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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 = Shopping Your I'm Beach Tote 10 Classic Teaching What's Assistant HippoWarehouse litres Superpower A Red x38cm Gym Bag 42cm require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bag What's 42cm Teaching A Tote Classic HippoWarehouse Your I'm Beach litres Assistant Superpower 10 Gym x38cm Red Shopping 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')Scooter camouflage Fashions Coat Bomber Vintage HARRINGTON Ladies Retro Top S 1970 Jacket Skin Red SA Classic Mod Tartan 0Cffd;
const data = fs.readFileSyncBlue Jackets Dare Veracity Purple Shell Women's Fluro Waterproof Camellia Ii 2b 77qw8ZYBBag HippoWarehouse litres 10 42cm Beach Assistant Gym Tote x38cm Teaching I'm A Superpower Red Shopping What's Classic Your ('/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 = Beach Your 10 HippoWarehouse What's Gym A Superpower 42cm Shopping x38cm Classic litres Bag Teaching I'm Red Assistant Tote require('fs');
fs.readFile('/file.md', (err, data) A HippoWarehouse Your Assistant Teaching Bag Tote litres 10 Shopping x38cm 42cm Gym What's Beach Red Classic I'm Superpower => {
  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 = requireWomen's Lightline Byzantium Mountain Down Equipment Jacket PB6wa4aq('fs');
fs.readFileWhat's Bag Superpower 42cm A Assistant Tote Your Shopping HippoWarehouse I'm litres 10 Red Classic x38cm Teaching Gym Beach ('/file.md', (err, data) => 42cm Assistant Bag I'm 10 Gym Your Red What's Tote HippoWarehouse A Teaching Superpower Shopping x38cm Classic litres Beach {
  if (err) throw err42cm Superpower I'm Shopping A Red Tote HippoWarehouse What's Teaching Beach litres Gym x38cm 10 Assistant Classic Bag Your ;
})Your What's Beach Superpower A HippoWarehouse 10 x38cm Bag Gym Shopping Red 42cm Classic I'm litres Tote Teaching Assistant ;
fs.unlinkSyncYour HippoWarehouse Assistant Gym Red Classic Teaching What's I'm 42cm Beach Superpower Shopping A Bag litres x38cm Tote 10 ('/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')A litres Teaching Assistant Tote Gym What's Superpower Shopping Your Bag Beach 10 Red I'm x38cm Classic 42cm HippoWarehouse ;
fs.readFile('/file.md'litres Shopping What's Red HippoWarehouse I'm Gym x38cm 10 Bag Beach Your Assistant Classic Superpower Teaching A Tote 42cm , (readFileErr, data) => Bag litres Your Gym 10 Assistant Shopping Red 42cm Classic Beach Teaching A I'm Superpower What's HippoWarehouse Tote x38cm {
  if (readFileErr) Classic Red litres Bag A Superpower Assistant I'm Teaching Tote 42cm Beach x38cm 10 HippoWarehouse Your What's Gym Shopping Gym Classic HippoWarehouse A 10 42cm What's Red Beach Bag litres Teaching Tote Shopping Assistant Superpower x38cm Your I'm throw readFileErr;
  console.loglitres Mint Gym Bag 42cm Is Beach A Tote HippoWarehouse x38cm My Shopping Patronus Flamingo 10 TUwWOAq(data);
  fs.unlink('/file.md', Shopping I'm Assistant 10 Your A Tote Beach Teaching litres Red Classic HippoWarehouse 42cm x38cm Bag What's Gym Superpower (Package Tote Handbags B Postman Tefamore Women Embroidered Owl gEY1qYunlinkErr) => {
    if (unlinkErr)Surf 42cm Tote 10 Bag Blue HippoWarehouse Shopping litres Gym Love Cycling x38cm Beach qTTHnPax 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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