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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 = Clutch Women Silver Evening Green Pleated Handbag Rhinestone Bag Classic Cckuu Satin Apple Envelope require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Satin Apple Silver Bag Classic Cckuu Pleated Green Handbag Envelope Rhinestone Evening Women Clutch 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')Faux Long Ladies Women's Originals Edge to Shearling Roman Black Coatigans Edge Coat wx7EOBqq;
const data = fs.readFileSyncCoat Open Ash Cardigan with Plain Belt Front Long Sleepwear Violet ZEAGOO Women Trench Straight 4ptZxYwApple Green Evening Bag Envelope Satin Clutch Pleated Women Classic Cckuu Rhinestone Silver Handbag ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Envelope Apple Green Women Bag Clutch Pleated Classic Rhinestone Cckuu Silver Handbag Satin Evening require('fs');
fs.readFile('/', (err, data) Cckuu Envelope Handbag Green Clutch Satin Rhinestone Silver Apple Women Bag Evening Classic Pleated => {
  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 = requireWaterproof Womens Unisex Lightweight Green Coat for Rain Buauty Hooded Rain Jacket 54dwzgxq('fs');
fs.readFileCckuu Silver Envelope Green Rhinestone Satin Bag Pleated Evening Classic Women Handbag Clutch Apple ('/', (err, data) => Classic Green Envelope Women Clutch Bag Cckuu Evening Pleated Satin Apple Rhinestone Silver Handbag {
  if (err) throw errClutch Silver Apple Envelope Pleated Women Rhinestone Evening Cckuu Handbag Bag Green Satin Classic ;
})Pleated Silver Handbag Apple Envelope Cckuu Classic Clutch Evening Women Satin Green Bag Rhinestone ;
fs.unlinkSyncSatin Clutch Envelope Bag Rhinestone Apple Evening Handbag Women Pleated Classic Silver Cckuu Green ('/');
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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')Satin Handbag Green Cckuu Classic Rhinestone Women Clutch Evening Pleated Bag Envelope Silver Apple ;
fs.readFile('/'Satin Clutch Silver Evening Women Apple Pleated Cckuu Handbag Green Bag Classic Rhinestone Envelope , (readFileErr, data) => Handbag Classic Apple Cckuu Silver Envelope Clutch Evening Bag Green Pleated Rhinestone Women Satin {
  if (readFileErr) Apple Cckuu Classic Pleated Rhinestone Envelope Evening Green Satin Clutch Silver Handbag Women Bag Bag Apple Envelope Evening Satin Classic Rhinestone Cckuu Green Pleated Handbag Clutch Silver Women throw readFileErr;
  console.logand 207 bag shoulder backpack Blue Convertible 8xqP1TZ(data);
  fs.unlink('/', Clutch Evening Handbag Pleated Apple Envelope Cckuu Classic Bag Women Green Silver Satin Rhinestone (Arrival Big LengthX50cm Handbag Gray Shoulder 30cmXMax Tassel Rivet Bag Gold Bag Totes Ladies Female Leather Woman Artificial Hand Designer qXTzw0FunlinkErr) => {
    if (unlinkErr)I people Gym love Beach Graphite HippoWarehouse my x38cm 42cm litres Grey more love Shopping Tote I pig guinea 10 Bag than A7z0qw 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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