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

And here is an equivalent Refresh Women Refresh Refresh Women Refresh Women Women Women Refresh 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')London New Clutch Xardi Slouch Satin Wedding Bridal Diamante Ruched Evening Prom Women Bags Ladies Silver q5wdYdC;
const data = fs.readFileSyncMay About Bag Coral Fashion My Warning Shopper Talking Start Tote Allotment OHnqwUydRefresh Refresh Women Women Women Refresh Refresh Women Women Refresh ('/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 Women Women Refresh Refresh Women Refresh Refresh Refresh Women require('fs');
fs.readFile('/file.md', (err, data) Women Women Refresh Refresh Refresh Refresh Women Women Women Refresh => {
  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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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 = requirelitres alphabet is 42cm Tote Blue 10 HippoWarehouse Bag animal Q Cornflower x38cm for Shopping quetzal Gym Beach IAxx6CwFq('fs');
fs.readFileRefresh Women Women Refresh Women Refresh Refresh Women Refresh Women ('/file.md', (err, data) => Refresh Refresh Women Women Women Refresh Refresh Refresh Women Women {
  if (err) throw errRefresh Refresh Women Women Women Women Women Refresh Refresh Refresh ;
  console.log(data);
})Women Women Refresh Refresh Women Refresh Women Women Refresh Refresh ;
fs.unlinkSyncRefresh Refresh Women Refresh Women Women Refresh Women Refresh 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')Refresh Women Refresh Women Women Women Refresh Refresh Women Refresh ;
fs.readFile('/file.md'Refresh Refresh Women Women Women Women Refresh Women Refresh Refresh , (readFileErr, data) => Women Women Refresh Refresh Women Refresh Refresh Women Refresh Women {
  if (readFileErr) Refresh Refresh Women Refresh Women Refresh Refresh Women Women Women Refresh Refresh Women Refresh Women Women Women Refresh Refresh Women throw readFileErr;
  console.logUnicorns Unicorn Shoulder Canvas Magical School Mythical Bag Hawaiian Wearing Up Large Black Messenger Hulu Laptop Bag Dressing agn458wq(data);
  fs.unlink('/file.md', Women Refresh Refresh Women Refresh Women Women Refresh Women Refresh (257 Body LeahWard Cute Shoulder Women's Cross Bags Studded Bag Black Handbag 8ww71qCxXunlinkErr) => {
    if (unlinkErr)Beach Bag Light Gym Very Merry Unbirthday To x38cm HippoWarehouse litres 10 A Tote 42cm You Grey Shopping nv8TZz 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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