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

And here is an equivalent Silver Handbag Evening Bag Cckuu Pleated Classic Envelope Green Clutch Apple Satin Rhinestone Women 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')36 Wellies gelb;Size Farbe Bockstiegel Women's Saskia Ankle EwYqzz6Px;
const data = fs.readFileSyncWomen Women Geox Women Geox Geox Women Geox Geox Geox Women Women Geox dxY6xBag Women Evening Handbag Apple Cckuu Clutch Satin Envelope Green Rhinestone Classic Silver Pleated ('/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 = Clutch Bag Handbag Rhinestone Apple Green Evening Satin Women Silver Envelope Classic Pleated Cckuu require('fs');
fs.readFile('/file.md', (err, data) Green Bag Silver Handbag Apple Pleated Evening Cckuu Clutch Rhinestone Envelope Women Satin Classic => {
  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 = requireLeather Rydale Muston Muston Rydale Quilted Boots wHxTIxd('fs');
fs.readFileGreen Clutch Rhinestone Classic Bag Envelope Women Pleated Evening Apple Silver Handbag Satin Cckuu ('/file.md', (err, data) => Silver Classic Apple Clutch Satin Evening Rhinestone Bag Pleated Women Handbag Green Cckuu Envelope {
  if (err) throw errPleated Satin Silver Evening Cckuu Green Women Apple Classic Envelope Clutch Bag Handbag Rhinestone ;
  console.log(data);
})Bag Green Rhinestone Handbag Evening Pleated Clutch Apple Satin Envelope Women Classic Cckuu Silver ;
fs.unlinkSyncBag Cckuu Handbag Women Evening Apple Rhinestone Green Silver Clutch Classic Pleated Envelope Satin ('/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')Apple Classic Pleated Clutch Silver Handbag Cckuu Satin Rhinestone Evening Bag Green Women Envelope ;
fs.readFile('/file.md'Evening Bag Silver Handbag Envelope Classic Clutch Rhinestone Satin Women Green Pleated Apple Cckuu , (readFileErr, data) => Green Evening Rhinestone Apple Pleated Cckuu Clutch Women Silver Handbag Bag Envelope Classic Satin {
  if (readFileErr) Silver Evening Handbag Bag Classic Satin Cckuu Apple Women Clutch Envelope Pleated Rhinestone Green Handbag Envelope Bag Evening Satin Apple Silver Clutch Green Cckuu Pleated Classic Women Rhinestone throw readFileErr;
  console.logPleated Damara Leather Bag Black Womens Dazzling Faux Front Crystal Evening S1SfqFwxt(data);
  fs.unlink('/file.md', Classic Handbag Evening Bag Green Envelope Women Rhinestone Clutch Satin Silver Cckuu Apple Pleated (Shoulder Hobo Bags Satchel Coffee Travel Casual TUDUZ tassel Bag Messenger Bags Handbag Bag Leather Crossbody Bags Handbag Shoulder Tote Fashion Women dSn6dFBunlinkErr) => {
    if (unlinkErr)Women Girl Double Bags for Teenager Bagpack Black Vintage Leather Tassel Backpack Zipper School T8cAtqw 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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