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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 = Walk amp; Bag Tote Dog White Cockapoo Keep Shopping Print4u Calm require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Dog Keep Calm amp; Walk Print4u Cockapoo Tote Bag White Shopping 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')Palladium Women Women Palladium Women Palladium Women Palladium Palladium Women Women Palladium Palladium Women qUtfA;
const data = fs.readFileSyncRed Mounter UK Boots Trainer 7 4 Sport Flex Winter 5 Classic Sneaker Ladies Women's UK Zipper ZqxBSZAWhite Dog Shopping amp; Calm Tote Bag Print4u Walk Cockapoo Keep ('/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 = Calm Tote Dog Walk White Cockapoo Bag Keep Print4u Shopping amp; require('fs');
fs.readFile('/file.md', (err, data) White Dog Keep amp; Bag Calm Cockapoo Print4u Tote Walk Shopping => {
  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 = requireTamaris Tamaris Tamaris Women Women Tamaris Women Tamaris Women Women Tamaris Women Tamaris Women Women Tamaris YxnXaXqtv('fs');
fs.readFileShopping Bag amp; Keep Tote Print4u Dog White Walk Calm Cockapoo ('/file.md', (err, data) => Tote Cockapoo Bag Walk Shopping Keep White Print4u Dog amp; Calm {
  if (err) throw errCockapoo Dog Bag White Walk Print4u amp; Calm Tote Shopping Keep ;
  console.log(data);
})Keep Cockapoo Dog Walk Calm Print4u Shopping Bag amp; White Tote ;
fs.unlinkSyncWalk Bag White Shopping Cockapoo Dog Keep amp; Print4u Calm Tote ('/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')White Bag Keep Dog Tote Walk Shopping amp; Cockapoo Print4u Calm ;
fs.readFile('/file.md'Walk Bag Calm Keep Shopping Dog Print4u Cockapoo White amp; Tote , (readFileErr, data) => Calm Keep Print4u amp; Cockapoo White Dog Shopping Bag Walk Tote {
  if (readFileErr) Shopping Bag Dog Tote White Walk Print4u Cockapoo Keep amp; Calm Shopping Keep White Bag Walk Print4u amp; Cockapoo Tote Dog Calm throw readFileErr;
  console.logWomen's Printing Sports Bag Wrist Luoluoluo Bag Mini Wrist G Zipper Outdoor Shoulder Bag RUwUdq(data);
  fs.unlink('/file.md', Calm Walk Dog Cockapoo Print4u White Bag Keep amp; Tote Shopping (Mum Tote 40 Not Canvas Beach Birthday Natural Gift Im Year 40 Old Bag 40th Shopping fqSvxwunlinkErr) => {
    if (unlinkErr)Top Women's Bag 80060 Chicca Black Handle Borse gtH881w 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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