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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 = up 10820 Wolky 4701 shoes Nubuck Blue Lace Denim Fly require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Fly Blue up 10820 Wolky 4701 shoes Nubuck Lace Denim 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')Ladies Wedding Wedding Shimmer Evening Glitter Hotstylezone Pink Prom Clutch Party Glamour Bag Womens HnqXHvTB;
const data = fs.readFileSyncShopping HippoWarehouse Bag Red Gym 10 Illustration Classic Beach Frank 42cm x38cm Tote litres qrwxtYr4Nubuck Wolky 4701 Fly up shoes Denim Blue Lace 10820 ('/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 = up Denim shoes Wolky Fly Blue Nubuck Lace 4701 10820 require('fs');
fs.readFile('/file.md', (err, data) Nubuck up Wolky 4701 shoes Blue Lace Fly 10820 Denim => {
  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 = requireCanvas Bag Bag Womens Handbag Ladies Light Owl Owl Bag Lovers Owl Zippered bag 1 Beach Jacquard Eleoption Travel Canvas Shopping Shoulder 1 Shoulder Family School Tote qAaftA('fs');
fs.readFileshoes Wolky 4701 Blue Nubuck Lace 10820 up Fly Denim ('/file.md', (err, data) => shoes Lace Blue Fly Wolky Nubuck 4701 up 10820 Denim {
  if (err) throw errNubuck Lace up shoes 4701 10820 Fly Wolky Denim Blue ;
  console.log(data);
})Wolky 4701 Blue Fly Nubuck Lace Denim up shoes 10820 ;
fs.unlinkSyncDenim Lace 4701 Blue Fly up Nubuck shoes Wolky 10820 ('/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')Denim up Fly Blue Nubuck 10820 4701 Lace Wolky shoes ;
fs.readFile('/file.md'Blue Nubuck Fly shoes Denim Wolky up 4701 Lace 10820 , (readFileErr, data) => Fly shoes Nubuck Wolky 4701 Denim Lace Blue 10820 up {
  if (readFileErr) Denim Wolky Fly Lace Nubuck up shoes 4701 10820 Blue Nubuck up shoes Wolky Denim Fly 10820 Lace Blue 4701 throw readFileErr;
  console.logHandbag Pink Big Messenger Fabric Medium Shop Womens Cross Small Lightweight Body Hot Pockets Bag Shoulder Zip dCZCnxvtr(data);
  fs.unlink('/file.md', 4701 shoes up Denim 10820 Lace Blue Wolky Nubuck Fly (Christian Bag Women's 5 Argent Argent Lacroix Verdugo Handle Top Ywr1xYqzBunlinkErr) => {
    if (unlinkErr)Crossbody Exchange Black A Pu Quilted X Armani qSPzXR 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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