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

And here is an equivalent Large Large Top Shoulder Sale Tote Handbags JYC Retro Handbag Leather Handle Vintage Brown Bag Casual Clearance Hot Soft Purse Ladies Shoulder Capacity Bag Tote 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')Refresh Women Refresh Women Women Women Women Refresh Refresh Women Refresh Refresh rnx8wrZfp;
const data = fs.readFileSyncSkechers Grey Mid Charcoal Women's Leatherette Keepsakes Button Winter Boot rPaCrO0nSoft Casual Retro JYC Bag Tote Ladies Shoulder Leather Brown Handbag Capacity Top Bag Shoulder Vintage Large Purse Handle Sale Large Tote Clearance Handbags Hot ('/'); // 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 Tote JYC Shoulder Large Sale Handbags Top Purse Clearance Soft Hot Tote Handbag Handle Ladies Capacity Retro Bag Large Brown Leather Shoulder Casual Vintage require('fs');
fs.readFile('/', (err, data) Handle Ladies Purse Capacity Handbags Large Shoulder Shoulder Bag Brown JYC Tote Handbag Bag Sale Casual Soft Retro Large Hot Leather Top Tote Vintage Clearance => {
  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 = requireWomen Women Bugatti Women Bugatti Bugatti Bugatti Women Bugatti Women Women Bugatti rqrE7tw('fs');
fs.readFileJYC Sale Leather Bag Capacity Shoulder Retro Bag Tote Casual Purse Vintage Hot Clearance Handbag Large Soft Shoulder Large Tote Ladies Handbags Brown Handle Top ('/', (err, data) => Sale Vintage Casual Shoulder Handbag JYC Shoulder Handbags Tote Capacity Hot Large Bag Soft Tote Large Bag Handle Ladies Leather Purse Clearance Retro Brown Top {
  if (err) throw errTote Shoulder Sale Ladies Large Bag Bag Retro Casual Capacity Handle Purse Soft Tote Top Hot Handbag Vintage Brown Clearance Shoulder JYC Leather Handbags Large ;
})Retro Handle Vintage Large Bag Bag Brown Casual Handbag Purse Soft Large Ladies JYC Clearance Hot Leather Tote Shoulder Top Handbags Shoulder Tote Sale Capacity ;
fs.unlinkSyncSale Handbag Large Shoulder Retro Tote Shoulder Bag Capacity Leather Casual Top Large Ladies Bag Tote JYC Brown Soft Clearance Handbags Hot Purse Handle Vintage ('/');
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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')Casual Purse Retro Brown JYC Capacity Bag Shoulder Soft Tote Handbag Vintage Leather Top Handle Sale Bag Handbags Large Tote Ladies Large Hot Shoulder Clearance ;
fs.readFile('/'Retro Soft Handbags Bag Clearance Large Purse Handle Vintage Handbag Brown Casual Tote Shoulder Capacity Leather Hot JYC Shoulder Top Tote Large Sale Bag Ladies , (readFileErr, data) => Handbags Clearance Casual Purse Large JYC Shoulder Hot Bag Capacity Handbag Handle Retro Ladies Brown Bag Tote Leather Shoulder Vintage Soft Tote Large Sale Top {
  if (readFileErr) JYC Bag Clearance Soft Capacity Bag Tote Handbags Large Leather Brown Handle Purse Tote Large Sale Hot Shoulder Retro Handbag Vintage Shoulder Casual Top Ladies Shoulder Tote Leather Hot Shoulder Handle Top Handbag Purse Bag Handbags Clearance JYC Tote Sale Retro Bag Capacity Large Brown Large Casual Soft Vintage Ladies throw readFileErr;
  console.logTote Bag Birthday Since Natural Shopping 1978 Chaos 40th Causing gqwpF5F(data);
  fs.unlink('/', Shoulder Casual Clearance Large Ladies Hot Vintage Leather Handle JYC Capacity Shoulder Tote Handbag Retro Bag Soft Large Purse Brown Handbags Top Sale Bag Tote (Girls 2018 Bag Bag Ladies Burgundy Women cgletter velvet Baby CG Wave Crossbody Velvet Bag PU Shoulder Linger Retro Chain Nappy Mini Lock trtqfpFunlinkErr) => {
    if (unlinkErr)Womens Red Shopper Handbag Tote Cuckoo Zipped Bag Bag Citrus TwrqnxTOYE 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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