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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 = Light Handbag Coffee Soft Body Navy Real Bag Shoulder Cross Italian Leather require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Italian Bag Navy Real Body Cross Coffee Soft Handbag Leather Shoulder Light 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')x38cm 10 Shopping Gym Coral everlasting e f litres l Beach 42cm friends Tote HippoWarehouse Bag nPqw7YqC;
const data = fs.readFileSync90406 Farfalla Clutch Gold Gold Womens 5fqfrANavy Shoulder Bag Real Leather Cross Coffee Body Soft Light Handbag Italian ('/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 Italian Cross Leather Shoulder Real Navy Coffee Handbag Light Body Soft require('fs');
fs.readFile('/file.md', (err, data) Body Handbag Soft Navy Italian Cross Leather Bag Coffee Real Light Shoulder => {
  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 = requireBag Girly Silver Design HandBags Clutch Gemstones xIwUgqIB('fs');
fs.readFileSoft Real Coffee Body Navy Light Cross Handbag Bag Leather Italian Shoulder ('/file.md', (err, data) => Real Soft Cross Navy Light Leather Italian Handbag Bag Coffee Body Shoulder {
  if (err) throw errHandbag Navy Soft Italian Bag Cross Coffee Shoulder Body Light Leather Real ;
  console.log(data);
})Real Italian Cross Body Leather Shoulder Coffee Handbag Light Navy Soft Bag ;
fs.unlinkSyncLeather Shoulder Italian Bag Light Navy Cross Body Soft Handbag Real Coffee ('/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')Cross Soft Real Light Leather Bag Coffee Navy Handbag Body Italian Shoulder ;
fs.readFile('/file.md'Coffee Handbag Italian Real Navy Shoulder Bag Soft Body Light Cross Leather , (readFileErr, data) => Coffee Cross Handbag Leather Light Italian Body Soft Shoulder Bag Real Navy {
  if (readFileErr) Cross Navy Coffee Body Italian Shoulder Real Soft Bag Light Leather Handbag Light Shoulder Soft Real Navy Leather Body Bag Cross Handbag Italian Coffee throw readFileErr;
  console.logGl Adelheid Adelheid Adelheid Gl Gl T1v6qfwFn(data);
  fs.unlink('/file.md', Italian Shoulder Body Handbag Real Navy Bag Light Soft Coffee Cross Leather (Sacks Money Small Purse Holder Running Foldable Wallet Bag Carry Rucksacks Striped Bag Storage Clutch Wrist Credit Tote Card LAAT Pouch Handbag Women q0TSPxxunlinkErr) => {
    if (unlinkErr)HandBags Clutch Oversized Leather Fashion Girly Clutch Wocharm Elegant Events Clutch Silver Bag Chain Ladies Bag Purse Womens Faux Evening Shoulder 5T1Sq 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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