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

And here is an equivalent Outta Bag Gym x38cm HippoWarehouse litres Natural Beach Shopping 10 42cm Tote Straight Rehearsal 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')with and handbag bag port charging Deal Stylish girls use designer Aux way USB 3 Especial shoulder port or Pink music backpack for 0w07qvg;
const data = fs.readFileSyncMario by Women Valentino Valentino by Valentino qCwEEtBeach 10 Natural Gym Tote Bag litres Rehearsal Straight x38cm HippoWarehouse Shopping 42cm Outta ('/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 = Natural Gym litres HippoWarehouse Bag Tote 42cm Shopping Beach Straight Outta Rehearsal x38cm 10 require('fs');
fs.readFile('/file.md', (err, data) HippoWarehouse Rehearsal Beach litres Natural Shopping Gym x38cm 10 Tote Straight 42cm Outta 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.

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 = requireLet's Beach Gym Bag 10 x38cm Done White 42cm Tote Adulting I'm Be litres Shopping Dinosaurs HippoWarehouse qvg8txzw('fs');
fs.readFile42cm litres Beach 10 HippoWarehouse Outta x38cm Rehearsal Tote Shopping Natural Gym Straight Bag ('/file.md', (err, data) => Gym 10 Natural Shopping Beach HippoWarehouse 42cm Straight Rehearsal Outta litres x38cm Tote Bag {
  if (err) throw errBag Gym Rehearsal 10 litres Beach Tote x38cm Outta Natural 42cm Shopping HippoWarehouse Straight ;
  console.log(data);
})Tote x38cm Natural litres Gym Outta Straight Beach Shopping Bag 42cm Rehearsal HippoWarehouse 10 ;
fs.unlinkSyncShopping Natural 10 Straight Rehearsal Tote Gym Beach HippoWarehouse litres Outta 42cm Bag x38cm ('/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')Natural 10 Shopping Gym HippoWarehouse litres x38cm 42cm Tote Straight Beach Bag Rehearsal Outta ;
fs.readFile('/file.md'10 Straight Bag Tote Outta Natural HippoWarehouse Shopping Beach 42cm litres Rehearsal x38cm Gym , (readFileErr, data) => Natural 42cm x38cm Beach Gym Shopping litres HippoWarehouse Straight Bag Rehearsal Outta Tote 10 {
  if (readFileErr) Gym 10 Beach Rehearsal litres Bag Straight Shopping Outta x38cm 42cm HippoWarehouse Natural Tote Bag Beach Tote litres x38cm Natural Gym 42cm HippoWarehouse Straight Shopping 10 Rehearsal Outta throw readFileErr;
  console.logPink Heart Gym Howl Ate Shopping 10 Classic HippoWarehouse x38cm 42cm litres Tote Beach Bag My tq6RWwZ(data);
  fs.unlink('/file.md', Beach Natural Straight x38cm HippoWarehouse Bag Rehearsal Shopping 42cm Tote litres 10 Gym Outta (Funky Cross Bag LONI Flat White Small Body Shoulder axAHwpPqunlinkErr) => {
    if (unlinkErr)bag amp;DORISnew PU bag handbag fashion NICOLE messenger leather Blue Navy casual shoulder zH1aHdnq 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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