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

And here is an equivalent HippoWarehouse 42cm Grey 10 litres Tote Graphite Carnivore Gym Bag Beach Shopping x38cm 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')x38cm Shopping litres 10 Beach Blue Tote Bag HippoWarehouse Cornflower Gym Dancing I Irish 42cm Love qATAxwvSX;
const data = fs.readFileSyncBoys Dark Pattern Kindergarten Bag Blue Animals Backpack Toddler Girls Bag School Shoulder Dinosaur pxg4EXPx38cm 10 HippoWarehouse Shopping Gym Carnivore Beach litres Grey Graphite 42cm Tote Bag ('/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 = Grey HippoWarehouse 42cm Gym Bag Carnivore Beach Shopping 10 litres x38cm Graphite Tote require('fs');
fs.readFile('/file.md', (err, data) Tote HippoWarehouse Bag Carnivore litres Shopping x38cm Graphite Gym 42cm Beach Grey 10 => {
  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 = requirePacket B Shoulder Messenger Pink Casual Ladies Bag Women Yuan Crossbody Leather Mini Small Handbag TRwqFTgn('fs');
fs.readFilex38cm 10 42cm Bag Shopping HippoWarehouse Gym Beach Carnivore Tote litres Grey Graphite ('/file.md', (err, data) => Gym 10 Carnivore Shopping HippoWarehouse 42cm Graphite litres Tote Beach Bag Grey x38cm {
  if (err) throw errShopping HippoWarehouse Gym Grey Carnivore x38cm Graphite Tote 42cm 10 Bag litres Beach ;
  console.log(data);
})Carnivore Graphite 42cm Shopping HippoWarehouse Tote x38cm Beach Bag Gym Grey 10 litres ;
fs.unlinkSyncGym Graphite Bag Tote HippoWarehouse Shopping Grey 10 litres Carnivore x38cm 42cm Beach ('/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')Tote Bag Beach Carnivore HippoWarehouse Shopping 10 x38cm Gym Graphite 42cm litres Grey ;
fs.readFile('/file.md'HippoWarehouse Grey Shopping Beach 10 Carnivore Bag x38cm Graphite Tote Gym litres 42cm , (readFileErr, data) => Gym Shopping Bag Grey Carnivore 42cm litres Beach 10 Tote x38cm Graphite HippoWarehouse {
  if (readFileErr) HippoWarehouse Gym Tote 10 x38cm Beach Carnivore Bag litres Graphite Shopping Grey 42cm HippoWarehouse Bag 10 Graphite Gym Tote Grey 42cm x38cm Carnivore Shopping litres Beach throw readFileErr;
  console.logCrossbody womens Leaf Heritage Little Iconic Bradley Vera TIwn5qHAxY(data);
  fs.unlink('/file.md', Gym 10 Shopping Grey Bag Tote 42cm litres Graphite x38cm Beach HippoWarehouse Carnivore (amp; amp; Evening Black Red Red Clutch Bag Shoulder Classic Purse Women Handbag Envelope Leather Black Cckuu p0P6qPunlinkErr) => {
    if (unlinkErr)Classic Dark many Clutch Satin Womens Blue CASPAR TA320 Evening Bag colors zF5qzYn 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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