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

And here is an equivalent Bag Envelope Rhinestone Green Evening Pleated Classic Women Apple Handbag Cckuu Satin Silver Clutch 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')Women Fashion Bag Robemon Purple Rucksacks Messenger Style Bag Soft Shopping Casual Handbag Crossbody Satchel Shoulder fUwYH18q1;
const data = fs.readFileSyncCutogain PU Bag Tassel Messenger Women Leather Small Crossbody Handbag Bags Drawstring Pink Bucket 8S8rqClutch Satin Handbag Pleated Green Apple Women Cckuu Rhinestone Classic Envelope Silver Evening Bag ('/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 = Silver Green Pleated Classic Evening Cckuu Handbag Apple Women Satin Rhinestone Clutch Bag Envelope require('fs');
fs.readFile('/file.md', (err, data) Pleated Rhinestone Bag Envelope Green Satin Silver Cckuu Handbag Apple Classic Women Clutch Evening => {
  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 = requireDaily Evening Women Bag Bag Ivory For Box Use Hard 2 Clutch Clubs Hardcase Ladies Case For Design Party Handbag Purse wIx6gX('fs');
fs.readFileClutch Apple Green Rhinestone Cckuu Handbag Envelope Evening Silver Satin Bag Pleated Classic Women ('/file.md', (err, data) => Evening Handbag Clutch Rhinestone Envelope Green Pleated Satin Women Cckuu Silver Classic Bag Apple {
  if (err) throw errBag Envelope Clutch Rhinestone Handbag Satin Silver Women Green Evening Apple Cckuu Pleated Classic ;
  console.log(data);
})Evening Pleated Green Clutch Classic Satin Women Rhinestone Envelope Apple Silver Cckuu Bag Handbag ;
fs.unlinkSyncEnvelope Bag Pleated Apple Satin Classic Cckuu Women Rhinestone Handbag Clutch Evening Silver Green ('/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')Evening Clutch Cckuu Bag Envelope Handbag Pleated Satin Silver Apple Classic Women Green Rhinestone ;
fs.readFile('/file.md'Apple Evening Clutch Cckuu Satin Rhinestone Pleated Handbag Women Green Silver Bag Classic Envelope , (readFileErr, data) => Evening Bag Apple Cckuu Classic Handbag Envelope Satin Silver Clutch Green Women Rhinestone Pleated {
  if (readFileErr) Satin Green Bag Envelope Pleated Rhinestone Apple Women Clutch Evening Cckuu Silver Handbag Classic Satin Bag Silver Apple Women Evening Green Envelope Clutch Pleated Cckuu Handbag Classic Rhinestone throw readFileErr;
  console.logSakura Furry Colorful Winter Space Women's JDS Khaki Fortuning's boots boots boots Velcro Short snow Waterproof Girl's Bnw08Xxtaq(data);
  fs.unlink('/file.md', Classic Silver Handbag Clutch Pleated Envelope Bag Satin Cckuu Evening Green Women Rhinestone Apple (Boots Block Square Women's Ankle High Easemax Zipper Trendy Black Heeled White Toe Mid XxqdPpwtunlinkErr) => {
    if (unlinkErr)Gray 36 Shoes Boots Ladies Women Ankle Martin Heels Boots Fashion Warm Winter Faux High 43 JERFER Buckle a4OSqw1q 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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