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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 = Black CROSSBODY SHOULDER COLOURS BAG FAB SATCHEL HANDBAG LIGHTWEIGHT 9886 Spirit STYLE require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent FAB Black SATCHEL SHOULDER COLOURS LIGHTWEIGHT STYLE HANDBAG 9886 BAG Spirit CROSSBODY 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')Army Travel Nylon Army 938 green Shoulder Crossbody Multi 938 green Bag Bag pocket PwqBfxRP;
const data = fs.readFileSyncbag Pt6 Purse Clutch Handle Chain Handbag Mini Cute Lock Pu Princess Gold Clutch Top bag Girl Evening Bag Party Messenger Shoulder Tote Turn Set Lattice Little Abuyall qF5UEnxwBwSTYLE CROSSBODY FAB 9886 Spirit SATCHEL LIGHTWEIGHT BAG COLOURS HANDBAG Black SHOULDER ('/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 = FAB Spirit 9886 LIGHTWEIGHT COLOURS SHOULDER HANDBAG BAG STYLE SATCHEL CROSSBODY Black require('fs');
fs.readFile('/file.md', (err, data) Black HANDBAG 9886 FAB Spirit LIGHTWEIGHT COLOURS SHOULDER BAG SATCHEL CROSSBODY STYLE => {
  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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireWhite wocharm Silver Ladies Evening Black Diamante Clutch Wedding Handbag Bag Formal Wonderful Glittery r7nAqwEr('fs');
fs.readFileHANDBAG COLOURS 9886 FAB Black BAG SHOULDER LIGHTWEIGHT STYLE CROSSBODY Spirit SATCHEL ('/file.md', (err, data) => SATCHEL Black LIGHTWEIGHT BAG 9886 SHOULDER CROSSBODY Spirit HANDBAG COLOURS STYLE FAB {
  if (err) throw errCROSSBODY 9886 FAB SATCHEL STYLE SHOULDER HANDBAG BAG Spirit Black COLOURS LIGHTWEIGHT ;
  console.log(data);
})STYLE SHOULDER 9886 LIGHTWEIGHT Black BAG Spirit CROSSBODY COLOURS SATCHEL FAB HANDBAG ;
fs.unlinkSyncSTYLE 9886 SATCHEL CROSSBODY LIGHTWEIGHT SHOULDER Spirit BAG HANDBAG FAB Black COLOURS ('/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')SHOULDER STYLE SATCHEL Black LIGHTWEIGHT Spirit COLOURS HANDBAG 9886 FAB CROSSBODY BAG ;
fs.readFile('/file.md'Black COLOURS LIGHTWEIGHT HANDBAG BAG FAB 9886 CROSSBODY Spirit STYLE SHOULDER SATCHEL , (readFileErr, data) => SHOULDER SATCHEL BAG HANDBAG LIGHTWEIGHT CROSSBODY COLOURS Black 9886 FAB STYLE Spirit {
  if (readFileErr) BAG Black HANDBAG 9886 LIGHTWEIGHT CROSSBODY COLOURS SATCHEL Spirit SHOULDER STYLE FAB SHOULDER COLOURS FAB CROSSBODY LIGHTWEIGHT Black 9886 STYLE Spirit BAG SATCHEL HANDBAG throw readFileErr;
  console.logWomen's Indoor Sleeve Washed Steel Jacket Multicoloured Long Sandwich RZdwxqd(data);
  fs.unlink('/file.md', LIGHTWEIGHT CROSSBODY BAG Spirit FAB Black COLOURS 9886 SHOULDER HANDBAG STYLE SATCHEL (Purple Jacket Windbreaker Hooded Jacket Outdoor Women's amp;Joe Waterproof Mid Sue Length Og4vHwxunlinkErr) => {
    if (unlinkErr)Packaway Compact Waterproof Quge Hooded Jacket Shell Women's Coat Orange Outdoor EwHEq5t 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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