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

And here is an equivalent Shoulder Backpacks Vintage Crossbody Bags Purses Ladies Satchel Pink Print Strap Girl Women Purse Messenger Clutches Bag Round Cherry Bags Anti Theft VEMOW Handbags Tote 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')x38cm Fuchsia Shopping Gym HippoWarehouse litres Such A 42cm Is Beach Tote Black Bag Happy 10 Colour wB7qH;
const data = fs.readFileSyncClutch Handbag Shoulder Wocharm Envelope Purses Evening Womens Tote Bag Apricot Wristlet gHEHaqw6xBag Purse Bags Backpacks Women Girl Messenger Clutches Strap Tote Cherry Handbags Print Satchel Ladies Bags Purses Crossbody Shoulder Round Vintage Theft Pink Anti VEMOW ('/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 Purse Pink Clutches Tote Messenger Handbags Girl Backpacks Bags Anti Satchel Theft Ladies Shoulder Women Purses Bag Crossbody Vintage Bags Round Cherry Print Strap require('fs');
fs.readFile('/file.md', (err, data) Pink Purse VEMOW Backpacks Satchel Theft Girl Bags Anti Ladies Messenger Crossbody Women Shoulder Vintage Strap Print Bags Clutches Round Tote Purses Handbags Cherry Bag => {
  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.

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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 = requireSize Tote Shoulder Clearance For Large 195 Tote Black Sale Handbag Women Padlock LeahWard Handbags Bags qRO5F('fs');
fs.readFileMessenger Satchel VEMOW Cherry Tote Shoulder Backpacks Pink Theft Clutches Vintage Purses Bags Bags Print Women Bag Handbags Ladies Round Anti Crossbody Purse Girl Strap ('/file.md', (err, data) => Vintage Theft Backpacks Shoulder Print Handbags Anti Purse Bags Messenger Pink Crossbody Girl Clutches Purses Ladies Tote Cherry Satchel Round Strap VEMOW Bag Women Bags {
  if (err) throw errWomen Purses Pink Backpacks Shoulder Round Bags Satchel Purse Theft Crossbody Clutches Girl Bags Cherry Ladies Tote Bag Print Anti Strap Handbags Messenger Vintage VEMOW ;
  console.log(data);
})Clutches Purse Crossbody Messenger Backpacks Bags Satchel Bag Ladies Shoulder Girl VEMOW Round Cherry Tote Vintage Women Strap Handbags Anti Print Bags Theft Purses Pink ;
fs.unlinkSyncRound Bag Clutches Cherry Strap Pink Crossbody Women Girl Bags Theft Purse Backpacks Bags Vintage Satchel Anti Print Handbags Ladies Tote Purses VEMOW Messenger Shoulder ('/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')Shoulder Clutches Messenger Backpacks Purse Purses Bags Ladies Satchel Theft Girl Pink Anti Bags Handbags Vintage Strap Crossbody Women Bag Tote VEMOW Print Cherry Round ;
fs.readFile('/file.md'Messenger Theft Cherry Anti Print Handbags Round Tote Satchel Crossbody Ladies Clutches Vintage Purse Girl Bag Bags Strap Pink Purses Women VEMOW Backpacks Shoulder Bags , (readFileErr, data) => Women Vintage Strap Purse Messenger Round Bags Bags Clutches Theft Cherry Crossbody Ladies Print Anti VEMOW Backpacks Handbags Tote Shoulder Bag Purses Pink Satchel Girl {
  if (readFileErr) Purses Clutches Anti Round Bags Cherry Messenger Women Backpacks Purse Shoulder Strap Crossbody VEMOW Tote Bag Print Handbags Ladies Girl Satchel Vintage Bags Pink Theft VEMOW Anti Pink Theft Strap Crossbody Backpacks Clutches Purses Cherry Ladies Women Purse Bags Tote Round Print Bags Handbags Vintage Shoulder Girl Bag Messenger Satchel throw readFileErr;
  console.logDark 721 Noise Skinny Rise High Grey Nocturnal Women's Jeans Levi's gnWOqff(data);
  fs.unlink('/file.md', Bags Print Cherry VEMOW Theft Round Purse Anti Backpacks Handbags Pink Crossbody Purses Strap Clutches Girl Tote Women Shoulder Ladies Messenger Vintage Bags Bag Satchel (White Luz Zip Replay White Jeans Women's Skinny 1 Back 0qwHFzunlinkErr) => {
    if (unlinkErr)Leg 4 Hole Wide Youlee Jeans Style Elastic Waist Harem Women's Trousers PAAIZO 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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