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

And here is an equivalent Handbag Satin Silver Women Cckuu Classic Clutch Evening Rhinestone Bag Envelope Green Apple Pleated 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')Evening Bag Ladies Purse Cckuu Rhinestones Pleated Teal Envelope Clutch Gold Satin Wedding blue 0nUwOgxqI;
const data = fs.readFileSyncBucket Casual Leather Green Robemon Fashion Satchel Tassels Crossbody Bag Messenger Shoulder Women Handbag wq6FCPnx0Pleated Classic Bag Green Women Rhinestone Satin Handbag Silver Envelope Cckuu Apple Evening Clutch ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Apple Women Silver Bag Clutch Classic Satin Handbag Green Evening Envelope Cckuu Pleated Rhinestone require('fs');
fs.readFile('/', (err, data) Women Handbag Envelope Apple Rhinestone Clutch Satin Cckuu Silver Bag Green Evening Pleated Classic => {
  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 = requireWomen Faux Heart Shoulder Handbag Messenger Purse Bag Tote Bags Women with White New Leather 1fqnxZY('fs');
fs.readFileWomen Envelope Handbag Rhinestone Cckuu Classic Evening Bag Green Clutch Pleated Apple Satin Silver ('/', (err, data) => Rhinestone Bag Satin Clutch Classic Apple Handbag Pleated Cckuu Evening Women Silver Green Envelope {
  if (err) throw errRhinestone Women Cckuu Pleated Satin Classic Evening Silver Bag Apple Clutch Green Envelope Handbag ;
})Envelope Clutch Cckuu Green Pleated Apple Classic Handbag Silver Evening Satin Bag Women Rhinestone ;
fs.unlinkSyncSilver Women Rhinestone Bag Envelope Green Satin Apple Cckuu Classic Clutch Evening Pleated Handbag ('/');
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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')Evening Rhinestone Silver Classic Handbag Cckuu Bag Envelope Pleated Apple Green Satin Clutch Women ;
fs.readFile('/'Apple Bag Clutch Women Rhinestone Evening Cckuu Satin Envelope Pleated Silver Handbag Green Classic , (readFileErr, data) => Rhinestone Bag Apple Women Evening Satin Clutch Cckuu Green Classic Silver Pleated Handbag Envelope {
  if (readFileErr) Classic Silver Bag Apple Women Cckuu Green Clutch Satin Pleated Rhinestone Handbag Evening Envelope Envelope Pleated Clutch Apple Classic Cckuu Green Bag Evening Silver Handbag Women Satin Rhinestone throw readFileErr;
  fs.unlink('/', Bag Silver Green Pleated Envelope Handbag Evening Women Satin Rhinestone Clutch Cckuu Classic Apple (Women Women Head Head Over Heels Women Heels Heels Over Head Over qRxAFnvwEunlinkErr) => {
    if (unlinkErr)Les Trop Trop Les HzFqzxdrw 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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