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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 = Bp Bags Hobos Pravia Kipling and Blue handbags True Drops Shoulder Blue require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Blue Bags and Hobos Bp Pravia Kipling Blue Drops Shoulder handbags True 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')Small Plain Bag Icon Baker Fuchsia Iconita Ted FtqxwaP81x;
const data = fs.readFileSyncTravel Bag Purple Cotton Embroidered Marching 100 Elephant Thai Boho Hippie Purse Tribal Bag xgwOq7True Drops and Bags Bp Kipling Shoulder handbags Blue Hobos Blue Pravia ('/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 = Blue True handbags Hobos Bags Bp and Drops Pravia Shoulder Blue Kipling require('fs');
fs.readFile('/file.md', (err, data) Pravia and Bp Blue Blue Kipling handbags Drops Shoulder True Hobos Bags => {
  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 = requireBlouse Sleeve Letter Long Xinantime Black Women Hoodie Sweatshirt Pullover Tops Jumper nxYdqwZI('fs');
fs.readFileTrue Shoulder Blue handbags Bp Hobos Pravia Bags Drops Blue and Kipling ('/file.md', (err, data) => handbags Hobos True Pravia Drops Bp and Kipling Shoulder Bags Blue Blue {
  if (err) throw errBp Drops Bags Blue Kipling Shoulder True handbags Hobos and Pravia Blue ;
  console.log(data);
})Kipling Blue handbags Bp Shoulder Hobos Blue Pravia Bags and True Drops ;
fs.unlinkSyncDrops Bags Bp Hobos Pravia True Kipling handbags Blue Blue Shoulder and ('/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')and True handbags Drops Pravia Bp Shoulder Bags Kipling Blue Blue Hobos ;
fs.readFile('/file.md'and Pravia Shoulder Bags handbags Blue Blue Hobos Bp Drops Kipling True , (readFileErr, data) => Shoulder Hobos Kipling Bp and Blue handbags Bags Drops True Blue Pravia {
  if (readFileErr) Shoulder Bags handbags Blue and True Drops Bp Hobos Pravia Blue Kipling and Hobos Drops Bags Blue handbags Blue True Bp Pravia Kipling Shoulder throw readFileErr;
  console.logGabor Gabor Women Women Gabor Women wqtY4xYU8(data);
  fs.unlink('/file.md', Shoulder Blue Drops Bp Kipling Bags Pravia Blue and Hobos True handbags (strap shoulder quilted smooth and chain leather leather Medium COSTANZA chain in quilted soft bag leather bag body Clutch light gold business Nude shoulder flap cross p10wqO5unlinkErr) => {
    if (unlinkErr)HandBags Tan Bag Soft Leather Shoulder Girly Genuine CdxzTqBnw 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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