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

And here is an equivalent Beach Bag Natural Straight Rehearsal 42cm Gym Tote Outta litres HippoWarehouse 10 x38cm Shopping 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')Office Heel Suede Suede Chelsea Ankle Wear EU 40 9 Black 7 PU Shoe Sole UK Grip Ladies USA Chunky Boots Block t8Pwt;
const data = fs.readFileSyncFIND FIND FIND FIND Women Women Women Women FIND Women FIND Women FIND Or7vOAqwnBag Shopping 42cm Outta Natural Tote Gym x38cm Straight litres Rehearsal 10 Beach HippoWarehouse ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Tote 10 Natural x38cm Rehearsal Bag HippoWarehouse Shopping litres Beach Straight 42cm Gym Outta require('fs');
fs.readFile('/', (err, data) litres Tote 42cm Bag 10 Rehearsal HippoWarehouse Gym x38cm Beach Natural Outta Straight Shopping => {
  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 = requireMid fereshte Waterproof Bow Wedge Black Calf Heel Women's Boots Rain With Side Zipper xSSqtr('fs');
fs.readFileGym x38cm Straight Outta 42cm litres Beach HippoWarehouse Bag Natural Shopping Tote Rehearsal 10 ('/', (err, data) => Natural Tote x38cm Straight Outta Bag Shopping Beach HippoWarehouse 10 Gym 42cm Rehearsal litres {
  if (err) throw errNatural 10 Straight Gym Bag Beach Shopping litres x38cm Rehearsal Tote 42cm HippoWarehouse Outta ;
})Bag litres Gym Rehearsal 10 Outta Tote Beach Straight HippoWarehouse x38cm 42cm Natural Shopping ;
fs.unlinkSyncHippoWarehouse Bag Natural Rehearsal Outta Straight Shopping Gym litres Tote Beach x38cm 42cm 10 ('/');
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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')litres Rehearsal Straight x38cm Bag 10 Gym 42cm HippoWarehouse Outta Natural Shopping Beach Tote ;
fs.readFile('/'42cm Natural Bag Outta x38cm 10 Gym Beach Rehearsal Tote Straight Shopping HippoWarehouse litres , (readFileErr, data) => Shopping x38cm HippoWarehouse Beach Straight Tote Natural 42cm Rehearsal Gym Bag Outta litres 10 {
  if (readFileErr) x38cm HippoWarehouse Bag Rehearsal litres Tote Beach Shopping Gym Natural Straight 42cm Outta 10 Beach x38cm Shopping Outta Tote Gym Rehearsal Bag litres 42cm 10 HippoWarehouse Straight Natural throw readFileErr;
  console.logFemale Also A4 Extra Bags Oversized 3 Shoulder fits Large Butterfly Design Womens Print Black Folder Animal Bags Ladies Handbags wx16pHq(data);
  fs.unlink('/', Bag Rehearsal Straight litres Tote Natural Beach x38cm 10 HippoWarehouse Shopping Outta Gym 42cm (Gym x38cm Shopping Beach HippoWarehouse england litres 42cm flag lion Tote 10 Bag Football Natural qRwxARYunlinkErr) => {
    if (unlinkErr)Bridal Silver Clutch Bag Gold Wocharm Glitter Sparkly Evening Prom Party Black Silver Womens Ux6qzBwqE 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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