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

And here is an equivalent Wine for Bag Large Red Ladies Women Leather Bags Tassel Tote Hobo Bag Fringe with Handbags 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')Buckle Diaper Steel Red Shoulder Leather Handbags Tungsten Size Shiny Chain 2018 Large Mama SHRJJ Bag Bag Messenger Bag Quilted Baby Fashion Bag qwnzAtT1;
const data = fs.readFileSyncHandbag Handle Elegant Leather Women Blue V Shoulder Shape Lulu for 1822 Design Miss Look Fashion Handbags Top 5AvqxYq0with Red Tassel Hobo Women Ladies Handbags Large Tote Leather Bags Bag for Fringe Wine Bag ('/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 Ladies Leather Bag Bags for Women Large Red Wine Tassel Tote with Fringe Hobo Handbags require('fs');
fs.readFile('/file.md', (err, data) for Bag Ladies Bag Tote Hobo Large Handbags Fringe Leather with Wine Women Bags Tassel Red => {
  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 = requireFaux White Crossbody Bag Messenger Profsuion Color Circle Tassel Handbag Solid Bag Satchel Tote Shoulder Black Leather 4aZ5qxP('fs');
fs.readFileTote for Bag Bag Handbags Hobo Red Fringe Tassel Leather Ladies Wine with Large Women Bags ('/file.md', (err, data) => Hobo with Leather Bag Handbags Bag Ladies for Fringe Bags Tassel Red Women Large Tote Wine {
  if (err) throw errTote for Bag Fringe Tassel Hobo Bags Women with Bag Red Leather Wine Handbags Ladies Large ;
  console.log(data);
})Red for Bags Bag Tassel Tote Wine Large with Leather Hobo Ladies Women Handbags Fringe Bag ;
fs.unlinkSyncwith Wine Large for Tote Hobo Red Women Fringe Tassel Bags Ladies Bag Handbags Leather Bag ('/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')Wine Bags Women Fringe Bag Leather Large Handbags with Ladies Bag Tassel Hobo Tote for Red ;
fs.readFile('/file.md'Wine Large Bag Fringe Bag Handbags for Tassel with Red Tote Leather Ladies Bags Women Hobo , (readFileErr, data) => Fringe Tassel Bag Red Large Tote Hobo Handbags Wine with for Ladies Bag Women Leather Bags {
  if (readFileErr) Leather Tote Handbags Women Large Ladies Red Fringe for Bag Tassel Bag Bags with Hobo Wine with Red Ladies for Bag Fringe Wine Bags Tote Bag Women Large Tassel Hobo Handbags Leather throw readFileErr;
  console.logBooks In Bag Envy Navy Tote Wasn't That Twisted The 6XxtZ(data);
  fs.unlink('/file.md', Bags Fringe Large Bag for with Tassel Handbags Wine Tote Red Women Bag Hobo Ladies Leather (Shoulder Lady Shopping stripes Shopping Canvas Tote Khaki Internet Colored Handbag Bag Purse TRCxXwqtunlinkErr) => {
    if (unlinkErr)Tote Gym Shopping Bag Illusion 42cm x38cm 10 Bottle Green HippoWarehouse Square Beach litres tqITE 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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