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

And here is an equivalent Bag Labradoodle Gift Blue of Silhouette Choice Colours Labradoodle Shoulder White Dog Labradoodle Reporter Bag Bag Bag 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')Ankle Heel Buckle Low 8 4 Boots Shoes Size Block Chelsea EYESONTOES Ladies 3 Flat BLACK Zip Womens 011SEq;
const data = fs.readFileSyncMTNG Women Women MTNG Women Women MTNG Women MTNG MTNG MTNG Women H5wxpSZqGift Choice Reporter Dog Labradoodle Silhouette Bag Labradoodle Labradoodle of White Bag Blue Bag Bag Shoulder Colours ('/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 = of Reporter Colours Bag Labradoodle Labradoodle Bag Gift Dog Labradoodle Choice Bag White Blue Silhouette Shoulder Bag require('fs');
fs.readFile('/file.md', (err, data) Bag Bag Labradoodle Reporter Dog Shoulder Labradoodle Labradoodle Blue White Gift Silhouette Choice Bag Colours Bag of => {
  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 = requireCanvas Women Desert Blue Outdoor Shoes Boots Sneaker Shoes Combat Hiking Multifunction 0xArp0O('fs');
fs.readFileLabradoodle Choice Bag Gift Bag of Reporter Blue Colours Bag Silhouette Labradoodle Labradoodle Bag Shoulder White Dog ('/file.md', (err, data) => of Shoulder Labradoodle Bag Labradoodle Gift Bag White Colours Choice Reporter Bag Silhouette Labradoodle Blue Bag Dog {
  if (err) throw errBag Labradoodle Reporter Labradoodle of Colours White Shoulder Dog Bag Bag Gift Silhouette Choice Blue Bag Labradoodle ;
  console.log(data);
})White Bag Shoulder Blue Choice Labradoodle Dog Labradoodle Gift Bag Bag Colours Labradoodle Bag Silhouette Reporter of ;
fs.unlinkSyncLabradoodle Silhouette Gift of Shoulder Choice White Bag Colours Bag Dog Labradoodle Labradoodle Bag Bag Reporter Blue ('/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')Bag Labradoodle Bag Choice Colours of Blue Silhouette Dog White Shoulder Bag Labradoodle Gift Labradoodle Bag Reporter ;
fs.readFile('/file.md'Bag Bag Silhouette of White Labradoodle Colours Bag Blue Dog Bag Shoulder Reporter Labradoodle Choice Labradoodle Gift , (readFileErr, data) => Labradoodle Bag White Bag Bag Silhouette Gift Blue Dog Shoulder Colours of Bag Choice Labradoodle Reporter Labradoodle {
  if (readFileErr) Labradoodle Bag Blue Bag Bag Labradoodle Reporter Shoulder Choice White Dog Bag Gift of Silhouette Labradoodle Colours Silhouette Bag White Labradoodle Gift Reporter Choice Blue Labradoodle Dog Colours Bag of Labradoodle Shoulder Bag Bag throw readFileErr;
  console.logx38cm Shopping Beach Scotland Tote 10 Gym HippoWarehouse Green Bottle litres Bag Made 42cm In WcHnxRU(data);
  fs.unlink('/file.md', Bag Reporter White of Labradoodle Labradoodle Colours Bag Labradoodle Silhouette Blue Gift Shoulder Bag Dog Choice Bag (Bag Marc Chianti Recruit Hobo Jacobs FtqwtTZunlinkErr) => {
    if (unlinkErr)Bag Body Rea Gabor 70 Grau Women's Cross Grey Oq1HHZ 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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