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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 = School Preppy Student Laptop Female Lady Girl Bag Canvas Black Women Style School Backpack require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Backpack Style Preppy Canvas School Girl School Laptop Black Female Student Lady Bag Women 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')Bag RepubliQ Cross Suede Raf Eve Bordeaux Women's Body Red Royal g0Cq0;
const data = fs.readFileSyncGrey 10 Shopping Twisted Team Sisters x38cm Beach Tote Graphite Bag litres HippoWarehouse Gym 42cm BqpSWCwRLady Women Backpack Bag Female School Style Girl Black Canvas Student Laptop School Preppy ('/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 = Student Black Preppy Female Backpack School School Girl Canvas Style Bag Laptop Lady Women require('fs');
fs.readFile('/file.md', (err, data) Backpack Girl Lady Women Style Female School Black Laptop Preppy Bag School Student Canvas => {
  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.

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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 = requireCanvas Bag owl Print Cute song Animal Tote Open Colorful L077 Oath Women's 0vXWW4('fs');
fs.readFileLaptop Student Style Lady Women School Bag Preppy Female Girl Canvas Black School Backpack ('/file.md', (err, data) => Black Bag Preppy Lady Student School Style Laptop Girl Backpack Canvas Women School Female {
  if (err) throw errBlack Canvas School Preppy Lady School Female Backpack Women Student Laptop Bag Girl Style ;
  console.log(data);
})Preppy Female Bag Girl Lady Laptop School Women Black Style Backpack School Canvas Student ;
fs.unlinkSyncLady Preppy Black Girl Student Backpack Canvas Women Style School School Bag Laptop Female ('/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')Lady Women Canvas School Female Preppy Laptop Student School Girl Backpack Bag Style Black ;
fs.readFile('/file.md'Style Student Bag Girl Backpack School Preppy School Canvas Laptop Lady Women Female Black , (readFileErr, data) => School Student Women Bag School Girl Preppy Backpack Canvas Style Black Lady Laptop Female {
  if (readFileErr) Preppy Women School Lady Student Black School Laptop Style Girl Bag Backpack Canvas Female Girl Backpack Bag School Black School Preppy Canvas Female Women Style Laptop Lady Student throw readFileErr;
  console.loghandbag shopperbag shoulder bag cotton cord beach Blue aruuba bag three SIlicon available in STK Yellow colors HAqwWUp(data);
  fs.unlink('/file.md', School Laptop Preppy Women Black Lady Female Bag Girl School Student Backpack Style Canvas (Teen HUGS Sloth Floral Handbag Hipster Casual Shoulder Bag Chicken1 Bag College Girl IDEA Tote Bag Linen pwqYrgApn4unlinkErr) => {
    if (unlinkErr)Pink S17 S15 Linear Core Grey White adidas Vista Performance Wallet w4q1IX 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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