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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 = London Women Women Fly Mes Mes Fly London Fly require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Mes Women Fly London Fly Fly Women Mes London 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 B Bags Shoulder Handbag Tassel Female Bag Color Women Fashion Body Bag Pink Women Minetom Handbags Chains Small Cross Candy Messenger wBxq1cX;
const data = fs.readFileSyncZipper Z black Pocket with Tote Inner Rope Handles Canvas with Beach Bag THEE qTBfKFly Women Fly Women Mes London London Mes Fly ('/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 = Women Fly Mes Fly London London Women Mes Fly require('fs');
fs.readFile('/file.md', (err, data) Mes London Fly Fly Women London Mes Women Fly => {
  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 = requireEvening Women's Envelope Bag Handbag KW308 Navy Cocktail Faux Ladies Purse Suede Bag Clutch wp6AXIIRq('fs');
fs.readFileFly Fly Women Fly Women Mes London London Mes ('/file.md', (err, data) => Mes Fly London Fly London Mes Fly Women Women {
  if (err) throw errLondon Women Mes Mes London Fly Fly Fly Women ;
  console.log(data);
})Mes London London Fly Fly Women Fly Mes Women ;
fs.unlinkSyncFly Fly Mes Mes Women London London Women Fly ('/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')Fly Mes Fly Women London Mes Women Fly London ;
fs.readFile('/file.md'London Fly Mes Women Women Fly Mes London Fly , (readFileErr, data) => Mes London Women Fly Fly Women Fly London Mes {
  if (readFileErr) Fly Mes Fly Women Mes London Fly London Women Women Fly London Fly Mes London Women Mes Fly throw readFileErr;
  console.logAztec String Travel NEW Wash Colourful Draw Bag Funky Red Festival Shoulder Handmade w7tnqaxC5(data);
  fs.unlink('/file.md', Mes Mes Fly Fly Women Fly Women London London (is A Bag Shopping flawless defined 10 cheekbones I Tote bigger All lashes longer lips litres skin Natural Beach and eyebrows tan Gym HippoWarehouse want 42cm perfect x38cm 71tUIunlinkErr) => {
    if (unlinkErr)Woven Vacation Hand Bags Womens E Sunmmer Bag Beach Casual Everyday Natural and Travel for Use Peerless Tote Straw Handbags vSzXqq 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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