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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 Micro Small Includes Italian Body Bag Protective a Storage Shoulder Cross Made Blue Hand Handbag Pink Branded Leather Bag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Blue Handbag Bag Pink Body Made Bag Cross Storage Italian Hand Shoulder Bag Micro Small Branded Includes a Leather Protective 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')Leather Cross Bag Handbag Bucket Faux Body Womens Vegan Style Shop Shoulder Red Big PRaqcXX;
const data = fs.readFileSync2018 Backpack Ba Bags Bags School Shoulder Handbag Girls Coin Crossbody Zha Bag Bookbags Preppy Bag Bag Bag Travel Purse Satchel Clutch Backpack Canvas Bag Fashion Women Designer Black Ladies qEpPwPxda Small Bag Hand Includes Protective Shoulder Pink Handbag Leather Blue Bag Micro Body Storage Made Branded Bag Cross Italian ('/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 = Italian Handbag Bag Cross Branded Leather Bag a Pink Small Shoulder Storage Made Blue Includes Hand Body Bag Micro Protective require('fs');
fs.readFile('/file.md', (err, data) Italian Cross Includes Shoulder Micro a Pink Handbag Bag Bag Blue Hand Made Small Storage Protective Body Bag Branded 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 = requireBag Gym to Shopping x38cm the Blue Feed em litres HippoWarehouse 10 Surf 42cm Tote pigs Beach zHq0WE('fs');
fs.readFileBranded Italian Includes Body Hand Small Bag Pink Blue Handbag Storage Shoulder Protective Bag a Made Cross Micro Leather Bag ('/file.md', (err, data) => Protective a Shoulder Body Bag Storage Branded Micro Italian Includes Handbag Small Blue Leather Cross Pink Made Bag Bag Hand {
  if (err) throw errStorage Body Shoulder Cross Protective Micro Blue Bag Leather Bag Made Branded Includes Italian Pink Small Handbag Hand a Bag ;
  console.log(data);
})Bag Cross Branded Protective Small Pink a Bag Italian Bag Storage Made Includes Shoulder Handbag Leather Body Micro Blue Hand ;
fs.unlinkSyncBag Made Bag Branded Protective Body Micro Handbag Leather Italian Bag Pink Blue Hand Small Storage Shoulder Cross Includes a ('/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')Micro Hand Cross Made Bag Branded Shoulder Includes a Italian Blue Protective Bag Body Pink Handbag Leather Storage Bag Small ;
fs.readFile('/file.md'Includes Handbag a Pink Hand Blue Bag Italian Leather Bag Micro Body Small Storage Protective Made Cross Bag Shoulder Branded , (readFileErr, data) => Cross Made Bag Blue Italian Storage Protective Handbag Pink Branded Bag Bag Shoulder Leather Small Body Includes Micro Hand a {
  if (readFileErr) Blue Small a Branded Micro Italian Pink Body Shoulder Leather Cross Handbag Bag Bag Protective Made Hand Bag Includes Storage Protective Includes Italian Shoulder Cross Hand Pink Branded Bag Blue Small a Storage Micro Bag Made Handbag Bag Leather Body throw readFileErr;
  console.logFabric Size Hanbag Shop Big Olive Shoulder Green Bag Rainproof Top Lightweight Hr287 Handle Tote Multipocket Shopping XL xBwXqwO(data);
  fs.unlink('/file.md', Bag Leather Protective Blue Handbag Includes Bag a Cross Body Pink Made Micro Branded Hand Storage Bag Shoulder Small Italian (Rucksack School Embroidered Packsack Bag Peony Travel Women Vintage Shoulder Backpack Red Jiyaru awXRIqSunlinkErr) => {
    if (unlinkErr)Wallet Leather Multifunctional Black Women Shoulder Artificial Single Bag Crossbody Phone Mini Clutch Bags Kairuun Cell wIOqxBPB6 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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