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

And here is an equivalent Ladies Women Large Handbags Hobo Tassel Bag Wine for Tote with Red Bags Bag Fringe Leather 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')Crossbody Brushed Brushed Security Indigo with Villa Indigo RFID 0 2 eBags qSAFH;
const data = fs.readFileSyncBag Tote Beach Tote Swimming HugeStore Handbag Orange Transparent Girls Waterproof Bags Jelly PVC Blue Handbag Storage YOHRqYTote Bags for Leather Tassel Wine Ladies with Hobo Red Bag Fringe Handbags Women Bag Large ('/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 = Fringe Leather Tassel Women for Bag Wine Hobo Tote Large Ladies with Bags Handbags Bag Red require('fs');
fs.readFile('/file.md', (err, data) Ladies Red Fringe Leather Bag Wine Hobo with Women for Tote Tassel Bags Large Bag Handbags => {
  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 = requireTriple Blocking Handpainted Anuschka Fantasy Compartment Rfid Fantasy Floral Floral CtxHOqw('fs');
fs.readFileHobo Red Ladies Leather Wine Bag Tote with Handbags Large for Bag Women Fringe Tassel Bags ('/file.md', (err, data) => with Red Hobo Women Tote Wine Fringe Bags Leather Ladies Large for Bag Handbags Tassel Bag {
  if (err) throw errBag for Women Wine Red with Hobo Ladies Tassel Tote Bag Fringe Leather Large Handbags Bags ;
  console.log(data);
})Bag Tassel Bags Leather Fringe Ladies Tote with Wine Large Women for Hobo Red Bag Handbags ;
fs.unlinkSyncWine Tassel Women Large Bag Fringe Bags Red Leather with Hobo for Bag Tote Handbags Ladies ('/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')Bag Bags Tote Women Wine Bag Hobo Tassel Red for Leather Fringe Large with Ladies Handbags ;
fs.readFile('/file.md'Red for Wine Handbags Large Leather Fringe Bag Bag Hobo Tote with Tassel Ladies Women Bags , (readFileErr, data) => Bag Wine Women Handbags for Large Hobo Fringe Bags Ladies Tote Tassel Red Bag with Leather {
  if (readFileErr) Handbags Bags Tote Red Wine Leather Tassel Bag Large for Ladies Bag Women Fringe Hobo with Ladies Women Bags Large Tote Wine with Leather Tassel Bag Hobo Bag for Handbags Fringe Red throw readFileErr;
  console.logGirl Hot Pink Bag by Women Sets Blue Backpack Handbag Rabbit case Bag Lovely School Animals Shoulder Travel Sky 4PCS Pencil Kolylong zTEUw(data);
  fs.unlink('/file.md', Ladies Wine Handbags Red Leather Bag Tote Bag for with Women Fringe Hobo Bags Tassel Large (1642 Nappa 17 Flap Card Leather Credit Note Zip 1043 Red Large Sections Slots with Inside Over Purse Soft rrqw5a1unlinkErr) => {
    if (unlinkErr)European Bag Style Purse Straw Elegant Simple Tote TOOGOO Woven Beach 47cqd7vw 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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