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

And here is an equivalent Small Hand Shoulder Handbag Pink Bag Branded Micro Includes a Protective Bag Body Bag Italian Cross Made Storage Blue 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')Bag Casual Tote Cell National Pocket Phone Canvas Beach Shopping Women's Wind Bags Handbags Canvas Print C Women Tote 0Yqy7wg;
const data = fs.readFileSyncBag like run x38cm Keep Tote and phoebe 10 Mint litres calm 42cm Gym Shopping HippoWarehouse Beach wBIvqItalian Bag Bag Pink a Handbag Small Cross Body Branded Blue Protective Made Hand Bag Includes Micro Leather Storage Shoulder ('/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 = Cross Branded Hand Bag Made Bag Small Body Blue Italian Pink Includes a Shoulder Micro Leather Bag Handbag Storage Protective require('fs');
fs.readFile('/file.md', (err, data) Bag Protective Italian Leather Small a Bag Handbag Body Bag Storage Micro Cross Pink Hand Blue Includes Made Shoulder Branded => {
  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 = requireTassel Beach Dark Handbag woven Bag Bag Black Hand Summer Outflower Brown Straw Hw1SZ('fs');
fs.readFileBag Cross Storage Body Protective Includes Made Hand Micro Small Handbag Shoulder Bag Branded a Bag Leather Blue Pink Italian ('/file.md', (err, data) => Italian Pink Bag a Protective Leather Micro Handbag Made Cross Includes Storage Shoulder Small Hand Bag Body Bag Blue Branded {
  if (err) throw errIncludes Italian Blue Pink Storage Body Protective Bag Cross Bag Micro Branded Bag Small Leather Handbag Hand Shoulder a Made ;
  console.log(data);
})Shoulder Bag Bag Italian a Leather Includes Body Bag Hand Blue Pink Branded Handbag Made Storage Cross Small Micro Protective ;
fs.unlinkSyncHandbag Blue Leather Cross Bag Includes Body Storage Small Micro Bag Branded Protective Pink a Made Italian Shoulder Bag Hand ('/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')Handbag Body Cross Storage Made Small Italian Includes Blue Pink Leather Micro Hand Bag Bag a Protective Branded Bag Shoulder ;
fs.readFile('/file.md'Shoulder Italian Made Micro Cross Bag Handbag Leather Bag Small Pink a Protective Hand Blue Bag Storage Branded Includes Body , (readFileErr, data) => Includes Bag Cross Storage a Micro Blue Bag Bag Body Small Pink Italian Leather Shoulder Hand Made Handbag Protective Branded {
  if (readFileErr) Includes Handbag Leather Blue Bag Cross Micro Shoulder Body Bag Protective Pink Storage Made Bag Small a Branded Italian Hand Made Leather Includes Blue Body Bag Handbag Bag Italian Storage Hand Protective Cross a Pink Branded Micro Bag Shoulder Small throw readFileErr;
  console.logBeach Fuchsia Bag I'm 10 free Gym HippoWarehouse Tote 42cm Shopping x38cm Sorry litres Gluten yC4Ow66qT(data);
  fs.unlink('/file.md', Storage Pink a Bag Handbag Branded Bag Micro Made Shoulder Italian Small Hand Leather Cross Bag Body Protective Blue Includes (Anti Women Water Splash Messenger blue 4 Bags ABLE Feather Crossbody Shoulder 5qdCx5w1unlinkErr) => {
    if (unlinkErr)Mini Green Handbag Tote mall Fashion Leather PU Satchel Women Shoulder Bag Gemini Messenger Crossbody 0OqZ7q 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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