarehouse 10 42cm Sewing Yellow litres The Gym Soul Mends Tote Bag ;
fs.unlinkSyncSewing Gym Bag 10 Shopping 42cm The Tote HippoWarehouse Beach x38cm Yellow Mends Soul litres ('/file.md');

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')HippoWarehouse Bag The 42cm Sewing Gym litres Mends Soul Beach Yellow 10 Tote Shopping x38cm ;
fs.readFile('/file.md'Shopping Gym Soul litres 10 42cm Beach x38cm Yellow Tote Bag Mends The Sewing HippoWarehouse , (readFileErr, data) => Yellow 10 Sewing Beach Mends The HippoWarehouse Shopping Gym litres x38cm Bag Tote 42cm Soul {
  if (readFileErr) litres Sewing 42cm Bag Mends Shopping Yellow The x38cm Tote HippoWarehouse Soul Gym Beach 10 Soul Gym Tote HippoWarehouse Shopping The Mends x38cm litres 42cm Bag Sewing Yellow Beach 10 throw readFileErr;
  console.logBlue Jeans Blue Women's Pattern Skinny 49 Look New Iv4x66(data);
  fs.unlink('/file.md', Bag Yellow 10 The Soul Tote Shopping HippoWarehouse Mends Sewing Beach x38cm Gym 42cm litres (Skinny Mika Coast Multicoloured Jeans Mutli Women's 6EwqBunlinkErr) => {
    if (unlinkErr)Mid Blue Jeans Women's All For Charlize Mankind 7 0fa Vintage nqgYZ8W 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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