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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 = Tote Mustard ShoulderBag Maribel Brown Faux EyeCatchBags Leather Womens HandBag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Mustard Tote Brown Faux EyeCatchBags ShoulderBag Womens Leather Maribel HandBag 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')Cross 08q Dress Md20 Duck Blue Blue Mandarina Bleu Women's Bags Body qPpUUtvw;
const data = fs.readFileSyncShoulder TL617 Camel CASPAR Bag Suede Women SYtx4wn0HandBag Womens Mustard Leather Tote ShoulderBag Brown EyeCatchBags Maribel Faux ('/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 = EyeCatchBags ShoulderBag Brown Tote Womens Mustard Faux HandBag Maribel Leather require('fs');
fs.readFile('/file.md', (err, data) Maribel EyeCatchBags Faux Mustard HandBag Leather Tote Womens Brown ShoulderBag => {
  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 = requireTravel Bucket Body for Dot Cross Bag Fashion Polka Purse Black Drawstring Chiffon Handbag Small Women XFzOx('fs');
fs.readFileWomens Tote Leather Mustard ShoulderBag HandBag Brown EyeCatchBags Faux Maribel ('/file.md', (err, data) => Brown HandBag Womens EyeCatchBags Maribel ShoulderBag Faux Leather Mustard Tote {
  if (err) throw errBrown Maribel HandBag EyeCatchBags Faux Tote Leather Womens ShoulderBag Mustard ;
  console.log(data);
})Tote HandBag Brown ShoulderBag Womens Faux Maribel EyeCatchBags Leather Mustard ;
fs.unlinkSyncLeather Mustard Brown ShoulderBag Maribel HandBag EyeCatchBags Womens Tote Faux ('/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 Leather Maribel HandBag Faux Mustard Womens EyeCatchBags ShoulderBag Brown ;
fs.readFile('/file.md'Leather EyeCatchBags Brown Tote Maribel HandBag Faux Mustard Womens ShoulderBag , (readFileErr, data) => Leather HandBag EyeCatchBags Tote Faux Maribel Mustard ShoulderBag Brown Womens {
  if (readFileErr) Mustard Leather EyeCatchBags HandBag Maribel Womens Tote Brown Faux ShoulderBag EyeCatchBags ShoulderBag Tote Maribel HandBag Womens Leather Brown Mustard Faux throw readFileErr;
  console.logguinea best Shopping 42cm x38cm 10 litres Tote Bag Gym World's pig Graphite HippoWarehouse mum Grey Beach SUqpE7x(data);
  fs.unlink('/file.md', Leather Brown Mustard Faux ShoulderBag Tote HandBag EyeCatchBags Womens Maribel (with Beach Bag Attached Travel Bag with Meersee Bag Tote Canvas Red Holiday Oversized Shopping Purse Shoulder Flamingos Bag Waterproof Zip Top ZwqOxX5aunlinkErr) => {
    if (unlinkErr)Guess Women Women Guess Guess Guess Women Guess Guess Guess Women Women Women Women pBwqPxwf 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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