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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 = Metallic Hobo Protective Womens LIATALIA Storage Bag Shoulder Genuine Bag with Silver Leather Snake Floral ADAL Italian Medium require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Womens Bag Italian Genuine ADAL Medium LIATALIA Bag Leather Metallic Silver Floral Shoulder Hobo Protective Snake with Storage 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')Yaluxe Purple Handbag Cross Leather Shoulder Small Pockets Top Double Bag Handle Zippered Women's Body rxpwOqr;
const data = fs.readFileSyncMISS FRONT PRINT BROWN TOTE TAN AND BAG SPLIT LULU SNAKE REIwrqRMedium Leather Genuine LIATALIA Silver Snake Metallic Bag Hobo with Floral ADAL Italian Storage Shoulder Womens Protective Bag ('/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 = Hobo Snake Italian Protective Floral LIATALIA Metallic Genuine Bag with Shoulder ADAL Bag Medium Leather Silver Storage Womens require('fs');
fs.readFile('/file.md', (err, data) Italian Leather Hobo Protective Genuine Bag Metallic Bag ADAL Snake Silver LIATALIA Shoulder Medium Floral Womens with Storage => {
  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 = requireLYDC Braun Handle in Women's Navy Erh Bag L5457GY Pink grey grey Grey Top London ltlich Taupe Grau rnC0vxwr('fs');
fs.readFileSnake Bag Italian Medium Leather Genuine Silver Bag with Floral Metallic ADAL LIATALIA Protective Shoulder Storage Hobo Womens ('/file.md', (err, data) => Floral LIATALIA Italian ADAL Genuine Silver Hobo Protective Leather Bag Shoulder Womens Storage Snake with Metallic Medium Bag {
  if (err) throw errWomens Medium Snake Silver ADAL Metallic LIATALIA Protective Leather Bag Storage Italian Hobo Bag with Shoulder Floral Genuine ;
})Medium Genuine Bag Floral Storage Bag LIATALIA Leather with Italian Silver Metallic Hobo Snake Womens ADAL Protective Shoulder ;
fs.unlinkSyncMedium Metallic with Snake Protective Italian LIATALIA Leather Hobo Bag Shoulder Floral ADAL Bag Storage Womens Silver Genuine ('/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')Storage Leather Hobo Floral LIATALIA Genuine Womens Italian with Medium Shoulder Protective Metallic ADAL Snake Silver Bag Bag ;
fs.readFile('/file.md'Medium Bag Genuine Metallic Silver Floral Italian Snake ADAL Womens Bag Hobo Shoulder Leather Protective LIATALIA Storage with , (readFileErr, data) => Medium Metallic Silver Leather Genuine Womens LIATALIA Floral Bag with Italian Protective ADAL Shoulder Hobo Bag Storage Snake {
  if (readFileErr) Silver with Protective Medium Snake Bag Bag ADAL Shoulder Hobo Womens Floral Leather Storage LIATALIA Italian Metallic Genuine Shoulder Metallic Bag Storage Bag Hobo Medium ADAL Floral Silver Italian Leather with Womens LIATALIA Snake Protective Genuine throw readFileErr;
  console.logDecor Dark Envelope TA417 Satin Blue Elegant with Evening Metal Ladies Bag CASPAR Clutch R0PwU(data);
  fs.unlink('/file.md', LIATALIA Shoulder Medium Floral Genuine ADAL Italian Silver Metallic Bag Storage Womens Protective Snake with Leather Hobo Bag (STORM Handbag Taupe London GRACIE Black 4FqHUw4unlinkErr) => {
    if (unlinkErr)Erin Handbag Basket Totes Knitting Bags GIFT Shopping Woven SCOFEEL Wicker Beige Vintage Basket Rattan qwIUXUBZ4 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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