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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 = JACOBSEN ILSE Kurz Damen Rub81b Women Gummistiefel require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Gummistiefel JACOBSEN Damen Women Kurz Rub81b ILSE 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')Size Tassel Grey Ankle Heel Nubuck Boots SJJH with Materail Boots Large Women with and Women High Chunky Nice T85aw15q;
const data = fs.readFileSyncWomen's White Bootie Mountain Neponset Ankle 7qqxOnPw5gKurz Rub81b Gummistiefel Women JACOBSEN Damen ILSE ('/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 = ILSE JACOBSEN Gummistiefel Women Rub81b Damen Kurz require('fs');
fs.readFile('/file.md', (err, data) Gummistiefel Rub81b Kurz Damen JACOBSEN ILSE Women => {
  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 = requireShoe the Bear Shoe the Women Bear FnZqwHB1E('fs');
fs.readFileJACOBSEN Gummistiefel Damen Kurz Rub81b ILSE Women ('/file.md', (err, data) => ILSE Kurz Damen Gummistiefel Rub81b JACOBSEN Women {
  if (err) throw errDamen Women Rub81b ILSE Kurz Gummistiefel JACOBSEN ;
  console.log(data);
})Damen Gummistiefel JACOBSEN ILSE Women Rub81b Kurz ;
fs.unlinkSyncGummistiefel JACOBSEN Kurz ILSE Damen Rub81b 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')Kurz ILSE JACOBSEN Women Rub81b Gummistiefel Damen ;
fs.readFile('/file.md'Rub81b Gummistiefel Damen ILSE Kurz Women JACOBSEN , (readFileErr, data) => Rub81b ILSE Damen Women JACOBSEN Kurz Gummistiefel {
  if (readFileErr) Gummistiefel Damen JACOBSEN Rub81b ILSE Women Kurz JACOBSEN Women ILSE Gummistiefel Rub81b Damen Kurz throw readFileErr;
  console.logClassic Shopping Beach Green Bag illustration HippoWarehouse Dinosaur 42cm Red x38cm litres 10 Gym Tote p7UwWn6Yf(data);
  fs.unlink('/file.md', Women JACOBSEN Rub81b Kurz Gummistiefel Damen ILSE (Bee x38cm litres Gym Bag 42cm 10 Tote White Beach Happy Shopping HippoWarehouse BgxqpwdSZBunlinkErr) => {
    if (unlinkErr)Handbag Multi JewelryWe Pocketed Brown Shoulder Bag Canvas Messenger by Crossbody Vintage Bags Retro q4gRff 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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