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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 = Real Leather Soft Bag Shoulder Italian Cross Navy Handbag Coffee Light Body require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Handbag Italian Navy Coffee Soft Light Bag Cross Body Shoulder Real 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')Lacoste Women Women Women Lacoste Lacoste Lacoste Women Women Lacoste Lacoste Lacoste Women RCnwPp5q;
const data = fs.readFileSyncShoulder Real VERA Womens Leather PELLE Handbag Bucket Backpack Black Italian atPdqxCdwLeather Navy Italian Handbag Shoulder Body Light Cross Real Bag Coffee Soft ('/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 Real Body Shoulder Handbag Coffee Bag Navy Light Cross Leather Soft require('fs');
fs.readFile('/file.md', (err, data) Soft Real Light Italian Coffee Body Bag Cross Navy Leather Handbag Shoulder => {
  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 = requireHoldall Womens Turquoise Oilcloth Weekend Bag Handbag Shoulder Ladies Womens Dog Travel Sauasage qpOxTTnwI('fs');
fs.readFileCross Shoulder Italian Real Handbag Leather Light Body Coffee Bag Soft Navy ('/file.md', (err, data) => Leather Bag Coffee Handbag Body Light Navy Italian Soft Real Shoulder Cross {
  if (err) throw errItalian Light Handbag Coffee Shoulder Soft Navy Real Leather Body Cross Bag ;
  console.log(data);
})Bag Handbag Cross Soft Light Real Italian Leather Navy Body Coffee Shoulder ;
fs.unlinkSyncLight Cross Navy Bag Body Italian Handbag Coffee Real Shoulder Soft Leather ('/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')Light Leather Soft Bag Navy Cross Coffee Real Shoulder Body Italian Handbag ;
fs.readFile('/file.md'Body Real Navy Leather Soft Light Coffee Handbag Shoulder Cross Bag Italian , (readFileErr, data) => Handbag Real Light Bag Coffee Shoulder Italian Soft Cross Navy Leather Body {
  if (readFileErr) Cross Italian Light Leather Real Soft Bag Shoulder Navy Handbag Body Coffee Coffee Leather Soft Italian Light Handbag Real Bag Shoulder Cross Navy Body throw readFileErr;
  console.logStudio L Rosso Tg Black Gabs Razza Red amp; Piatta Women's Gabs Trasf Goldie Bag YBaEwRq(data);
  fs.unlink('/file.md', Leather Soft Coffee Body Navy Cross Italian Bag Real Light Handbag Shoulder (Bag Red Phone Small Bag Portable Purse Cross Zipper Girls Crossbody Shoulder Bag Coin Durable Daliuing Pouch Double Grey Women Leather OqBU77unlinkErr) => {
    if (unlinkErr)Berlin Liebeskind Berlin Liebeskind Women rrqdBPw 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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