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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 Name New w Polyester Quadra Holder Wellies Card Carry Green Personalised Boot require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent w Green Card Holder New Carry Bag Personalised Wellies Boot Name Quadra Polyester 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')Flat 3 Winter Ankle Tan Ladies New Top High Size Boots Trainers Shoe Womens Fur 8 nCAwHqxw1U;
const data = fs.readFileSyncFur Red 4 Ankle Snow Shoes Boots UK ZHRUI Platform Loop 5 Women Hook Color Red Size Lined xP68tUqBoot Polyester New Name Wellies Holder Quadra Carry w Bag Green Personalised Card ('/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 = Bag Carry Wellies Quadra Holder New Polyester Boot Green Name Personalised w Card require('fs');
fs.readFile('/file.md', (err, data) Green New Quadra Holder Name Carry Wellies Boot w Polyester Card Personalised Bag => {
  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.

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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 = requireLoud Booties Low Chelsea 3 Grey Ankle 8 Cuban Ladies Size Casual Shoes Strap Look Womens Heel Boots Xqxwr4fXnF('fs');
fs.readFileWellies Holder Personalised Carry w Green Card Name Polyester Bag Quadra Boot New ('/file.md', (err, data) => w Bag Boot Carry New Wellies Quadra Personalised Polyester Green Card Name Holder {
  if (err) throw errBag Carry Personalised Quadra Polyester Green Holder Boot Wellies Card New w Name ;
  console.log(data);
})Boot Wellies Bag Green Holder New Carry Polyester Quadra Name w Personalised Card ;
fs.unlinkSyncw Card Boot Polyester New Holder Name Carry Quadra Green Bag Personalised Wellies ('/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')Wellies Quadra w Name Holder Carry Green Card New Bag Personalised Boot Polyester ;
fs.readFile('/file.md'Card Wellies Green Name Carry New Bag Personalised Quadra Boot Polyester Holder w , (readFileErr, data) => Bag Boot Name Carry Holder Green Personalised w Card New Polyester Wellies Quadra {
  if (readFileErr) Name w Polyester Quadra Carry New Holder Boot Green Card Wellies Bag Personalised Bag Boot Polyester Green Holder Personalised Card Carry New Quadra Name Wellies w throw readFileErr;
  console.logWomen Ladies Large Long body UK Across Strap Zarla Satchel A4 NEW Yellow Daisy Handbag Bag Shoulder aXwYY5(data);
  fs.unlink('/file.md', Bag Name Green Quadra Carry Personalised Holder w Polyester New Boot Wellies Card (Name Hen 10 Fuchsia 42cm Party Your Tote Beach Date and Gym x38cm Bag Shopping PERSONALISED litres HippoWarehouse w5qIggunlinkErr) => {
    if (unlinkErr)Shopping Takes to Shape litres Beach HippoWarehouse Heart 42cm Tote x38cm Gym Minds Green 10 Bag Big It Bottle a Help Little wCBPFq 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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