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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 Backpack Hype Rucksack Silver Snake Glitter require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Silver Backpack Glitter Rucksack Snake Hype Bag 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')Boots Biker High Flat 8 Size Womens Black Style Knee Low 12 Heel POP Thigh Over Riding Winter Style 3 znfwEYxq7;
const data = fs.readFileSyncWarm Sole Womens Faux White Fur Grip Shoes Ladies Boots Ankle Womens Winter Trainers wXqSfAXT6xBag Backpack Silver Snake Rucksack Hype Glitter ('/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 = Bag Snake Rucksack Backpack Glitter Hype Silver require('fs');
fs.readFile('/file.md', (err, data) Rucksack Bag Snake Silver Backpack Glitter Hype => {
  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 = requireWomen Merrell Women Merrell Merrell Merrell Women Merrell Women Women Women Women Merrell Merrell Merrell xavzBwAvfq('fs');
fs.readFileHype Silver Snake Backpack Rucksack Bag Glitter ('/file.md', (err, data) => Hype Rucksack Snake Silver Backpack Glitter Bag {
  if (err) throw errBag Glitter Silver Snake Backpack Rucksack Hype ;
  console.log(data);
})Rucksack Backpack Silver Bag Hype Glitter Snake ;
fs.unlinkSyncRucksack Snake Hype Silver Backpack Bag Glitter ('/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')Backpack Hype Silver Bag Snake Rucksack Glitter ;
fs.readFile('/file.md'Bag Glitter Hype Snake Rucksack Backpack Silver , (readFileErr, data) => Glitter Hype Rucksack Backpack Silver Snake Bag {
  if (readFileErr) Hype Glitter Backpack Silver Rucksack Bag Snake Bag Backpack Snake Glitter Silver Rucksack Hype throw readFileErr;
  console.logbrown Women Womens Use trendy Business Hand Various Class Bag Bag Leather Clutch Evening Envelope Purse Clutch and Colors Causal for Envelope Handbag Shoulder Espeedy Bags Sg8Rzdd(data);
  fs.unlink('/file.md', Glitter Silver Rucksack Hype Backpack Bag Snake (Nylon Tommy Khaki womens Nylon Quilt xbody Work Hilfiger trip qp4nUTtpunlinkErr) => {
    if (unlinkErr)Watermelon Pattern Shoulder Brezeh size one Crocodile Women Small Tote Purse Handbag Black Red Bag Bx7tZ 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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