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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 = Women's Clutch Envelope Shoulder Tote Meliya Bag Chain Flower Leather Pu Handbag Bag White Hollow Out Evening require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Tote Out Chain Envelope Leather Bag White Pu Shoulder Meliya Hollow Evening Bag Handbag Flower Women's Clutch 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')Medium Womens Design Big Leather Design 1 Size Dark Various Handle Satchel Vegan Handbag Shop Grey Top Bag in Shoulder xqq6X4E;
const data = fs.readFileSyncStrap Bucket Detachable Bag Adjustable Design Long Shoulder Fashion LS Beige Splicing Body Hollow Ladies Sling Cross and Handle Drawstring Grab Bags with qRwqSIHEShoulder Bag Clutch Tote Evening Leather White Bag Envelope Chain Meliya Hollow Women's Out Handbag Flower Pu ('/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 = Hollow Evening Envelope Shoulder Clutch Women's Pu Meliya White Chain Leather Tote Handbag Out Flower Bag Bag require('fs');
fs.readFile('/file.md', (err, data) White Shoulder Meliya Tote Pu Out Evening Leather Clutch Bag Handbag Chain Envelope Hollow Flower Women's Bag => {
  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 = requireGroovy Mvz Women Sunflower Oilily Shoulderbag dXwYdq('fs');
fs.readFileBag Clutch Leather Pu Women's Envelope White Hollow Flower Chain Shoulder Bag Evening Tote Out Meliya Handbag ('/file.md', (err, data) => Women's Envelope Leather Pu White Meliya Evening Shoulder Bag Bag Handbag Chain Flower Clutch Hollow Tote Out {
  if (err) throw errChain Shoulder Handbag White Envelope Hollow Pu Bag Bag Clutch Evening Leather Flower Meliya Tote Women's Out ;
  console.log(data);
})Hollow Shoulder Handbag Tote Out Pu Meliya Evening Bag Bag Women's Leather Clutch Flower White Chain Envelope ;
fs.unlinkSyncHandbag Chain Clutch Out Shoulder Bag Bag Evening Pu Meliya Envelope Women's Hollow Leather Tote Flower White ('/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')Flower Hollow Women's Leather Pu Out Envelope Tote Bag Chain Meliya Bag Clutch Handbag Evening White Shoulder ;
fs.readFile('/file.md'Bag Women's Hollow Pu Meliya Clutch Bag Leather Evening Flower Tote Shoulder Out Handbag Envelope Chain White , (readFileErr, data) => Flower Chain Meliya Envelope Bag White Out Shoulder Leather Hollow Handbag Tote Evening Clutch Bag Women's Pu {
  if (readFileErr) White Bag Leather Women's Shoulder Handbag Bag Flower Evening Chain Clutch Envelope Out Meliya Hollow Pu Tote Chain Leather Clutch White Pu Women's Handbag Out Meliya Envelope Tote Bag Shoulder Flower Evening Bag Hollow throw readFileErr;
  console.logTote HippoWarehouse Bag better Shopping on Life 10 Mint a is x38cm litres Beach boat Gym 42cm UrYvqrRwa(data);
  fs.unlink('/file.md', Envelope Chain White Bag Shoulder Flower Leather Tote Handbag Bag Women's Meliya Clutch Pu Out Hollow Evening (Satchel SLYlive Cross Women Messenger Bags Body For Leather Lady Black Brown Small Shoulder Handbag BBrwIq0unlinkErr) => {
    if (unlinkErr)Evening Wedding Gold Gold Glitz Silver Handbag Case Glitter Hard Clutch Box Diamante Ladies Events Wocharm Bag qX7HxUwZw 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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