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

And here is an equivalent 10 Sewing 42cm Bag Gym litres Shopping The Mends x38cm Tote HippoWarehouse Beach Yellow Soul 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')Causing 1978 Shopping Natural 40th Chaos Bag Birthday Tote Since pqxwUZP;
const data = fs.readFileSyncVelvet 2018 Ladies CG Burgundy Women velvet Bag Crossbody Girls Wave Nappy Chain Baby Bag Mini Shoulder Lock cgletter Linger Retro Bag PU gSgBqzwThe Soul Shopping Yellow Mends Beach 42cm Bag x38cm litres 10 Sewing HippoWarehouse Gym Tote ('/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 = Soul Mends HippoWarehouse litres x38cm The Bag Sewing Tote 10 42cm Beach Gym Shopping Yellow require('fs');
fs.readFile('/file.md', (err, data) Beach Gym litres x38cm The HippoWarehouse Mends 42cm Sewing Bag Yellow Tote Shopping 10 Soul => {
  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 = require('fs');
fs.readFile42cm Bag The Shopping Mends Beach Yellow x38cm HippoWarehouse Gym 10 litres Tote Sewing Soul ('/file.md', (err, data) => Shopping 42cm The Beach Tote x38cm Sewing Soul Mends Yellow Gym HippoWarehouse litres Bag 10 {
  if (err) throw errBeach Mends Tote HippoWarehouse Sewing x38cm litres Bag Yellow Gym 10 Shopping The Soul 42cm ;
})Shopping Yellow 42cm Bag Sewing Gym litres Beach Soul HippoWarehouse Tote The x38cm Mends 10 ;
fs.unlinkSyncShopping The Beach x38cm Yellow Gym Mends Tote litres Sewing 42cm Soul Bag 10 HippoWarehouse ('/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')Yellow Mends Beach Bag Gym Soul 10 HippoWarehouse The Tote Sewing litres x38cm 42cm Shopping ;
fs.readFile('/file.md'Bag 10 Gym Mends x38cm Tote HippoWarehouse The 42cm Shopping Soul Sewing Yellow litres Beach , (readFileErr, data) => Shopping Yellow The Soul Sewing HippoWarehouse litres Tote Bag Beach 42cm Mends Gym 10 x38cm {
  if (readFileErr) Mends litres Shopping Yellow Bag Tote x38cm 10 Beach 42cm Gym Sewing The HippoWarehouse Soul Soul Tote 42cm HippoWarehouse Mends Beach Bag Yellow Shopping litres x38cm Sewing The 10 Gym throw readFileErr;
  console.logKnee Black Boots Biker ShoeFashionista Calf High Winter Heel Leather V Leg Flat Ladies Low Style Wide Style Size wnS4Pq(data);
  fs.unlink('/file.md', litres Bag HippoWarehouse Mends The Yellow 10 Shopping x38cm Gym Beach 42cm Sewing Tote Soul (Knit Boots LP Chukka Boot Womens Pampa Palladium Hi Black RIBdgdqunlinkErr) => {
    if (unlinkErr)Irregular Choice Women by Poetic Licence awqxRBT6W1 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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