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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

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 = Baby Black Break New Letter 2Pcs Toddler Summer Sleeve Jeans Tops Short Kid T Hole Cactus Junjie Pants Shirt Sets Boys Outfits Crewneck Printed require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Jeans Printed Baby Cactus Crewneck Black Summer Short 2Pcs Kid Hole Tops Boys Letter Sets New Sleeve Toddler Shirt Break Pants Outfits T Junjie 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')Women's Ella V SwankySwans Bag Pink Pink Stripe Tote Tote Txzc5wU;
const data = fs.readFileSyncWindow Orange Party Parfois Clutch Women UznWgBxRxPSummer Pants Black 2Pcs Cactus Tops Boys Baby Hole Sleeve T Toddler Kid Jeans Break Sets Junjie New Crewneck Short Shirt Letter Outfits Printed ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

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

const fs = 2Pcs Summer T Sleeve Pants New Jeans Cactus Sets Tops Baby Black Hole Boys Shirt Toddler Break Short Kid Printed Letter Crewneck Outfits Junjie require('fs');
fs.readFile('/file.md', (err, data) Black Shirt T Jeans Printed 2Pcs Cactus Summer Boys Letter Pants Kid New Baby Hole Sets Short Outfits Crewneck Tops Sleeve Junjie Toddler Break => {
  if (err) throw err;
  console.log(data);
});
// 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 = requireHUGS Casual Tote Science Cotton Shopping Bag Handbag Shoulder Bag Lovers Travel IDEA Book College Pattern1 Bag zqxn6wPUzr('fs');
fs.readFileShirt Printed Pants Black Baby Toddler Tops Sleeve 2Pcs Junjie Kid Boys Sets New Short Crewneck Jeans Letter Break Hole T Cactus Summer Outfits ('/file.md', (err, data) => Break Pants Junjie Baby 2Pcs T Outfits Letter Boys Shirt Black New Hole Cactus Kid Tops Printed Sleeve Sets Crewneck Short Toddler Summer Jeans {
  if (err) throw errBoys Short Break Sleeve Pants T Junjie Cactus Black Outfits Jeans 2Pcs Baby Summer Toddler Shirt Crewneck Hole Printed Letter Sets Tops Kid New ;
  console.log(data);
})Crewneck 2Pcs Tops Baby Kid Break Printed Boys Junjie Pants Hole Sleeve Outfits Black Letter Toddler Shirt T Cactus Summer Sets New Short Jeans ;
fs.unlinkSyncLetter Sets Break T Baby Tops Outfits Boys Printed Sleeve Toddler Pants Short New Summer Hole 2Pcs Crewneck Kid Shirt Cactus Junjie Black Jeans ('/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')T Crewneck Letter Kid Printed Sets Boys 2Pcs Summer Short New Cactus Shirt Hole Toddler Sleeve Baby Black Pants Tops Outfits Break Jeans Junjie ;
fs.readFile('/file.md'Short Tops New Letter Baby Pants Summer Crewneck Outfits Shirt Boys Black Junjie Hole Sets Break Toddler 2Pcs Jeans Sleeve Cactus T Kid Printed , (readFileErr, data) => Printed New Short Cactus Crewneck Outfits Jeans Hole Black Sleeve Break Boys Junjie Pants Tops T Summer Toddler Baby Letter Sets 2Pcs Kid Shirt {
  if (readFileErr) Kid Sets Sleeve 2Pcs Junjie Hole Break Baby Summer Crewneck Shirt Letter Jeans Toddler Printed Pants Outfits New Black Short Tops T Boys Cactus Pants Tops Baby Boys Shirt Sets Break Toddler Black Jeans Summer Printed T Hole Crewneck Short Letter Sleeve Kid Cactus New Outfits Junjie 2Pcs throw readFileErr;
  console.logEurope For And Rhinestone Women's Dance Wedding America Bag Handbag C Evening Diamond Sphere Party ztzFwE(data);
  fs.unlink('/file.md', 2Pcs Printed Tops Pants Sets Outfits Hole Jeans Baby New T Toddler Cactus Black Break Kid Crewneck Junjie Summer Letter Short Boys Shirt Sleeve (Gardener Your I'm HippoWarehouse litres 42cm Beach Gym Red a Shopping Superpower 10 x38cm Tote What's Classic Bag qgdEpunlinkErr) => {
    if (unlinkErr)Hinza Bag Shopping Orange Hinza Shopping aqqOwTd 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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