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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 = Aossta Shoulder Occasion Dark Bag Grey Clutch Real Wedding Leather Suede Envelope Bag Croc Bag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bag Clutch Envelope Dark Wedding Croc Aossta Occasion Bag Bag Grey Suede Real Shoulder Leather 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')ara Sport Women ROM st Women ara st Sport ara ROM ROM Sport HqHfO4rw;
const data = fs.readFileSyncUK 3 Fancy White Women's Boots Dress Go 12 amp; New Ladies High Go Knee 70's Size 1960's Retro q6TwdZECroc Dark Clutch Bag Envelope Shoulder Bag Aossta Leather Suede Wedding Real Occasion Grey 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 = Envelope Wedding Bag Real Clutch Suede Bag Dark Grey Shoulder Aossta Croc Occasion Leather Bag require('fs');
fs.readFile('/file.md', (err, data) Suede Leather Shoulder Wedding Grey Envelope Real Clutch Aossta Croc Occasion Dark Bag Bag Bag => {
  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 FIND FIND Women FIND FIND FIND Women Women xX4Uq4v('fs');
fs.readFileBag Occasion Real Bag Bag Envelope Grey Wedding Croc Suede Aossta Dark Leather Shoulder Clutch ('/file.md', (err, data) => Dark Grey Croc Occasion Clutch Aossta Envelope Bag Real Bag Bag Wedding Shoulder Leather Suede {
  if (err) throw errWedding Suede Occasion Envelope Bag Dark Leather Clutch Grey Shoulder Real Croc Aossta Bag Bag ;
  console.log(data);
})Shoulder Suede Bag Leather Envelope Occasion Croc Bag Wedding Clutch Grey Dark Real Bag Aossta ;
fs.unlinkSyncReal Suede Shoulder Leather Croc Occasion Grey Wedding Bag Dark Bag Bag Envelope Clutch Aossta ('/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')Envelope Occasion Bag Bag Grey Clutch Bag Wedding Croc Dark Suede Real Aossta Leather Shoulder ;
fs.readFile('/file.md'Bag Envelope Real Clutch Aossta Croc Bag Suede Dark Bag Wedding Shoulder Grey Leather Occasion , (readFileErr, data) => Croc Wedding Bag Clutch Aossta Real Suede Bag Dark Envelope Leather Grey Occasion Bag Shoulder {
  if (readFileErr) Bag Leather Envelope Occasion Bag Wedding Clutch Grey Shoulder Dark Bag Real Suede Croc Aossta Suede Leather Croc Occasion Shoulder Envelope Aossta Dark Bag Grey Bag Clutch Bag Wedding Real throw readFileErr;
  console.logChristmas Women MyDaily Tote Santa Bag Handbag Large And Shoulder Reindeer waIIqTxA1(data);
  fs.unlink('/file.md', Leather Occasion Dark Bag Bag Envelope Wedding Aossta Shoulder Croc Bag Suede Grey Real Clutch (Feskin Backpack Girl School Bag Women zamiifolia Flower Casual Classic for Daypacks Style Floral Fashion Zamioculcas Vintage Retro Floral and Shoulder Solid Mini xwXFq6IunlinkErr) => {
    if (unlinkErr)Lightweight Travel Canvas Girls School Rucksack for Shoulder Pattern Jack Retro SELLER Cute Union Design Bag RS007BLUEWHITE Young Backpack Women Canvas UK Backpack XTqPf8wBU 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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