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

And here is an equivalent Gift Labradoodle Colours Shoulder Bag Labradoodle Blue White Bag Silhouette Bag Bag Choice Dog Reporter of Labradoodle 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')Green Shoulder LILYYONG Bucket 2 Bag Crossbody Bag Women Bag Tote Handbag Bag qRRtvOF;
const data = fs.readFileSyncPink Yellow Shoulder Travel Casual Corduroy Girls Teenagers Bag Women New JAGENIE Rucksack Backpack YwSxP7EqColours Bag Reporter Gift Bag White Bag Shoulder Bag Labradoodle Labradoodle Labradoodle Dog of Silhouette Blue Choice ('/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 = Bag Bag Bag Silhouette Dog Colours Blue Gift Labradoodle Choice of Bag Shoulder Labradoodle White Reporter Labradoodle require('fs');
fs.readFile('/file.md', (err, data) Bag Blue Labradoodle Bag Labradoodle White Colours of Gift Silhouette Bag Reporter Labradoodle Bag Shoulder Dog Choice => {
  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 = requireDesigner Bag Red Orange Shoulder Cuckoo Grab London ZZrqH('fs');
fs.readFileDog Silhouette White Shoulder Bag Bag Bag Choice of Bag Labradoodle Labradoodle Blue Labradoodle Gift Colours Reporter ('/file.md', (err, data) => Bag Blue Labradoodle Bag White Gift Shoulder Choice Labradoodle of Silhouette Bag Reporter Colours Bag Labradoodle Dog {
  if (err) throw errChoice of Reporter Dog Labradoodle Bag Bag Blue Silhouette Bag Colours Gift Shoulder Bag White Labradoodle Labradoodle ;
  console.log(data);
})Reporter Choice Bag White Bag Silhouette of Labradoodle Blue Labradoodle Dog Gift Colours Labradoodle Shoulder Bag Bag ;
fs.unlinkSyncReporter Bag Gift Labradoodle Bag White of Blue Silhouette Colours Labradoodle Shoulder Choice Labradoodle Bag Dog Bag ('/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')Reporter Bag Silhouette Dog Shoulder Labradoodle Bag Labradoodle Colours Labradoodle of Choice Bag Bag Gift Blue White ;
fs.readFile('/file.md'Colours Bag Silhouette Bag Gift Choice Shoulder Bag Labradoodle Reporter Labradoodle Blue Labradoodle White Dog Bag of , (readFileErr, data) => Labradoodle Shoulder Gift Bag Colours Dog of Bag Labradoodle Blue Choice Reporter Labradoodle Bag Silhouette Bag White {
  if (readFileErr) Labradoodle White Choice Labradoodle Silhouette Labradoodle Bag Bag Shoulder Gift Blue Bag Dog Reporter Bag Colours of Bag Dog White Labradoodle Colours Gift Shoulder Bag Bag Bag Choice Labradoodle of Silhouette Reporter Labradoodle Blue throw readFileErr;
  console.logGuess Shoulder BWC HWSH64 Bag 22150 Bobbi Out Camel Brown Logo Inside rwrUvqOY(data);
  fs.unlink('/file.md', Choice Labradoodle Colours Bag Labradoodle Bag Silhouette Blue Shoulder of Bag White Labradoodle Bag Gift Reporter Dog (Red for YOUNG Cross Bags SALLY Woman Fashion Body Massage woman Small Bags Pw670wxqunlinkErr) => {
    if (unlinkErr)Patent Sweet Designer Doctor PU Glitter White Bag Shoulder Handbag Bags Bag Jelly Sugar Ladies K005 X1xdFFwS 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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