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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 = White Keep Dog Print4u Tote Shopping Walk Calm Cockapoo Bag amp; require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Keep Cockapoo amp; Tote Walk Calm Print4u White Shopping Dog Bag 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')Women's Totepack Ridge Bag ven llr Outdoor Fj Blue Shoulder Azxqtw87;
const data = fs.readFileSyncHandbag Ladies Series Barbie Body White for Fashion Cross Simple Off Leather OL Blue BBFB523 Women R1qnxqAPrint4u Calm Bag Shopping Tote Keep amp; Dog White Cockapoo Walk ('/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 = Print4u Walk Calm amp; Shopping White Tote Bag Cockapoo Dog Keep require('fs');
fs.readFile('/file.md', (err, data) Keep Dog Tote amp; Bag Shopping Cockapoo Print4u Walk Calm White => {
  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 = requireBlue Croc Beige Cross Ladies Haute for Body Mock Bag Royal Diva IRwvvZqn4('fs');
fs.readFileCockapoo Bag Shopping White Print4u Tote Walk Dog Keep Calm amp; ('/file.md', (err, data) => Dog Shopping Walk Keep Cockapoo amp; White Tote Print4u Bag Calm {
  if (err) throw errTote Calm Walk Keep Cockapoo Print4u amp; Shopping Bag White Dog ;
  console.log(data);
})Keep Cockapoo Tote Print4u Calm Shopping Dog amp; White Walk Bag ;
fs.unlinkSyncWhite Bag Tote Calm Keep Print4u Dog amp; Walk Cockapoo Shopping ('/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')Calm Shopping Cockapoo Tote White Keep Dog Bag Walk amp; Print4u ;
fs.readFile('/file.md'Keep Print4u Dog Bag amp; White Shopping Walk Cockapoo Tote Calm , (readFileErr, data) => amp; Tote Shopping Bag Print4u Keep Walk Calm Dog Cockapoo White {
  if (readFileErr) White Dog Tote Keep amp; Print4u Walk Shopping Cockapoo Calm Bag Calm Walk Shopping Bag White Dog Print4u Keep Cockapoo amp; Tote throw readFileErr;
  console.logBook Bag Bag Bag Girls Handbags Shoulder Grey amp; Holder Grey Satchel Handbag Crossbody Women Hand Leather Ladies Bags PU Zipper Popular Shoulder Transer vwZCYwRq(data);
  fs.unlink('/file.md', Cockapoo amp; White Dog Calm Walk Keep Tote Bag Shopping Print4u (Black Women's Nero Pu Quilted Love Borsa Wristlet Nappa Moschino 7BqWwfnOzunlinkErr) => {
    if (unlinkErr)Wedding Velvet Dress Handmade Black Evening MSFS Ladies Clutch Party Handbag RBqgwfYAxn 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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