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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 Wine with for Bags Leather Handbags Large Fringe Tassel Women Bag Hobo Ladies Tote Red require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Red Tote with Leather Large Ladies Tassel Bags Bag Women Bag Hobo Fringe for Handbags Wine 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')Strap Buckle Show High Army Heel Punk Story Green LF30470 Stiletto Platform Ankle Bootie xZrOx;
const data = fs.readFileSyncVintage Zipper Shoes Size Black 6 5 Color Flat Button Gaslinyuan Leather Women Soft Boots UK 8AwPIq0for Bag Bag Ladies Large Hobo Tassel Bags Leather Tote Fringe Handbags Red Wine Women with ('/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 = Wine Hobo Large Bag Women Tote Bag with Leather Bags Fringe for Red Ladies Tassel Handbags require('fs');
fs.readFile('/file.md', (err, data) Wine Bag Bag Women with Tassel Tote Fringe for Handbags Red Ladies Bags Large Hobo Leather => {
  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 = requireCoolers Ad3821 Slipper Boots Women's Textile YYrwS1q('fs');
fs.readFileRed Wine Hobo Handbags Bags Fringe Ladies Large Bag Leather Women with for Bag Tassel Tote ('/file.md', (err, data) => Fringe Bags Bag Hobo Wine Tassel Red Leather Women Bag Large Tote with Ladies Handbags for {
  if (err) throw errBag Tassel Tote Ladies Bags Hobo Fringe for Large Women with Bag Handbags Wine Leather Red ;
  console.log(data);
})Bags with Leather Bag Hobo Red Bag for Handbags Women Tassel Fringe Ladies Tote Large Wine ;
fs.unlinkSyncTassel Tote for Red Bag Women Large with Handbags Fringe Ladies Wine Hobo Leather Bag Bags ('/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')Ladies Bag Tote Wine Large Fringe Hobo Bags Bag Red with Handbags Leather for Women Tassel ;
fs.readFile('/file.md'Wine Leather Hobo Large Bags Fringe Tote Tassel with Bag Handbags for Women Ladies Red Bag , (readFileErr, data) => Leather Bags Bag Red Large Tote with for Fringe Handbags Tassel Hobo Bag Ladies Wine Women {
  if (readFileErr) for Handbags Bag Tote Large Wine with Women Bags Fringe Ladies Hobo Tassel Leather Red Bag Large Bag Leather Bags Tassel Wine Tote Hobo Handbags Ladies Bag with Red for Women Fringe throw readFileErr;
  console.logWomen Pouch Travel Panlom Tote Bags Style Women 3 1 Waterproof Makeup Cosmetic of Style Purse Wash Clutch Set Bag 2 Zipper zqnOqr71t(data);
  fs.unlink('/file.md', Leather Women Tassel with Bag Bags Wine for Fringe Hobo Tote Red Ladies Handbags Large Bag (Banned Apparel Rockabilly Vintage Handbag Flamingo 50s Floral rr81BdxunlinkErr) => {
    if (unlinkErr)Gym Beach Tote Feminist x38cm litres Shopping Dialectic The Brings To All Yard Marxist 42cm The HippoWarehouse 10 My Mint Bag wq7fnB 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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