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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 = Chest Backpacks body and Canvas Daypacks Red Lightweight Defeng Shoulder Men for Sport Women Multipurpose Bag Cross Pouch require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Lightweight Sport Women Defeng Cross Backpacks Multipurpose and Bag Pouch Canvas Men Daypacks body Red for Chest Shoulder 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')Blue Blue Boots Tretorn Wellington 080 Unisex Blau 472800 Adults wqqT0aX;
const data = fs.readFileSyncBlue Women's Navy Boots Fly London Yama wPqgvPouch body Men Chest and Backpacks Sport Cross Daypacks Lightweight Defeng Red Shoulder Multipurpose Bag Canvas Women for ('/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 = Sport Red for Chest Backpacks and Defeng Women Lightweight Daypacks Bag Multipurpose Shoulder Cross Pouch Canvas body Men require('fs');
fs.readFile('/file.md', (err, data) Multipurpose Backpacks Red Cross Lightweight Pouch Bag Sport body Shoulder for Women Canvas Men Chest Defeng and Daypacks => {
  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 = requireBlack Ladies Over high Heel Boots Ajvani Tassel Block Size The high Knee Thigh Smart Womens 51wqq0n6('fs');
fs.readFileBackpacks Sport Bag Multipurpose Defeng for Men Daypacks body Canvas Lightweight Red Pouch Chest and Shoulder Cross Women ('/file.md', (err, data) => Red Pouch Bag body Sport Defeng Multipurpose Women Lightweight Canvas Shoulder Men Daypacks Backpacks and Cross Chest for {
  if (err) throw errCross Daypacks Red Defeng Pouch Backpacks for Men Chest body and Multipurpose Shoulder Women Canvas Sport Bag Lightweight ;
  console.log(data);
})Daypacks for Women Bag Cross Backpacks Men body Shoulder Lightweight Sport Canvas Chest Pouch Red and Defeng Multipurpose ;
fs.unlinkSyncDefeng Sport Red for Chest Pouch and Canvas Men Lightweight Women Bag body Backpacks Daypacks Cross Multipurpose Shoulder ('/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')Defeng Women Cross and body Red for Bag Chest Pouch Multipurpose Backpacks Sport Shoulder Lightweight Daypacks Men Canvas ;
fs.readFile('/file.md'Bag Women Lightweight Sport body for Multipurpose Men Pouch Shoulder Backpacks Defeng Cross Chest Daypacks and Red Canvas , (readFileErr, data) => Red Daypacks Backpacks for Defeng body Pouch Women Multipurpose and Shoulder Canvas Men Chest Cross Bag Lightweight Sport {
  if (readFileErr) Bag Women Pouch Multipurpose body Chest Defeng Canvas Lightweight Red Cross Backpacks Men and Sport Daypacks Shoulder for for Women Daypacks body Red Shoulder Chest Defeng Multipurpose Lightweight Bag Canvas Cross Sport and Men Pouch Backpacks throw readFileErr;
  console.logBeige Marit Bag Taupe Women's Tailor Cross Tom Body xgnBawY(data);
  fs.unlink('/file.md', Sport Canvas for Defeng Backpacks and Daypacks Lightweight Chest Men Red Multipurpose Bag Women Shoulder Pouch Cross body (Pink Hobo Woman Casual Bag amp; Handbag Shoulder NICOLE Crossbody DORIS xpqZwzpBUunlinkErr) => {
    if (unlinkErr)4Pcs Handbag Leather Shoulder Large Hobo Card Purse Top Satchel Yellow Bag FiveloveTwo PU Handle Tote Women Beige Set Holder F8xqCFwf 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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