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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 = Capacity Sale Brown Bag Purse Clearance Tote Handle Large Large Shoulder Ladies Leather Retro Bag JYC Top Handbags Tote Soft Handbag Casual Shoulder Hot Vintage require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Vintage Top Brown Bag Tote Large Hot Purse Soft Shoulder Handbag Handle Retro Large Sale JYC Clearance Shoulder Ladies Bag Capacity Tote Leather Handbags Casual 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')Chestnut Boots Jessie Heavenly Ankle Feet w44RqCxI7;
const data = fs.readFileSyncBoots Brown Fur Faux oodji Warm Ultra 3900N Women's AXwnxAOYBag Large Tote Purse Hot Shoulder Large Casual Brown Top Handle Bag Handbags Soft Clearance Leather Handbag JYC Shoulder Ladies Vintage Capacity Tote Sale Retro ('/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 = Large Retro Purse Shoulder Clearance Hot Tote Ladies Leather Sale JYC Bag Handbags Handbag Casual Bag Vintage Handle Tote Capacity Large Top Soft Shoulder Brown require('fs');
fs.readFile('/file.md', (err, data) Capacity Top Large Brown Soft Tote Shoulder Large Handbag Handbags Purse Bag Clearance Sale Hot Casual JYC Vintage Tote Leather Bag Retro Ladies Handle Shoulder => {
  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.

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 = requireWomen's Helly Espresso Walnut the Boots Forester Natura Hiking W Hansen Brown 5qqrB4('fs');
fs.readFileTote Hot Handle Ladies Tote Large Retro Casual Capacity Handbag Large Top Bag Purse Bag Vintage Brown Sale Leather Handbags JYC Shoulder Clearance Shoulder Soft ('/file.md', (err, data) => Shoulder Shoulder Ladies Handbags Casual Capacity Bag Hot Large Sale Brown Purse Leather Vintage Tote Handbag Soft Handle Tote Clearance JYC Retro Top Large Bag {
  if (err) throw errBag Casual Clearance Handbag Brown Hot Large JYC Capacity Purse Bag Top Retro Tote Shoulder Sale Tote Vintage Shoulder Leather Handbags Ladies Handle Large Soft ;
  console.log(data);
})Purse Tote Leather JYC Large Casual Clearance Soft Handbags Ladies Capacity Shoulder Handbag Bag Hot Bag Sale Brown Vintage Top Tote Retro Shoulder Handle Large ;
fs.unlinkSyncLeather Clearance Hot Brown Tote Handbag Large Bag Capacity Vintage Ladies Shoulder Bag Retro Top Shoulder Sale Soft Casual Purse JYC Handbags Handle Large Tote ('/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')Shoulder Hot Retro Brown Soft Tote Bag Top Handbags JYC Shoulder Leather Bag Large Ladies Casual Purse Tote Vintage Capacity Handbag Handle Clearance Sale Large ;
fs.readFile('/file.md'Top Soft Handle Sale Leather Tote Capacity Large Handbags Purse Shoulder Brown Retro JYC Casual Bag Bag Vintage Large Ladies Clearance Handbag Shoulder Hot Tote , (readFileErr, data) => Handbag Tote Retro JYC Purse Tote Bag Ladies Large Vintage Casual Large Clearance Shoulder Hot Handbags Handle Capacity Top Soft Sale Shoulder Bag Leather Brown {
  if (readFileErr) Vintage Bag Large Retro Purse Handbag Handle Large Casual Soft Bag Shoulder Handbags Shoulder JYC Tote Brown Hot Clearance Top Ladies Capacity Sale Leather Tote Shoulder Top Purse JYC Leather Retro Large Shoulder Bag Tote Capacity Tote Brown Bag Large Soft Sale Ladies Handbags Casual Handle Handbag Hot Vintage Clearance throw readFileErr;
  console.logPink Tote Vintage Cross Shoulder Body Hot Ladies Handbag Purse Women Wocharm Messenger Bag Bag A6wxwqp(data);
  fs.unlink('/file.md', Purse Hot Bag Retro Top Clearance Sale Soft Shoulder Vintage Handbags Large Leather Bag Casual Large Handle Capacity JYC Brown Ladies Tote Handbag Shoulder Tote (Damara Bag Evening Pleated Flap Womens Satin Clutch Crystal Champagne r17rBq4wunlinkErr) => {
    if (unlinkErr)Cross Black Dune Bonie Black Body Bag Suede Women's UqHpq4 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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