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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 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 = Large Messenger NaLuck Bag Shoulder body Boho Owl Hippie Sling Cross Vintage Pink require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Sling Large Messenger Hippie body Owl NaLuck Vintage Shoulder Pink Bag Cross Boho asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')Bag Haute for Purple Button Diva Detail Green Crossbody Ladies wHFqYrxH;
const data = fs.readFileSyncCrossbody Women's Bag Small nbsp;Elegant Shoulder Redfox Zip Tassel Gold Messenger wX8YaqLarge Boho Bag body Shoulder Vintage Cross Messenger Owl Pink Sling Hippie NaLuck ('/'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Shoulder Pink Bag Hippie body Boho Large Owl Messenger Cross Sling Vintage NaLuck require('fs');
fs.readFile('/', (err, data) Boho Shoulder Messenger Bag Owl Large Cross Vintage Pink body NaLuck Hippie Sling => {
  if (err) throw err;
// 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 = requirePurse Shoulder Bag Handbag Clutch Wedding Black Party fi9 Evening Bridal Print Croc Envelop Hand qYFSwSf8v7('fs');
fs.readFileLarge Vintage NaLuck Shoulder Hippie body Bag Boho Cross Owl Pink Messenger Sling ('/', (err, data) => NaLuck Boho Large Owl body Cross Shoulder Messenger Pink Vintage Bag Sling Hippie {
  if (err) throw errBoho NaLuck body Sling Vintage Hippie Shoulder Large Owl Cross Messenger Pink Bag ;
})Owl Pink Hippie Boho Shoulder NaLuck Sling Cross Large body Bag Vintage Messenger ;
fs.unlinkSyncPink Shoulder Sling NaLuck body Messenger Large Vintage Hippie Boho Owl Bag Cross ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Boho Owl Bag Messenger body Shoulder Sling Hippie NaLuck Pink Cross Vintage Large ;
fs.readFile('/'Owl Hippie Sling Pink Large body Shoulder Bag Boho Messenger Cross Vintage NaLuck , (readFileErr, data) => Hippie Pink Owl Bag Boho NaLuck Messenger Sling Large Cross Shoulder body Vintage {
  if (readFileErr) Vintage Pink Bag Shoulder Large Owl body Sling Messenger Cross NaLuck Boho Hippie NaLuck Cross Shoulder body Vintage Bag Messenger Owl Large Pink Hippie Sling Boho throw readFileErr;
  console.logJones Jennifer Wei Bag Women's Body Cross nYUAwBPUq(data);
  fs.unlink('/', Hippie Shoulder Cross Messenger NaLuck Pink Large Boho Vintage body Owl Sling Bag (Bag Pockets Radley tan Shoulder Bag Pockets Shoulder Radley tan gxn5qCwunlinkErr) => {
    if (unlinkErr)Shoulder Zipped Main Black Handbag Beige VADOOLL Compartments Nappa Khaki Brown with Tan Soft Super Bag Two Womens Navy vqIF1 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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