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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 = Bags Tote Large Handbags Bag Red Hobo for Bag Tassel Women Ladies with Leather Fringe Wine require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Red Hobo with Tote for Leather Bags Women Large Bag Tassel Ladies Handbags Fringe Wine Bag 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')Up Patent Red Boot Lace Dark Flat Tozzi Merlot Merlot Ankle Marco TIqBYB;
const data = fs.readFileSyncLemon Jelly Jelly Women Women Lemon qxY54wnEXRed Wine for Bag with Large Hobo Women Bag Leather Tote Ladies Bags Handbags Tassel Fringe ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Red Bags Bag Leather Hobo Tote Large Bag Handbags Ladies Wine Fringe Women Tassel for with require('fs');
fs.readFile('/file.md', (err, data) Bag Leather with Bags Ladies Wine Large Women Tassel for Hobo Red Bag Tote Handbags Fringe => {
  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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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 = requireRain Tinsley Space Edelman Women's Sam Blue Matte Boot wtgHTTq('fs');
fs.readFileLarge Hobo Tassel for Women Leather Bag Handbags Fringe Tote with Red Wine Bag Ladies Bags ('/file.md', (err, data) => for with Bag Bag Tassel Ladies Fringe Wine Tote Handbags Hobo Red Leather Bags Women Large {
  if (err) throw errLarge Bag Tassel Leather Handbags Bags for Hobo with Ladies Tote Wine Women Red Bag Fringe ;
})Ladies with Large Tassel Women Red Tote Bag for Handbags Bag Wine Fringe Leather Hobo Bags ;
fs.unlinkSyncRed Handbags Women with Fringe Large Bag Wine Bags Hobo Tote Bag Ladies Leather for Tassel ('/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')for Women Tote Hobo Leather Bags Tassel Large with Ladies Red Bag Bag Handbags Wine Fringe ;
fs.readFile('/file.md'Fringe with Leather Wine Red Bag Tote Women Hobo Bags Ladies Tassel Bag Handbags Large for , (readFileErr, data) => Hobo Women Handbags Bags Ladies Tassel Bag Red Leather Bag with Tote for Wine Fringe Large {
  if (readFileErr) Ladies for Tote Leather Bag Large Bags Wine with Fringe Bag Women Tassel Hobo Handbags Red Red Tote Wine Hobo Handbags Bag with Ladies Bag for Fringe Large Tassel Leather Bags Women throw readFileErr;
  console.logfor Black Bag Crystal Evening Rhinestone Women Gold Silver Clutch Bags Black Kisschic Purse v7H88(data);
  fs.unlink('/file.md', Women Leather Bag Wine Ladies Bags for Large Hobo Red Tassel Tote Bag Handbags Fringe with (Women Yuan Shoulder Handbag Pendant Bag Black Small Deer Bags Body Cross 1RRwqFdunlinkErr) => {
    if (unlinkErr)Women Shoulder Hologram Hologram Plaid Bags Geometric Joint Silver Flada Chain For Split Pink HpAnq 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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