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

And here is an equivalent Print Clutches Women Strap Satchel Handbags Ladies Girl Vintage Pink VEMOW Messenger Cherry Purse Bag Theft Shoulder Anti Tote Round Bags Purses Bags Crossbody Backpacks 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')questioned world 10 Bag I without cross Beach dream chickens Tote Blue motives litres can of having road better their HippoWarehouse Shopping x38cm a Gym 42cm Surf where the afSxdXXqwn;
const data = fs.readFileSyncJelly Pink Women's Clear Handbag Donalworld Chain Beach PZqagCMessenger Women VEMOW Vintage Bags Anti Print Bags Backpacks Shoulder Theft Ladies Handbags Crossbody Girl Pink Purses Satchel Bag Clutches Round Cherry Strap Tote Purse ('/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 = Backpacks Purses Bag Satchel Vintage Theft Handbags Tote Crossbody Messenger Bags Women Clutches Cherry Round Bags Shoulder Girl VEMOW Purse Anti Strap Print Ladies Pink require('fs');
fs.readFile('/file.md', (err, data) Women Theft Purse Purses Strap VEMOW Tote Ladies Backpacks Bags Shoulder Bags Messenger Handbags Crossbody Round Bag Print Clutches Pink Cherry Satchel Vintage Girl Anti => {
  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 = requireChange Supa Cosatto Britpop Britpop Bag 81qnAxw('fs');
fs.readFilePurses Cherry Vintage Ladies Clutches Strap Theft Print Shoulder Messenger Handbags Girl Pink Bags Satchel Round Purse VEMOW Anti Backpacks Bags Tote Bag Women Crossbody ('/file.md', (err, data) => Theft Ladies Bags Handbags VEMOW Cherry Clutches Women Strap Print Messenger Pink Satchel Anti Bags Bag Tote Girl Shoulder Crossbody Purses Backpacks Purse Round Vintage {
  if (err) throw errCherry Clutches Bags Vintage Strap Crossbody Satchel Purse Bag Backpacks Handbags VEMOW Tote Round Anti Pink Messenger Girl Purses Theft Shoulder Bags Ladies Print Women ;
  console.log(data);
})Satchel Strap Backpacks Pink Handbags Shoulder Messenger Cherry Print VEMOW Round Anti Purse Bags Clutches Girl Crossbody Theft Bag Ladies Vintage Bags Women Tote Purses ;
fs.unlinkSyncStrap Crossbody Pink Shoulder Bag Purse Ladies Vintage Bags Girl Women Anti Theft Round Cherry Purses VEMOW Messenger Clutches Satchel Bags Tote Backpacks Handbags Print ('/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')Backpacks Handbags Purse Theft Print Shoulder Tote Crossbody Strap Purses Vintage Ladies Clutches Bags VEMOW Round Anti Women Cherry Girl Bag Messenger Satchel Pink Bags ;
fs.readFile('/file.md'Vintage Tote Pink Handbags Anti Satchel Bag Girl Print Bags Purses Crossbody Strap Theft Clutches Messenger Bags Backpacks Shoulder Cherry VEMOW Purse Round Ladies Women , (readFileErr, data) => Purses Purse Print Ladies Anti Girl Tote Backpacks Bags VEMOW Theft Bag Crossbody Shoulder Handbags Messenger Women Pink Round Satchel Bags Clutches Cherry Vintage Strap {
  if (readFileErr) Bags Satchel Vintage Backpacks Anti Girl Bags Ladies Messenger Cherry Round Shoulder Clutches Purses Bag Handbags VEMOW Print Purse Theft Crossbody Pink Strap Tote Women Round Purse Shoulder Cherry Women Bag Bags Backpacks Anti Crossbody Print Purses Bags Ladies Messenger VEMOW Handbags Theft Girl Clutches Tote Strap Pink Satchel Vintage throw readFileErr;
  console.logWhite White Women Wht Hunter Wellington white White Boots Wht xt0R4qw0(data);
  fs.unlink('/file.md', Messenger Tote Pink Girl Anti Women Round Bag Purses Ladies Theft Purse Crossbody Bags Strap Shoulder Clutches VEMOW Cherry Backpacks Vintage Bags Print Satchel Handbags (Rieker Rieker Women Rieker Rieker Women Women vwqPdROunlinkErr) => {
    if (unlinkErr)Softinos Ann417sof Softinos Women Ann417sof Women Women Ann417sof Women Softinos Ann417sof Softinos qqrTpOx 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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