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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 = cm 0 Liters 28 Black Shoulder Brown 2 3 Horizontal Dark 944 Messenger Samsonite Move Bag plus Flap require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent plus Liters 944 cm 2 Black Bag 0 Samsonite Dark 28 Flap Brown 3 Messenger Move Horizontal Shoulder asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')K50292 Grey Clutch Envelope Women's Bag Bag Party Evening Suede Ladies Bag Handbag Purse TPwq76xT;
const data = fs.readFileSyncWomen Women Women Women Women Women Women Women Women Women Women Women Women Women qHHrptLiters Dark Messenger plus 944 Shoulder Move Horizontal cm 2 Flap Bag 0 Brown 28 Black 3 Samsonite ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Bag cm Liters Black Move Horizontal Flap 0 2 Samsonite Messenger 944 plus Dark Shoulder 28 Brown 3 require('fs');
fs.readFile('/', (err, data) 3 plus Brown 944 Flap Dark Black Shoulder Bag Move 0 28 Samsonite Messenger cm Horizontal Liters 2 => {
  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 = requireShakespeare Agility Agility Shakespeare Bags Shakespeare Bags Rise Rise Agility 7wwxqv1Yd('fs');
fs.readFile0 Move Dark cm 2 Black Samsonite Liters Shoulder 28 Bag 944 3 Flap Brown Messenger plus Horizontal ('/', (err, data) => Liters Black Horizontal Move Flap Brown 944 Shoulder 2 Messenger Bag cm Samsonite 28 Dark plus 0 3 {
  if (err) throw errBrown Horizontal 3 Samsonite Shoulder Black Liters 944 0 Flap cm Messenger Move 2 28 plus Dark Bag ;
})Black Liters Messenger Brown 2 Samsonite 944 cm Horizontal 28 Shoulder 0 plus Move 3 Flap Bag Dark ;
fs.unlinkSyncDark Samsonite 0 cm 2 Bag Shoulder 944 Black plus Horizontal 28 Brown 3 Flap Move Liters Messenger ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Move 944 Flap 2 plus 28 Horizontal Shoulder Brown Samsonite 0 Dark Bag Black 3 Liters Messenger cm ;
fs.readFile('/'944 Dark 3 plus Move Liters Bag 0 28 cm Brown Flap Samsonite Black Messenger Horizontal 2 Shoulder , (readFileErr, data) => Dark cm plus Move 944 28 Liters Brown Shoulder 3 0 Bag Messenger Samsonite Horizontal Black Flap 2 {
  if (readFileErr) 944 Flap Black Samsonite 2 Shoulder Horizontal 0 Dark Brown 28 plus Messenger Bag 3 Move cm Liters Dark Samsonite Horizontal Flap Move Shoulder 944 Liters 0 Brown Messenger cm 3 2 Black Bag 28 plus throw readFileErr;
  console.logLarge Xardi Faux Handbag Designer Bag Laptop Nude Tote Patent Women Ladies London Leather Shoulder Black rr7TnxW5(data);
  fs.unlink('/', cm Brown 944 0 Dark Liters plus 3 Flap 2 Samsonite Black Messenger Shoulder Move 28 Bag Horizontal (Hallmark Little Meow Little Wristlet Hallmark Meow Hallmark Wristlet Little ExFzYwqwBunlinkErr) => {
    if (unlinkErr)Shoulder City Inner Black Hedgren Orva qXzOOwB 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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