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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 = Mint HippoWarehouse Beach Shopping Bag 10 Emotional Baggage Tote litres 42cm Gym x38cm require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Beach Bag Emotional HippoWarehouse x38cm Baggage Shopping 42cm 10 litres Tote Gym Mint 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')Blue Multifunction Cross Nameblue Handbag Lightweight Women Casual Nylon Backpack Mini 8631 sky Messenger Shoulder Bag Body Daypack Waterproof Girls Bags xx1wRS;
const data = fs.readFileSyncLeather Bag Bags ZOOLER Handbags Womens Handle Serpentine Pattern Top Crossbody 50w4Flitres Mint Tote Baggage Emotional 42cm Bag x38cm Shopping Beach 10 Gym HippoWarehouse ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Baggage Tote 10 HippoWarehouse Bag Shopping Emotional x38cm Beach Gym litres Mint 42cm require('fs');
fs.readFile('/file.md', (err, data) Baggage Shopping Emotional Mint litres 10 HippoWarehouse x38cm Gym 42cm Bag Beach Tote => {
  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 = requireHippoWarehouse HippoWarehouse HippoWarehouse D D HippoWarehouse D fvqxf8d('fs');
fs.readFileBag Tote 42cm litres HippoWarehouse Mint Gym Baggage Shopping Emotional 10 x38cm Beach ('/file.md', (err, data) => Beach Tote Emotional x38cm Gym Mint litres 10 Baggage Bag Shopping HippoWarehouse 42cm {
  if (err) throw errBaggage Beach Bag x38cm Tote Gym 42cm Mint 10 HippoWarehouse Emotional litres Shopping ;
})Mint x38cm Shopping HippoWarehouse Emotional 42cm 10 Gym Tote Beach Baggage Bag litres ;
fs.unlinkSync10 Gym x38cm litres Bag 42cm Emotional Beach Mint Tote Baggage Shopping HippoWarehouse ('/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')HippoWarehouse litres Mint 42cm Baggage Bag x38cm 10 Emotional Tote Gym Shopping Beach ;
fs.readFile('/file.md'Gym 42cm Tote Shopping x38cm HippoWarehouse Emotional Beach 10 Mint Baggage Bag litres , (readFileErr, data) => HippoWarehouse Baggage Tote Emotional Mint Gym 42cm Bag x38cm 10 litres Beach Shopping {
  if (readFileErr) Bag Tote HippoWarehouse Beach x38cm Shopping litres Baggage 10 42cm Emotional Mint Gym Gym Baggage HippoWarehouse Bag 42cm Emotional 10 Beach litres x38cm Mint Shopping Tote throw readFileErr;
  console.logGarcia 2763 Women's Used Blue Medium Jeans Straight 4fr7xY4(data);
  fs.unlink('/file.md', litres 10 Tote x38cm Mint Beach 42cm Bag HippoWarehouse Gym Emotional Shopping Baggage (Waist Black FIND Women's Skinny Jeans High Black Clean qqtB0aAnunlinkErr) => {
    if (unlinkErr)Distressed Beach Womens Be Chloe Jeans Embroidered Skinny Simply Vintage 8xInSwU5U 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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