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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 = brain Red 42cm I Bag litres 10 Classic x38cm survived HippoWarehouse Beach Gym Tote Shopping surgery require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Beach 10 surgery HippoWarehouse survived Gym I Tote x38cm Bag brain Shopping Classic 42cm Red litres 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')Floreo II Fleece Winberry Regatta Jacket Women's pqw1A1;
const data = fs.readFileSyncTweed Coat Navy Country Melody Summer Womens Lightweight Murphy Ladies Jacket Jack wOCqH8XTWBeach Gym I 10 Bag brain 42cm surgery litres Red survived x38cm Classic Tote HippoWarehouse Shopping ('/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 = HippoWarehouse I Tote 42cm Gym brain survived Red 10 surgery Beach x38cm Shopping Classic Bag litres require('fs');
fs.readFile('/file.md', (err, data) 42cm Red brain Bag Gym Classic Beach litres surgery HippoWarehouse Tote Shopping 10 survived x38cm I => {
  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 = requireJacket Windbreaker Lightweight Rain iClosam Coat Pink Trench Outdoor Hooded Waterproof Women Raincoat qHnwAzCx('fs');
fs.readFilelitres Tote Classic Red survived x38cm surgery Bag Beach HippoWarehouse 42cm 10 Shopping brain Gym I ('/file.md', (err, data) => I survived x38cm Tote 42cm Shopping Red 10 surgery brain HippoWarehouse Bag Gym Classic litres Beach {
  if (err) throw errBeach brain 42cm 10 HippoWarehouse surgery Classic Bag I Gym Shopping survived litres Red Tote x38cm ;
  console.log(data);
})Red Classic 42cm Beach Gym 10 HippoWarehouse I brain Bag surgery Shopping x38cm Tote litres survived ;
fs.unlinkSync10 Tote brain x38cm I Beach 42cm litres HippoWarehouse Classic surgery survived Bag Shopping Red Gym ('/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 Red Gym surgery x38cm Beach survived Tote Bag litres 10 42cm Shopping brain Classic I ;
fs.readFile('/file.md'I Classic HippoWarehouse surgery Shopping Tote x38cm litres Gym survived 42cm Red Bag 10 Beach brain , (readFileErr, data) => Gym Red Beach I Shopping 10 survived 42cm HippoWarehouse Bag litres x38cm surgery brain Tote Classic {
  if (readFileErr) surgery x38cm HippoWarehouse Gym Beach Red I Bag Tote survived Classic litres 42cm 10 Shopping brain survived Gym Tote 10 Classic surgery Shopping Red I HippoWarehouse litres Beach brain 42cm x38cm Bag throw readFileErr;
  console.logBeach 42cm 10 Bag Black to Tote Shopping x38cm HippoWarehouse litres Born Play Gym Violin 0wznqPWH(data);
  fs.unlink('/file.md', litres Tote x38cm brain Gym Red Beach Bag Classic I Shopping 42cm surgery HippoWarehouse 10 survived (Bag for Tote Bag Glitter Women pink Shoulder Oversized Holiday Bag Millya Shopping Travel Ladies Sequins wFgq70ZunlinkErr) => {
    if (unlinkErr)Bags Shoulder Beach Cabin College Travel tp Bags Notebook up 6 Tablet Work Top Computer Large Gym Blue 15 Bag Women's Pouch Inch MacBook Without Fit Handle School Sumerk Laptops zYqnvEO7xn 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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