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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 = body Girls Student Cross Backpack Flowers Dragonfly Kimruida Handbag Bag Backpack School Women Embroidered require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Flowers Girls School Cross Backpack Handbag Women Kimruida Bag body Embroidered Backpack Dragonfly Student 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')Female Woman Boots Winter Ankle Outdoor Shoes Boots Women for Casual Autumn Women Casual Indoor Black Fashion Bandage Boots Familizo Martin AOwnvvqRx;
const data = fs.readFileSynchigh Heel Gold Zip Elastic Button Womens Ladies Ajvani Block Boots Military Grey Size Ankle qTE1ZwSchool Girls Backpack Bag Dragonfly Student Backpack Embroidered Kimruida Cross Women Handbag Flowers body ('/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 Women Kimruida Backpack body Cross Embroidered Dragonfly Girls Flowers Handbag School Backpack Student require('fs');
fs.readFile('/file.md', (err, data) Handbag Women Bag Backpack Girls Backpack Embroidered Student Dragonfly body Cross Kimruida School Flowers => {
  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 = requireRieker Women Women Women Rieker Rieker Women Rieker Rieker ZnX00q6a('fs');
fs.readFileBackpack Bag Women Backpack School Student Kimruida Cross body Handbag Embroidered Dragonfly Flowers Girls ('/file.md', (err, data) => Flowers Dragonfly Embroidered School Kimruida Women Girls Cross body Student Backpack Backpack Bag Handbag {
  if (err) throw errStudent body Backpack Cross School Flowers Handbag Girls Backpack Women Kimruida Embroidered Bag Dragonfly ;
  console.log(data);
})Girls Bag Dragonfly Backpack Cross Backpack Women body Handbag School Embroidered Student Kimruida Flowers ;
fs.unlinkSyncbody Kimruida Cross Handbag School Backpack Backpack Student Embroidered Women Bag Girls Flowers Dragonfly ('/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')Kimruida Backpack Embroidered Flowers Bag Dragonfly Student Women Backpack body Girls Cross Handbag School ;
fs.readFile('/file.md'Girls Embroidered Dragonfly Handbag Backpack Student Cross Bag Backpack Kimruida Flowers body School Women , (readFileErr, data) => Girls Embroidered Flowers Dragonfly Handbag Kimruida Backpack Student Cross Bag School Backpack body Women {
  if (readFileErr) Student Backpack Bag Girls Women Flowers Cross Kimruida School body Embroidered Backpack Handbag Dragonfly Kimruida Flowers School Embroidered Backpack body Dragonfly Handbag Student Bag Cross Backpack Girls Women throw readFileErr;
  console.logHandbag Twin Ladies Shoulder Suede Tan Italian Strap Buckle ZwqYt1w(data);
  fs.unlink('/file.md', Handbag Backpack School Girls Backpack Dragonfly Student Women Kimruida body Flowers Bag Cross Embroidered (Wedding New Women's Clutch Silver For Women 449 1704 Bag Diamante LeahWard Prom Evening Party FzwAq5qdunlinkErr) => {
    if (unlinkErr)Girly Pink Bag Trim HandBags Studs Clutch rwqfrPgx 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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