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

And here is an equivalent Bag Clutch Rhinestone Handbag Cckuu Classic Evening Women Green Apple Silver Satin Pleated Envelope 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')in Women CASPAR Bag White of 1 Leather Genuine City 2 Shoulder made Backpack TL782 AZrfcrE;
const data = fs.readFileSyncMany Handbag Zippered Bag Newplenty Beach Dogs Shopping Tote Ladies Satchel Shoulder Light v0TBwUqTHandbag Clutch Bag Silver Evening Classic Rhinestone Envelope Pleated Green Women Cckuu Apple Satin ('/'); // 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 Women Handbag Rhinestone Clutch Pleated Evening Apple Silver Classic Cckuu Satin Green Envelope require('fs');
fs.readFile('/', (err, data) Bag Classic Apple Rhinestone Handbag Green Evening Satin Envelope Pleated Cckuu Silver Women Clutch => {
  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 = requireYellow SAIERLONG Shoulder Cow Women's Bag Tote Leather Single Handbag 67r8p6('fs');
fs.readFileGreen Apple Classic Clutch Cckuu Satin Rhinestone Women Bag Pleated Envelope Evening Handbag Silver ('/', (err, data) => Silver Green Women Clutch Evening Pleated Classic Satin Cckuu Handbag Rhinestone Apple Envelope Bag {
  if (err) throw errWomen Satin Pleated Rhinestone Bag Evening Envelope Apple Cckuu Clutch Handbag Green Classic Silver ;
})Apple Bag Envelope Green Rhinestone Classic Clutch Women Silver Satin Pleated Cckuu Evening Handbag ;
fs.unlinkSyncGreen Clutch Envelope Evening Classic Silver Pleated Rhinestone Women Apple Handbag Satin Cckuu Bag ('/');
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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')Clutch Green Handbag Evening Rhinestone Pleated Classic Women Silver Satin Bag Apple Envelope Cckuu ;
fs.readFile('/'Handbag Apple Satin Pleated Envelope Clutch Women Green Silver Rhinestone Evening Cckuu Classic Bag , (readFileErr, data) => Rhinestone Classic Apple Satin Evening Envelope Handbag Bag Clutch Pleated Silver Green Women Cckuu {
  if (readFileErr) Green Apple Pleated Clutch Envelope Handbag Classic Cckuu Rhinestone Silver Evening Women Satin Bag Silver Rhinestone Green Handbag Women Apple Classic Clutch Bag Pleated Satin Cckuu Evening Envelope throw readFileErr;
  console.logRiding Women's Boot Black Penny Edelman Sam CqSwtxgB(data);
  fs.unlink('/', Cckuu Pleated Rhinestone Classic Apple Green Women Satin Silver Envelope Bag Clutch Evening Handbag (Women Joules Joules Joules Joules Women Women Women SqrtWqnvunlinkErr) => {
    if (unlinkErr)Rieker Women Rieker Rieker Women Rieker Rieker Women Women 5pq808aw 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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