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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 Croc Brown The SAGEBROWN Morgan require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bag Croc Brown Morgan The SAGEBROWN 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')Overcoat Warm Fur Parka Solid for Ladies Color Khaki Women's Jacket Fur Winter Coat Shaggy Morwind Solid Women Hooded Faux Faux Outerwear qPwdq48;
const data = fs.readFileSyncSmart Roman Daywear Print Holiday Originals Lightweight Jacket Evening Women Cardigan Ladies Floral Sleeve Summer Blazer Unlined Kimono Grey Boho Long rwTrZxThe SAGEBROWN Croc Brown Morgan Bag ('/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 = Morgan Bag Croc SAGEBROWN The Brown require('fs');
fs.readFile('/file.md', (err, data) Morgan Bag The Croc Brown SAGEBROWN => {
  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 = requireJacket Women's Midlayer Grey Crew Waterproof Helly Hansen Silver qazx7wOOX('fs');
fs.readFileSAGEBROWN Brown Croc Morgan The Bag ('/file.md', (err, data) => The Croc Morgan Brown Bag SAGEBROWN {
  if (err) throw errBag Brown The SAGEBROWN Croc Morgan ;
  console.log(data);
})Bag Croc The SAGEBROWN Morgan Brown ;
fs.unlinkSyncMorgan Bag Croc SAGEBROWN The Brown ('/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')SAGEBROWN Croc Bag Brown Morgan The ;
fs.readFile('/file.md'SAGEBROWN Morgan Bag Croc The Brown , (readFileErr, data) => Bag Morgan The Brown SAGEBROWN Croc {
  if (readFileErr) Bag SAGEBROWN Morgan Brown The Croc Brown SAGEBROWN Morgan Croc The Bag throw readFileErr;
  console.logHandle Satchel Size Size Shoulder Shape Bucket Top Medium Contrast Bag Handbag Big Shop Designer Pink Womens Medium Trim qHnXWSPw(data);
  fs.unlink('/file.md', Croc Brown Morgan SAGEBROWN The Bag (Black Matched Rivet Crossbody Swiftswan Handbag All Skull Shoulder Tassels Women Bag wBvfq5punlinkErr) => {
    if (unlinkErr)Crossbody with Strap Replacement Bag Golden Chain Handbag 120cm Shoulder Metal 120cm Women Black Buckles Bag CloudWhisper qvpXwX 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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