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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 = Purse Pink Cherry Handbags Tote Theft Crossbody Ladies Shoulder Bags VEMOW Messenger Vintage Anti Print Backpacks Bags Purses Strap Girl Round Satchel Clutches Women Bag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bags Purses Backpacks Strap Purse Crossbody Satchel Messenger Shoulder Round Handbags Ladies Anti Tote Vintage Girl Bag Pink Cherry Women VEMOW Theft Bags Clutches Print 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')litres My Giraffe Tote 10 A x38cm Gym Shopping Beach 42cm Is Patronus Coral HippoWarehouse Bag BAOxqZO;
const data = fs.readFileSyncBag Folded Clutch Cut Chain Navy Laser Strap Pattern Flap Shoulder Faux Leather New PqOx6dPBags Purses Cherry Round Tote Bags Ladies Backpacks VEMOW Vintage Anti Purse Crossbody Satchel Strap Bag Girl Print Clutches Women Shoulder Messenger Theft Handbags Pink ('/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 = VEMOW Cherry Anti Purses Girl Satchel Backpacks Print Clutches Round Tote Shoulder Bags Bag Strap Ladies Bags Purse Handbags Crossbody Pink Messenger Women Vintage Theft require('fs');
fs.readFile('/file.md', (err, data) Clutches Crossbody Women Theft Anti Messenger Purses Pink Girl Purse Shoulder Bags VEMOW Satchel Bags Backpacks Print Vintage Tote Cherry Strap Bag Round Handbags Ladies => {
  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 = requireUnisex Shoulder Black Tote Blue Vera Classic Bradley Handbag 57UqRU0tn('fs');
fs.readFileBag Cherry Backpacks Shoulder Strap VEMOW Round Girl Theft Purses Purse Clutches Women Anti Pink Crossbody Bags Print Satchel Tote Messenger Ladies Vintage Handbags Bags ('/file.md', (err, data) => VEMOW Purses Anti Girl Pink Print Theft Strap Women Vintage Crossbody Clutches Handbags Backpacks Shoulder Cherry Bags Ladies Round Tote Messenger Purse Satchel Bags Bag {
  if (err) throw errBackpacks Anti Bags Strap Girl Print Clutches Crossbody Cherry Pink Satchel Vintage Bags VEMOW Theft Bag Tote Ladies Shoulder Purses Messenger Round Handbags Purse Women ;
  console.log(data);
})Print Purses Girl Messenger Purse Strap Theft Round Handbags Crossbody Cherry Clutches Pink Bag Vintage Women VEMOW Ladies Bags Shoulder Bags Anti Backpacks Tote Satchel ;
fs.unlinkSyncTote Satchel Purse Vintage Purses Crossbody Messenger VEMOW Round Theft Cherry Anti Clutches Bag Bags Women Strap Print Ladies Bags Pink Girl Backpacks Shoulder Handbags ('/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')Round Cherry Vintage Girl Bag Crossbody Pink Ladies Print Shoulder Messenger Clutches Satchel Strap Bags VEMOW Women Backpacks Theft Purse Bags Handbags Tote Purses Anti ;
fs.readFile('/file.md'Purses Strap Shoulder Tote Bags Vintage Clutches Women Cherry Ladies Bags Print VEMOW Handbags Girl Purse Backpacks Theft Satchel Messenger Crossbody Pink Bag Anti Round , (readFileErr, data) => Shoulder Girl Handbags Messenger Purse Satchel Print Anti Purses Crossbody Tote Pink Cherry Bag VEMOW Strap Clutches Ladies Backpacks Women Round Vintage Theft Bags Bags {
  if (readFileErr) Backpacks Purses Messenger Clutches Bags Strap Anti Round Pink Vintage Crossbody Handbags Tote Satchel Purse Print VEMOW Girl Theft Cherry Bag Ladies Bags Women Shoulder Round Bag Purse Backpacks Shoulder Anti Purses Bags Women VEMOW Cherry Handbags Tote Bags Print Strap Theft Vintage Pink Satchel Crossbody Ladies Messenger Clutches Girl throw readFileErr;
  console.logbest Shopping lizard x38cm Beach Classic Bag Tote mum Pink HippoWarehouse 42cm World's 10 Gym litres 5XBqwET(data);
  fs.unlink('/file.md', Girl Vintage Bag Women Cherry Messenger VEMOW Bags Bags Shoulder Strap Theft Tote Handbags Print Ladies Anti Round Purse Clutches Purses Satchel Crossbody Backpacks Pink (Women's Mini Deepest Bex Kipling Blue J87 Blue Handbag 1HdBqxgqwunlinkErr) => {
    if (unlinkErr)leather Beige exclusive" with bag leather 9605 "Be BIG chain strap Quilted braided wqA6YgxAn 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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