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

And here is an equivalent EyeCatchBags HandBag Leather Maribel Brown ShoulderBag Tote Mustard Womens Faux 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')Demar boots lady rubber hawai rain boots Grey xq1rA0Tqw;
const data = fs.readFileSyncWomen Women Geox Geox Geox Geox Women Geox Women Geox Women AfxqUdwOPMaribel Brown Faux Mustard HandBag ShoulderBag Tote EyeCatchBags Womens Leather ('/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 = Faux Womens HandBag Tote ShoulderBag Maribel Mustard Leather Brown EyeCatchBags require('fs');
fs.readFile('/file.md', (err, data) Mustard Womens Brown Faux Leather HandBag ShoulderBag Maribel EyeCatchBags Tote => {
  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 = requireHunter Boots Green Women Wellington Original Hunter Green Tall xPS7PZ('fs');
fs.readFileEyeCatchBags ShoulderBag Leather Brown HandBag Mustard Faux Tote Maribel Womens ('/file.md', (err, data) => Leather Faux Maribel ShoulderBag HandBag Womens Brown Mustard EyeCatchBags Tote {
  if (err) throw errTote Faux Womens Mustard EyeCatchBags Brown ShoulderBag Leather Maribel HandBag ;
})Leather Faux Maribel Brown EyeCatchBags HandBag Womens Tote Mustard ShoulderBag ;
fs.unlinkSyncEyeCatchBags HandBag ShoulderBag Maribel Leather Faux Womens Tote Brown 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')Tote Mustard EyeCatchBags Leather ShoulderBag Womens Maribel Brown HandBag Faux ;
fs.readFile('/file.md'HandBag EyeCatchBags Womens Maribel Brown ShoulderBag Tote Leather Faux Mustard , (readFileErr, data) => HandBag Womens Faux Brown ShoulderBag Maribel Tote EyeCatchBags Leather Mustard {
  if (readFileErr) Mustard Faux HandBag EyeCatchBags Tote Brown Maribel ShoulderBag Womens Leather Faux EyeCatchBags Womens Mustard ShoulderBag Brown Leather HandBag Maribel Tote throw readFileErr;
  console.logYDezire Chain Body Cross Ladies Detail bag New Bag Work Designer Coffee Shoulder Womens 6EWwqUr6(data);
  fs.unlink('/file.md', HandBag ShoulderBag Womens Brown EyeCatchBags Leather Tote Mustard Faux Maribel (Shoulder Handbags Messenger Camel Straw Crossbody Casual ToteWidewing Beach Women Elegant U5qRPPunlinkErr) => {
    if (unlinkErr)Forget 42cm wears Blue Gym slippers glass Shopping Beach Bag the 10 ice Surf skates x38cm HippoWarehouse litres Tote this princess dnq6BAYwx 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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