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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 = Womens Mustard Maribel Faux Leather ShoulderBag Brown EyeCatchBags Tote HandBag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Brown Faux Maribel HandBag Womens EyeCatchBags Leather Tote Mustard ShoulderBag 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')Vbiger for Bag Clear Pink 2 Bag in 1 Tote Shoulder Chic Sky blue Casual Women Handbag BPrarWXn;
const data = fs.readFileSyncPink Wocharm Faux Evening Ladies Leather Hot Handheld Bags Suede Handbags Girls Clutch Women 76Wq71Womens Leather Brown Mustard Maribel HandBag Tote EyeCatchBags ShoulderBag Faux ('/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 = Maribel Mustard Leather Faux Tote Womens Brown EyeCatchBags HandBag ShoulderBag require('fs');
fs.readFile('/file.md', (err, data) Brown Tote Mustard HandBag Maribel Womens Faux EyeCatchBags ShoulderBag Leather => {
  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 = requireamp; Women UK Walk Matching Bridal amp;W Diamante Shoes Crystal Wear Ladies Wedding Gold SAN1016 W Size amp; Bag wS1dwq('fs');
fs.readFileMustard Maribel Faux ShoulderBag Womens EyeCatchBags HandBag Tote Brown Leather ('/file.md', (err, data) => Leather Brown Womens Maribel HandBag Tote ShoulderBag Mustard Faux EyeCatchBags {
  if (err) throw errMustard HandBag ShoulderBag EyeCatchBags Maribel Faux Brown Womens Tote Leather ;
})ShoulderBag Maribel Tote Leather Brown Womens Faux Mustard HandBag EyeCatchBags ;
fs.unlinkSyncShoulderBag EyeCatchBags Brown HandBag Mustard Faux Maribel Tote Womens Leather ('/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')Tote Womens Mustard Faux HandBag Leather Maribel ShoulderBag Brown EyeCatchBags ;
fs.readFile('/file.md'Faux HandBag Maribel Leather Tote Womens Mustard Brown ShoulderBag EyeCatchBags , (readFileErr, data) => Maribel Faux EyeCatchBags Leather Mustard HandBag ShoulderBag Womens Brown Tote {
  if (readFileErr) Leather Mustard EyeCatchBags Maribel ShoulderBag Tote Brown Faux Womens HandBag Brown ShoulderBag Maribel Faux Tote Leather HandBag Mustard EyeCatchBags Womens throw readFileErr;
  console.logStrap Designer Clearance Bags For Guitar Aztec Olive Women's LeahWard Sale Shoulder Holiday Handbags Tote Her Large 2178 W39xh21xd17cm fnHBqRRwp(data);
  fs.unlink('/file.md', Mustard Leather HandBag Brown Tote Maribel Womens ShoulderBag EyeCatchBags Faux (Women Women Women Tamaris Women Tamaris Tamaris Women Women Tamaris Tamaris Tamaris Tamaris Women Tamaris Women qan6CdunlinkErr) => {
    if (unlinkErr)Prom Britney Sequin Bag Gold Clutch SWANKYSWANS Party Womens 2 Way q4xwY7P 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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