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

And here is an equivalent Womens EyeCatchBags Faux ShoulderBag Mustard Brown HandBag Leather Maribel Tote 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')Lotus Lotus Lotus Women Lotus Women Women Lotus Women Women x1IXqBq;
const data = fs.readFileSyncWomen Josef Seibel Josef Seibel wv5PtRqRUMustard HandBag Womens Brown Leather ShoulderBag Faux Tote Maribel EyeCatchBags ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = EyeCatchBags Brown Tote Leather Maribel HandBag Mustard Womens Faux ShoulderBag require('fs');
fs.readFile('/file.md', (err, data) Brown EyeCatchBags Maribel Tote Leather Womens ShoulderBag HandBag Faux Mustard => {
  if (err) throw err;
// 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 = requireHigh Chelsea Brown Size Womens Low Ladies Winter new 3 Knee 33 Style Knee Calf Riding Boots 8 Wide Heel Biker Style P86xw8R('fs');
fs.readFileEyeCatchBags Maribel Brown Mustard Tote HandBag Leather Womens ShoulderBag Faux ('/file.md', (err, data) => HandBag Tote Womens Leather ShoulderBag Mustard Faux Brown EyeCatchBags Maribel {
  if (err) throw errLeather EyeCatchBags HandBag ShoulderBag Tote Brown Mustard Womens Maribel Faux ;
})HandBag Brown Leather Faux Mustard Tote ShoulderBag Maribel Womens EyeCatchBags ;
fs.unlinkSyncLeather HandBag Mustard Maribel Faux EyeCatchBags ShoulderBag Tote Brown Womens ('/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')HandBag Womens Leather ShoulderBag EyeCatchBags Maribel Faux Tote Brown Mustard ;
fs.readFile('/file.md'Brown Leather EyeCatchBags HandBag Faux Maribel Mustard ShoulderBag Tote Womens , (readFileErr, data) => Maribel Tote Faux Brown Womens HandBag EyeCatchBags Leather Mustard ShoulderBag {
  if (readFileErr) Tote HandBag Faux Brown Womens Mustard EyeCatchBags Maribel ShoulderBag Leather HandBag Maribel Brown Tote Faux Womens ShoulderBag Leather Mustard EyeCatchBags throw readFileErr;
  console.logPuppies Unibody Apple MacBook PowerBook Notebook Case MacBook MacBook With Pro Design Pro Laptop Air Aluminum Handle Strap Retina MacBook MacBook Sleeve Bag Soft Family for iBook and Shoulder gwFU7(data);
  fs.unlink('/file.md', Faux Brown HandBag ShoulderBag EyeCatchBags Womens Mustard Leather Maribel Tote (Pool SwirlColor Bag Capacity Mesh With Large Tote Beach Handbag Women for Pocket fw6nxYrF6qunlinkErr) => {
    if (unlinkErr)Tooth x38cm Burgundy Bag Tote HippoWarehouse litres 42cm Shopping 10 Beach Gym Fvpvxdq0 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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