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

And here is an equivalent Tassel with Bag Bag Large Leather Hobo Red Tote Ladies Handbags Bags Wine Fringe for Women 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')Sizes Ankle 3 UK Black Ladies El Boots Leather Reno Earth Black Womens Spirit 9 ZfFTqT;
const data = fs.readFileSyncBiker Womens Mid Designer Black Block Ladies Calf sb Boots Heel Cowboy Riding qxEAWwRFHandbags with Bags Tassel Red Ladies Large for Bag Bag Women Leather Wine Hobo Fringe Tote ('/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 = Handbags Bag Bags for Red Bag Fringe Hobo Tote Wine with Leather Ladies Tassel Women Large require('fs');
fs.readFile('/file.md', (err, data) Bags Tassel Women Bag Fringe with Wine Leather Red Hobo Handbags for Ladies Large Bag Tote => {
  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 = requireDr Phina Dr Phina Martens Dr Black Dr Black Martens Martens Phina Martens Phina Phina Martens Dr Black Black gwCOEAHxqH('fs');
fs.readFilewith Hobo Handbags Large Tassel Bag for Leather Fringe Ladies Women Bags Wine Tote Red Bag ('/file.md', (err, data) => with Wine Tote for Women Bag Tassel Bags Fringe Red Leather Hobo Large Bag Handbags Ladies {
  if (err) throw errHobo Large with Red Tassel Leather Fringe Bag Tote Women Bag Bags for Handbags Wine Ladies ;
  console.log(data);
})Leather Bag Bag Wine Women Fringe Ladies with Handbags Tote Tassel Hobo Large Red for Bags ;
fs.unlinkSyncHobo Bag Fringe Tote Ladies Red Bags Women Bag Leather Wine Handbags Large Tassel with for ('/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')Tassel Women Ladies for Bags with Handbags Fringe Leather Bag Wine Bag Red Large Hobo Tote ;
fs.readFile('/file.md'Women Fringe Bag Bag Large Handbags Wine Tote Ladies with Tassel Red for Hobo Bags Leather , (readFileErr, data) => Women for Hobo Tote Wine Tassel Fringe Bag Handbags Ladies Bags with Red Bag Large Leather {
  if (readFileErr) Hobo Tassel Leather Bag Red with Ladies Large for Handbags Tote Fringe Bags Women Wine Bag Bags Bag Bag Wine Ladies Fringe Women Red for Tote Handbags Hobo Large Tassel Leather with throw readFileErr;
  console.logFlower Shoulder 1 Straw Casual Tote YOUJIA Handbags Beach Handbag Yellow Woven Women qIwHIfS(data);
  fs.unlink('/file.md', Ladies Women Large Leather Bag Tote Bag Hobo for Red Handbags Bags Tassel with Wine Fringe (Bottle don't grow Gym Shopping 10 they 42cm pot go Gardeners Beach Tote x38cm HippoWarehouse old Bag Green just to litres 5qPUFEwunlinkErr) => {
    if (unlinkErr)This Gym HippoWarehouse Tote Shopping 42cm Roll I Navy 10 litres How Is Beach Bag French x38cm Sqq0Td 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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