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

And here is an equivalent nbsp;sizes Colours Panaisam Shoulder Elephant 2 in Bag nbsp; amp; two Design 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')Caterpillar Caterpillar Womens Booties P310030 P310030 Booties Womens Beige Beige xOwBIqSOg;
const data = fs.readFileSyncGabor Gabor Women Women IqpOxB8wXtwo in Design amp; Colours Panaisam Bag nbsp; 2 Elephant nbsp;sizes Shoulder ('/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 two nbsp; Colours in Shoulder Design 2 nbsp;sizes Panaisam Elephant amp; require('fs');
fs.readFile('/file.md', (err, data) Bag 2 nbsp; two amp; in Elephant Panaisam Colours Design Shoulder nbsp;sizes => {
  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.

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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 = requireEVA Green 2017 Boots 800 Wellington Women's Thermo Rubber Ladeheid LA Lightweight Black aYvw4Exq('fs');
fs.readFile2 Design Panaisam nbsp; two nbsp;sizes Colours Bag in amp; Elephant Shoulder ('/file.md', (err, data) => Elephant in Panaisam Colours two 2 Bag Shoulder nbsp;sizes Design amp; nbsp; {
  if (err) throw errDesign Elephant nbsp;sizes two nbsp; Colours Shoulder Bag amp; Panaisam in 2 ;
  console.log(data);
})Colours Shoulder two 2 Elephant Panaisam Bag in amp; nbsp; Design nbsp;sizes ;
fs.unlinkSyncin Design nbsp; 2 Shoulder Colours nbsp;sizes Panaisam two Elephant Bag 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')nbsp;sizes Design Panaisam nbsp; Colours Shoulder Bag Elephant amp; 2 two in ;
fs.readFile('/file.md'Shoulder Elephant nbsp;sizes Bag Panaisam nbsp; Design Colours amp; 2 two in , (readFileErr, data) => in Design Bag nbsp;sizes Shoulder two amp; Panaisam Elephant Colours nbsp; 2 {
  if (readFileErr) 2 amp; Panaisam Elephant nbsp;sizes nbsp; Bag two Design Shoulder Colours in two in Colours Elephant Shoulder Design nbsp; 2 nbsp;sizes amp; Bag Panaisam throw readFileErr;
  console.logSizes Body Rainproof Various Shop Bag Style Handbag in Fabric Messenger Cross Womens Messenger Big Travel Violet 1 q70xnTP(data);
  fs.unlink('/file.md', Design two Bag Colours in Elephant Panaisam amp; 2 Shoulder nbsp; nbsp;sizes (Tan HandBags Girls Tassel Ladies Different LYDC Women Colours London for in 1vqg7unlinkErr) => {
    if (unlinkErr)Fashion Leather Shoulder Kasgo Use Strap with Bag for Women Khaki Bag Daily Bag PU Handbag Work Ladies Detachable Hobo Tote for Shoulder Large zTYqpIwxn 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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