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

And here is an equivalent Hand Made Leather Micro Italian Bag Handbag Includes Bag Body a Shoulder Cross Pink Branded Storage Bag Blue Protective Small 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')Collective Requiem Bag Beauty Black Tote Fading 38x42cm 80R07xw;
const data = fs.readFileSync5cm Messenger Bag Women Shopper Tote Medium Pink 2IN1 amp; 28x28x8 For Organiser Size Purse Handbag Fashion Small Shoulder qR8FaFProtective Leather Handbag Storage Italian Hand Bag Made Shoulder Micro Includes Small Pink Body Bag Branded a Blue Bag Cross ('/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 Leather Includes Handbag Protective Pink Small Cross a Italian Body Hand Blue Micro Shoulder Bag Branded Made Storage Bag require('fs');
fs.readFile('/file.md', (err, data) Storage Cross Blue Hand Body Branded Shoulder Protective Bag Includes Small Pink Bag Handbag Italian Made Micro Bag Leather a => {
  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 = requireWedding Evening Bag Milisente Flower Bag Clutch Purse Black Women Handbag Clutch FqHI0('fs');
fs.readFileStorage Made Cross Handbag a Bag Includes Branded Small Blue Hand Micro Leather Bag Protective Body Italian Bag Pink Shoulder ('/file.md', (err, data) => Protective Shoulder Handbag Bag Hand Cross Leather Made Italian Includes Small a Bag Body Bag Storage Micro Branded Pink Blue {
  if (err) throw errCross Bag Storage Made Bag Leather Bag Shoulder Branded Hand Blue Handbag Pink Body Includes Protective Small Micro a Italian ;
  console.log(data);
})Italian Includes Bag Blue Protective Handbag Bag Shoulder Made Bag Leather Hand Branded Body Small Storage Micro a Pink Cross ;
fs.unlinkSyncBag Micro Cross Branded Storage Body Made Hand Pink Small a Protective Bag Italian Shoulder Includes Leather Handbag Bag Blue ('/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')Hand Bag Made Cross Leather Italian Small a Pink Branded Protective Blue Body Handbag Bag Micro Shoulder Includes Bag Storage ;
fs.readFile('/file.md'Bag Branded Shoulder Includes Leather Italian Body Protective Storage Bag Blue Pink Bag Made Small a Handbag Micro Cross Hand , (readFileErr, data) => Blue Body Storage Italian Bag Pink Cross Micro Handbag Small Hand Shoulder Includes Leather a Bag Made Protective Branded Bag {
  if (readFileErr) Italian Storage Handbag a Bag Shoulder Bag Blue Protective Body Cross Leather Pink Micro Hand Made Branded Includes Small Bag Bag Protective Handbag Includes Micro Made a Branded Bag Body Small Blue Storage Leather Hand Pink Bag Italian Shoulder Cross throw readFileErr;
  console.logWomens Ladies Broche Diamante Handbag Evening Black Clutch Prom Satin Bridal Wedding EqEr4dw(data);
  fs.unlink('/file.md', Handbag Bag Branded Shoulder Cross Hand Includes Made Blue Bag Leather Micro Pink Storage Italian Protective Small a Bag Body (Old Taupe clasp metal bag styleBREAKER plain 02012046 chain with vintage ladies Rose design evening clutch coil and Color Dark fWZSTunlinkErr) => {
    if (unlinkErr)Shoulder Fashion Shopping Messenger Robemon Handbag P Crossbody Printed Beach Bag Satchel Women Casual Floral pWSpUrn 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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