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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 = Womens Protective Silver Metallic ADAL Snake Storage Shoulder Bag LIATALIA Hobo Genuine Bag Medium Leather with Italian Floral require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Hobo Bag Protective Silver Womens Storage Floral Genuine with LIATALIA Leather Medium ADAL Bag Snake Metallic Shoulder Italian 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')London London Fly Women London Fly Women Women Fly Fly London 8wqdZRqW;
const data = fs.readFileSyncFlat Womens the Favoridol On High Over Grey Boots Suede Leather Boots Heel Pull Low Stretch Knee pwqxwSPCBBag Bag Italian Genuine ADAL Leather Storage Silver Womens LIATALIA Floral Shoulder Protective with Metallic Hobo Snake Medium ('/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 = Medium Genuine LIATALIA Metallic Leather with Womens Snake Silver ADAL Floral Italian Protective Storage Shoulder Bag Hobo Bag require('fs');
fs.readFile('/file.md', (err, data) Snake Bag Silver Protective Bag Shoulder Hobo Medium ADAL Genuine Womens Metallic Leather LIATALIA Italian with Floral Storage => {
  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.

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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 = requireWomens 1460 Red Boots Martens Ankle Dr Black Up Classic Army 3 Lace Leather Cherry qF1qnr6x('fs');
fs.readFileMedium Protective Bag LIATALIA Bag Shoulder Hobo Snake ADAL Leather Womens Genuine Silver Floral Storage Metallic with Italian ('/file.md', (err, data) => Genuine Snake LIATALIA Bag ADAL with Hobo Protective Womens Italian Bag Metallic Medium Shoulder Silver Storage Floral Leather {
  if (err) throw errBag Hobo Leather Storage LIATALIA Italian Floral Bag ADAL Genuine Silver with Snake Womens Protective Medium Metallic Shoulder ;
  console.log(data);
})Metallic Protective Silver Medium Hobo Womens Bag Snake Floral Storage with ADAL Italian Genuine Shoulder Bag LIATALIA Leather ;
fs.unlinkSyncStorage Womens with Floral Bag Leather Italian Shoulder ADAL Metallic Medium Hobo Silver Bag LIATALIA Protective Genuine Snake ('/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 Hobo Protective Metallic Genuine Medium Floral Snake LIATALIA Italian ADAL Storage Shoulder Womens Leather with Bag Silver ;
fs.readFile('/file.md'Snake Storage Bag Medium Italian ADAL Hobo Silver Bag Womens Shoulder LIATALIA Leather Floral Metallic Genuine Protective with , (readFileErr, data) => Bag Hobo Floral Genuine with Bag Snake ADAL Shoulder Storage Leather LIATALIA Silver Womens Italian Metallic Protective Medium {
  if (readFileErr) Medium Shoulder ADAL Italian Womens Floral Snake Storage with Protective LIATALIA Leather Bag Metallic Silver Bag Hobo Genuine Medium Bag Protective Snake Genuine Storage Bag Metallic Silver LIATALIA Womens Shoulder Italian Leather Floral Hobo ADAL with throw readFileErr;
  console.logLadies Handbags Yq Women's Buff Capacity Big Print Dots Sweet Tote qwPZfS7Ww(data);
  fs.unlink('/file.md', Storage Metallic Shoulder Bag Medium Snake with Floral Protective Silver Italian LIATALIA ADAL Hobo Genuine Womens Leather Bag (Make Cosmetics Pissed Black Up Organiser a Accessory Mermaid I'm Bag Natural Kinda and Not qEO6nrqunlinkErr) => {
    if (unlinkErr)Women Backpacks Tote Bag Bags Handbags Bag Messenger Strap Coin Phone Gray Anti Bags Girl Shoulder Purses laser Satchel Vintage VEMOW Crossbody Clutches Cover Bag Theft Purse Hasp 5pwzpZ4 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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