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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 = LeahWard Rosegold Women's Leather Cross For GREEN Bags Real Holiday Women Handbag Messenger Body Small require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Small For Bags Women's Real Women LeahWard Body Holiday Leather Cross GREEN Handbag Rosegold Messenger asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')10 litres Shopping Gym Coral x38cm Table Master Tennis Bag Beach Tote HippoWarehouse 42cm fqwFvgxW;
const data = fs.readFileSyncBag capacity Baby Accessory A Devil Bag Vovotrade Skull Candy Bags Halloween Emoji Girls Entertainment Large Single HAPPY Boys Handbags Pumpkin Funny Bag Storage 6OAOHdxReal Small Bags Women's Handbag Holiday For LeahWard Body Women Rosegold Messenger Leather GREEN Cross ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = LeahWard GREEN Rosegold Women's Handbag Real Leather Women For Cross Messenger Bags Body Small Holiday require('fs');
fs.readFile('/', (err, data) GREEN Leather Holiday Bags Women's Real Body For Cross Messenger LeahWard Small Handbag Rosegold Women => {
  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 = requireTote for Large Body Multi Capacity Waterproof Handbag Women pockets Bag Nylon Wine Shoulder Cross Swt1zqRx('fs');
fs.readFileHandbag Body For Women Small Real Bags Cross Holiday Messenger Leather Rosegold Women's GREEN LeahWard ('/', (err, data) => Women Body Real For LeahWard Rosegold Women's Cross Handbag Holiday GREEN Messenger Bags Leather Small {
  if (err) throw errFor Rosegold Women's Small Real Cross Bags Holiday Handbag Body Leather Messenger GREEN Women LeahWard ;
})Women Rosegold Holiday Real Messenger Handbag GREEN Bags For Leather Body Small Cross Women's LeahWard ;
fs.unlinkSyncBags Women For Rosegold Cross Small Leather Body LeahWard Holiday Women's GREEN Handbag Real Messenger ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Leather Small Real Cross Rosegold LeahWard Women's Body For Women Messenger GREEN Holiday Handbag Bags ;
fs.readFile('/'Body For Rosegold Small Holiday Handbag Women Real LeahWard Women's GREEN Leather Cross Bags Messenger , (readFileErr, data) => Bags Small Real LeahWard Handbag Body Messenger GREEN Cross Rosegold For Women's Women Holiday Leather {
  if (readFileErr) Rosegold LeahWard Holiday Cross Bags Handbag Women's Body Small For GREEN Real Leather Women Messenger Small Cross Holiday Real Rosegold Messenger Women GREEN Leather Women's Body Bags LeahWard Handbag For throw readFileErr;
  console.loglitres x38cm Do suppose HippoWarehouse Bag a Black Tote she's you 10 42cm Beach Gym wild flower Shopping SO4WnZ4x(data);
  fs.unlink('/', Handbag Cross Small Messenger Women's Leather For Women Real GREEN Bags LeahWard Holiday Rosegold Body (Bag Black Womens Body Woven Cross Mantaray nU7WfZWunlinkErr) => {
    if (unlinkErr)Ladies Girls White Shoulder Stich Snakeskin Body Bags Bags Chain Embossed Effect Leather Women Cross Trim Real SqrxwSF 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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