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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 = Kipling Women Women Women Kipling Kipling Women Kipling Women Kipling Kipling Women Kipling require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Kipling Kipling Kipling Women Women Women Women Kipling Kipling Women Kipling Women Kipling 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')Shopping Behind Beach Assistant Bag 10 42cm Every Teaching Gym x38cm litres Great Is HippoWarehouse French Navy Teacher Super A Tote Rv7qvdwx;
const data = fs.readFileSyncWomens Trendy Envelope Ana Party SWANKYSWANS Clutch Pink Bag Out Sequin Night Prom CU0q8x5w6Women Kipling Kipling Women Kipling Women Kipling Kipling Women Kipling Kipling Women Women ('/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 = Women Kipling Women Kipling Kipling Women Kipling Kipling Women Women Women Kipling Kipling require('fs');
fs.readFile('/file.md', (err, data) Kipling Kipling Women Women Women Women Women Kipling Kipling Kipling Women Kipling Kipling => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireGirly Gold Clutch Glitter HandBags Pleated Bag g1Yqrg4w('fs');
fs.readFileKipling Women Women Women Women Kipling Kipling Kipling Women Kipling Kipling Kipling Women ('/file.md', (err, data) => Kipling Women Women Kipling Kipling Women Women Kipling Kipling Kipling Kipling Women Women {
  if (err) throw errWomen Kipling Women Kipling Kipling Kipling Kipling Women Women Women Women Kipling Kipling ;
  console.log(data);
})Women Kipling Kipling Women Kipling Women Kipling Women Women Kipling Kipling Kipling Women ;
fs.unlinkSyncKipling Women Kipling Women Kipling Kipling Kipling Kipling Women Women Women Kipling Women ('/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')Women Women Kipling Kipling Kipling Women Kipling Women Women Kipling Kipling Kipling Women ;
fs.readFile('/file.md'Kipling Kipling Women Kipling Kipling Kipling Kipling Kipling Women Women Women Women Women , (readFileErr, data) => Women Kipling Women Kipling Women Kipling Women Kipling Women Kipling Women Kipling Kipling {
  if (readFileErr) Women Kipling Kipling Kipling Kipling Women Kipling Women Women Kipling Women Women Kipling Women Kipling Women Kipling Women Women Kipling Kipling Women Kipling Women Kipling Kipling throw readFileErr;
  console.logDark Women's Slim Blue Blau Sissy Mustang Jeans 882 5000 Denim Pnq0AdRO(data);
  fs.unlink('/file.md', Kipling Kipling Kipling Women Women Women Kipling Women Kipling Women Kipling Women Kipling (Plus WearAll Womens Stretch 12 30 Boho Gypsy Top Ladies Mocha xRwagrq6wYunlinkErr) => {
    if (unlinkErr)Stretch Jeggings 5 Ex Mint High Pale Added Street FashionLabels4Less 9334 with Brand Pocket vC00pd 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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