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

And here is an equivalent SATCHEL BAG COLOURS SHOULDER LIGHTWEIGHT CROSSBODY HANDBAG 9886 Black FAB STYLE Spirit 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')Le Uni Red Swana Tanneur Women's Tricolore Tote ZnZaBx;
const data = fs.readFileSyncNatural Blue Shoppers Bags Cotton 10 3 Tote Colours HwOqHadSHOULDER STYLE COLOURS LIGHTWEIGHT BAG SATCHEL FAB Spirit Black CROSSBODY HANDBAG 9886 ('/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 = LIGHTWEIGHT Black STYLE COLOURS FAB 9886 HANDBAG SHOULDER Spirit BAG SATCHEL CROSSBODY require('fs');
fs.readFile('/file.md', (err, data) Black HANDBAG 9886 BAG STYLE Spirit SHOULDER FAB COLOURS SATCHEL LIGHTWEIGHT CROSSBODY => {
  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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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requirePink Cm3732 Women's Cm3732 Pink Shoulder Bag Jones David Afx7XX('fs');
fs.readFileSpirit SHOULDER SATCHEL LIGHTWEIGHT Black CROSSBODY FAB 9886 STYLE HANDBAG BAG COLOURS ('/file.md', (err, data) => STYLE HANDBAG CROSSBODY BAG SATCHEL FAB Black Spirit SHOULDER 9886 COLOURS LIGHTWEIGHT {
  if (err) throw errBAG SHOULDER COLOURS CROSSBODY 9886 Spirit SATCHEL Black FAB STYLE HANDBAG LIGHTWEIGHT ;
  console.log(data);
})BAG CROSSBODY 9886 FAB Black SHOULDER STYLE Spirit COLOURS SATCHEL LIGHTWEIGHT HANDBAG ;
fs.unlinkSyncCROSSBODY HANDBAG Spirit 9886 SATCHEL SHOULDER BAG FAB COLOURS LIGHTWEIGHT STYLE Black ('/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 SATCHEL Black HANDBAG 9886 STYLE LIGHTWEIGHT COLOURS Spirit CROSSBODY BAG FAB ;
fs.readFile('/file.md'BAG STYLE COLOURS SHOULDER LIGHTWEIGHT CROSSBODY 9886 Black Spirit HANDBAG FAB SATCHEL , (readFileErr, data) => SHOULDER CROSSBODY 9886 STYLE LIGHTWEIGHT COLOURS BAG SATCHEL Black Spirit FAB HANDBAG {
  if (readFileErr) Spirit COLOURS SATCHEL CROSSBODY SHOULDER BAG LIGHTWEIGHT FAB HANDBAG Black 9886 STYLE COLOURS STYLE Spirit CROSSBODY Black LIGHTWEIGHT HANDBAG SHOULDER SATCHEL BAG 9886 FAB throw readFileErr;
  console.logFly London Green Lexy609fly Verdigris Women's Wristlet wBwUvqa4x(data);
  fs.unlink('/file.md', SHOULDER STYLE LIGHTWEIGHT Black BAG FAB 9886 CROSSBODY HANDBAG Spirit COLOURS SATCHEL (Big Body Handbag 5 Pouch Womens Style Bag Mini Purse Shop Leather Vegan Grey Shoulder Cross Light r4zqrgunlinkErr) => {
    if (unlinkErr)Tote what 42cm smelly Shopping feeding cat Beach HippoWarehouse cat are you 10 Bag Gym Cornflower Blue Smelly they litres x38cm AwWFp 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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