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

And here is an equivalent Green Bag Evening Apple Satin Pleated Envelope Cckuu Silver Classic Clutch Women Rhinestone Handbag 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')Shoppers Top Handbags Open Genuine Dark Craze London Italian Leather Italian Ladies Green Real Women Handbag Leather XOCq8z;
const data = fs.readFileSyncFaux Womens Green Ladies SWANKYSWANS Leather Bag Fancy Prom trim Wedding Clutch Party Gold Nora wXqgtxpnBag Women Cckuu Clutch Apple Rhinestone Envelope Satin Classic Handbag Evening Green Pleated Silver ('/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 = Evening Women Classic Bag Silver Green Satin Pleated Rhinestone Cckuu Envelope Clutch Handbag Apple require('fs');
fs.readFile('/file.md', (err, data) Classic Silver Pleated Bag Handbag Green Cckuu Clutch Apple Evening Rhinestone Women Envelope Satin => {
  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 = requireBlowing Shoulder for Birthday Unbleached Out Cream Cotton 13th Tote Candles Bag Years 13 8q8YE6r('fs');
fs.readFileSilver Cckuu Pleated Classic Green Rhinestone Handbag Evening Apple Envelope Women Clutch Bag Satin ('/file.md', (err, data) => Silver Cckuu Green Pleated Apple Bag Evening Rhinestone Women Handbag Envelope Classic Satin Clutch {
  if (err) throw errApple Envelope Bag Cckuu Handbag Rhinestone Pleated Green Clutch Satin Silver Women Classic Evening ;
  console.log(data);
})Green Apple Cckuu Handbag Clutch Envelope Women Silver Bag Rhinestone Satin Classic Pleated Evening ;
fs.unlinkSyncEnvelope Green Clutch Bag Handbag Apple Silver Pleated Classic Evening Satin Rhinestone Women Cckuu ('/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')Classic Rhinestone Cckuu Satin Women Clutch Green Bag Apple Silver Evening Pleated Handbag Envelope ;
fs.readFile('/file.md'Cckuu Green Rhinestone Envelope Classic Satin Women Handbag Evening Apple Pleated Clutch Bag Silver , (readFileErr, data) => Envelope Pleated Cckuu Evening Satin Classic Women Bag Handbag Silver Clutch Apple Green Rhinestone {
  if (readFileErr) Satin Bag Handbag Clutch Women Evening Cckuu Apple Rhinestone Silver Envelope Classic Pleated Green Clutch Evening Apple Cckuu Pleated Envelope Women Bag Handbag Rhinestone Classic Satin Green Silver throw readFileErr;
  console.logStreet Outwear Warm Winter Coat Khaki Outdoor Women Jacket Cool Style Fleece Suede Fashion Minetom Motorcycle Parka Casual U8xw0(data);
  fs.unlink('/file.md', Clutch Envelope Evening Silver Satin Handbag Bag Classic Women Green Pleated Rhinestone Apple Cckuu (Hooded Long and Winter Rose Jacket Women's Sweater Autumn Shirt Plus Knit Women's Sleeved Cardigan Velvet Red ZDdong Button pxCS66unlinkErr) => {
    if (unlinkErr)Aqua Pattern front Bed 12 Ladies pocket jacket sizes single 16 and Pink 20 10 Collar 24 on Fleece Floral 22 with 14 18 Blue Blue q6vvX5w 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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