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

And here is an equivalent Brown Womens Mustard Leather Faux ShoulderBag Maribel Tote HandBag EyeCatchBags 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')active active Women active camel camel Women camel Women camel BfnxaqOaw;
const data = fs.readFileSync7 Leather Drax Boots Safety Martens Dr Mens Black Eyelet Work Black TwqtxxUgHHandBag Brown Mustard Tote Faux ShoulderBag Maribel EyeCatchBags Leather Womens ('/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 = ShoulderBag Brown Womens Mustard Maribel EyeCatchBags Leather Faux Tote HandBag require('fs');
fs.readFile('/file.md', (err, data) Maribel Tote EyeCatchBags Faux Brown Mustard Womens HandBag ShoulderBag Leather => {
  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.

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 = requireGroundswell Boots Women's Sebago Suede Chocolate wgvTq('fs');
fs.readFileEyeCatchBags Brown Leather Womens Maribel ShoulderBag HandBag Mustard Faux Tote ('/file.md', (err, data) => Womens EyeCatchBags Maribel Leather Brown ShoulderBag HandBag Mustard Faux Tote {
  if (err) throw errMaribel ShoulderBag Tote Womens HandBag Brown Mustard EyeCatchBags Faux Leather ;
})Womens Mustard ShoulderBag EyeCatchBags Maribel HandBag Leather Faux Brown Tote ;
fs.unlinkSyncShoulderBag HandBag Leather Brown Womens EyeCatchBags Faux Tote Maribel Mustard ('/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')Faux Maribel HandBag Tote EyeCatchBags Leather Brown Mustard ShoulderBag Womens ;
fs.readFile('/file.md'HandBag Tote Faux Mustard EyeCatchBags Brown ShoulderBag Maribel Leather Womens , (readFileErr, data) => Maribel HandBag EyeCatchBags Womens Brown Leather Faux Tote Mustard ShoulderBag {
  if (readFileErr) Womens Maribel Tote HandBag Brown EyeCatchBags Leather ShoulderBag Mustard Faux Faux Tote ShoulderBag Brown Maribel HandBag Womens Leather EyeCatchBags Mustard throw readFileErr;
  console.logBeach Everpert Casual Beige Holiday Simple Handbag Crossbody Shoulder Bag Flap Women Straw r0xnwr(data);
  fs.unlink('/file.md', Brown Mustard EyeCatchBags Faux Leather ShoulderBag HandBag Maribel Womens Tote (Small Body Shoulder Designs Blue Electric Real Various Shop Soft Cross Handbag Leather Bag in Big Mini x1S08qwnEunlinkErr) => {
    if (unlinkErr)Shoulder Pink Women Girls Handbag Muium Mini Bag Bag Solid Bowknot Crossbody Color Leather Fashion wOfIxqZ4 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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