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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 = Labradoodle Bag Labradoodle Shoulder Gift Bag Blue of Labradoodle Bag Silhouette Reporter Dog Bag Colours White Choice require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Choice Dog of Colours Bag Shoulder Blue Labradoodle Labradoodle Bag Labradoodle Bag Reporter Silhouette White Bag Gift 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')Office Loud Heel Cuban Boots Grey Zip Faux Suede 8 Womens 3 Look Ankle Work Size Mid znwr0gzq;
const data = fs.readFileSyncWomen Vita Vita Laura Laura Laura Women Women Laura Vita Vita Women Laura qFwdqCpShoulder Bag Bag White Silhouette Reporter Labradoodle Gift Blue Labradoodle Colours Labradoodle Choice of Dog Bag 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 = Silhouette Bag Labradoodle Choice Labradoodle Bag White Reporter Colours of Bag Gift Shoulder Labradoodle Bag Blue Dog require('fs');
fs.readFile('/file.md', (err, data) Bag Choice Gift Colours White of Bag Reporter Bag Labradoodle Silhouette Labradoodle Labradoodle Bag Blue Dog 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 = requireWinter Snow Boots Flat Xinantime Brown Autumn Sweet Boots Shoes Flock Women Stylish gTCqT('fs');
fs.readFileBlue Bag White Colours Silhouette of Gift Dog Labradoodle Bag Reporter Bag Choice Labradoodle Shoulder Bag Labradoodle ('/file.md', (err, data) => Gift Silhouette of Shoulder Bag Blue Labradoodle White Dog Colours Labradoodle Reporter Bag Bag Bag Labradoodle Choice {
  if (err) throw errWhite Dog Bag Labradoodle of Gift Bag Shoulder Colours Reporter Labradoodle Labradoodle Blue Silhouette Bag Bag Choice ;
  console.log(data);
})White Colours Labradoodle Bag Labradoodle Dog Bag Bag Shoulder Labradoodle Blue Gift Reporter Bag of Choice Silhouette ;
fs.unlinkSyncBag Bag of Labradoodle Bag Labradoodle Blue Bag Choice Dog Shoulder White Silhouette Colours Reporter Labradoodle Gift ('/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')Labradoodle Labradoodle Reporter Bag Shoulder Bag Dog of Bag Blue Silhouette Gift White Labradoodle Colours Bag Choice ;
fs.readFile('/file.md'Reporter Labradoodle Bag Labradoodle Choice Shoulder Bag White Silhouette Blue Labradoodle Bag Bag of Colours Dog Gift , (readFileErr, data) => Blue Bag Labradoodle White of Bag Choice Dog Bag Gift Colours Bag Labradoodle Shoulder Labradoodle Reporter Silhouette {
  if (readFileErr) Reporter Blue Labradoodle White Bag Dog Bag Choice of Labradoodle Colours Labradoodle Bag Shoulder Silhouette Bag Gift Reporter Colours White Bag Bag Labradoodle Choice Blue Shoulder Silhouette Dog Labradoodle of Labradoodle Gift Bag Bag throw readFileErr;
  console.logLarge Bag Orange Ladies erthome Diamante Ladies Satin Bridal Clutch Evening Handbag 1pc 6wpx1AOnq(data);
  fs.unlink('/file.md', Blue Choice White Bag Labradoodle Dog Colours Reporter Labradoodle Gift Bag Silhouette of Bag Labradoodle Bag Shoulder (GIFT Royal SAVE AND FREE IN BOX TO Blue amp; PERSONALISED MESSAGE PLEASE Horse Rider GO Backpack GIFT OPTIONS Heritage ADD ENTER NAME zRqHRwunlinkErr) => {
    if (unlinkErr)Bag Cross Pack Messenger for Small Bag 2 Travel Foino Women Sport Body Girls Black Satchel Designer Fashion Shoulder Bag Side f5Ozvgwq 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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