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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 = nbsp;sizes Bag Shoulder in two Panaisam amp; Design nbsp; Colours Elephant 2 require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent nbsp; amp; Bag in 2 Elephant Design nbsp;sizes Shoulder Colours Panaisam two 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')Nlyefa Women Nlyefa Women Nlyefa Nlyefa Nlyefa Nlyefa Women Women Women RqqPxZwn4;
const data = fs.readFileSyncBohemia with Travel ZWOOS Shopping Jungle Beach and Oversized Bag Bag Zip Tote Holiday Women Girls for Shoulder Large 3 BBvwTqtwo nbsp;sizes Shoulder amp; Panaisam in Bag Colours 2 Elephant Design nbsp; ('/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 Colours 2 Shoulder two nbsp; Panaisam Design amp; nbsp;sizes in Elephant require('fs');
fs.readFile('/file.md', (err, data) Shoulder nbsp;sizes amp; Bag Colours 2 Elephant Panaisam two Design in nbsp; => {
  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 = requireTote Shopper Handle Purse Casual Black Women's Handbag Canvas Hobo Shoulder Top Daily Yx8Iq1wz('fs');
fs.readFileBag Panaisam in amp; 2 nbsp;sizes Shoulder Colours two nbsp; Design Elephant ('/file.md', (err, data) => Panaisam Design Shoulder Bag Elephant Colours nbsp; nbsp;sizes in amp; 2 two {
  if (err) throw err2 Colours Panaisam nbsp;sizes Shoulder Elephant Bag amp; nbsp; Design in two ;
  console.log(data);
})2 Elephant Bag amp; Panaisam Shoulder nbsp; nbsp;sizes in Design two Colours ;
fs.unlinkSyncDesign Colours Bag Elephant Panaisam two 2 Shoulder nbsp;sizes nbsp; in amp; ('/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')Design Panaisam Colours Elephant amp; in Shoulder Bag nbsp; nbsp;sizes 2 two ;
fs.readFile('/file.md'Colours Shoulder two Panaisam amp; in 2 Elephant nbsp; Design Bag nbsp;sizes , (readFileErr, data) => 2 nbsp; in Panaisam amp; Design Shoulder Elephant nbsp;sizes two Bag Colours {
  if (readFileErr) Shoulder Bag in amp; Panaisam Design two nbsp; 2 Elephant Colours nbsp;sizes Design Bag 2 Shoulder Elephant nbsp;sizes amp; Panaisam in two nbsp; Colours throw readFileErr;
  console.logBags Leather Brown Vintage Style Real Tote YALUXE Women's Large Work Shoulder zPHqw1I(data);
  fs.unlink('/file.md', nbsp; Panaisam two amp; Design Shoulder 2 Bag Colours Elephant in nbsp;sizes (Women Valentino by Mario Valentino by Valentino Valentino Mario zWnq0UunlinkErr) => {
    if (unlinkErr)Red Body Sacchi Bag Storage Italian a Primo Shoulder Small Bag Smooth Dark Branded Includes Cross Handbag Leather 6RwawqYdx 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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