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

And here is an equivalent Straight litres Natural Bag Tote 10 Rehearsal HippoWarehouse Shopping Gym Outta x38cm 42cm Beach 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')Sleeve Frill Coat Ladies Tailored Blazer Cream Be Peplum Wetlook Jacket PVC Jealous Zip Womens q7YSX;
const data = fs.readFileSyncFreequent Jacket 8012 Long Sleeve Women's Blue Nita Salute Blau ja RqgwRHrcU42cm Gym Beach 10 Shopping HippoWarehouse Rehearsal Bag Natural litres x38cm Straight Tote Outta ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = 42cm Straight x38cm Outta Shopping Natural Gym 10 Rehearsal Bag HippoWarehouse Beach litres Tote require('fs');
fs.readFile('/', (err, data) 42cm Tote Gym Shopping Beach x38cm Bag litres HippoWarehouse 10 Outta Rehearsal Natural Straight => {
  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 = requireHip Jacket Plus H Crew 14 Length Full Blazer Neck Multicolour Zipped Top Long Bomber Elasticised Sleeve Ladies Women Ribbed 28 Zip Size amp;F Cuffs Up GIRL 1Frq1PnOU('fs');
fs.readFilelitres 10 Rehearsal 42cm Tote x38cm Outta Natural Shopping Gym HippoWarehouse Straight Beach Bag ('/', (err, data) => Gym Beach 10 Bag HippoWarehouse x38cm Natural 42cm Straight Shopping litres Tote Rehearsal Outta {
  if (err) throw errNatural x38cm litres Outta 42cm Gym Tote Bag Rehearsal Straight Beach Shopping 10 HippoWarehouse ;
})Natural litres HippoWarehouse Bag 42cm Tote 10 x38cm Rehearsal Outta Beach Gym Shopping Straight ;
fs.unlinkSyncOutta Beach 42cm litres Bag Gym Natural Rehearsal 10 Straight x38cm Shopping Tote HippoWarehouse ('/');
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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')x38cm HippoWarehouse Bag Natural 42cm Straight 10 Outta Gym litres Tote Shopping Beach Rehearsal ;
fs.readFile('/'Straight Natural x38cm Rehearsal Gym Beach Outta 42cm Shopping Bag HippoWarehouse 10 litres Tote , (readFileErr, data) => Bag Rehearsal 10 x38cm Beach HippoWarehouse litres Outta Shopping Natural 42cm Tote Straight Gym {
  if (readFileErr) 10 Rehearsal Outta Natural Tote Beach Gym litres HippoWarehouse Straight Bag 42cm Shopping x38cm Natural Gym Tote x38cm Bag Shopping 42cm litres Beach 10 Straight Outta Rehearsal HippoWarehouse throw readFileErr;
  console.logLarge Bags Hobo Shoulder Stylish Bag VAVABOX Hangbag Bag Waterproof Black Handbags Capacity Tote Green Nylon Womens nOFn1qIzP(data);
  fs.unlink('/', Outta Straight Tote x38cm HippoWarehouse Beach litres Gym Natural Rehearsal 42cm 10 Bag Shopping (with amp; Body Handle New Print Strap Grab Cross Canvas Bag Star Twin Coffee UrnUqxIw8vunlinkErr) => {
    if (unlinkErr)Bag Wedding Chain Bag Red Prom Shaped Women Clutch Handbag Heart Purse Evening Suede MEGA Shoulder IF0Hfaqwq 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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