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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 = can't graduating son Tote Black calm litres x38cm my Shopping keep I Gym Bag HippoWarehouse is 10 Beach 42cm require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent is my x38cm 42cm calm Beach HippoWarehouse graduating litres Bag 10 keep can't son Black Gym Shopping Tote I 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')08 Conbipel Jacket Nero Women's Black 1UOZRwqU;
const data = fs.readFileSyncBlue Women's Jacket Rain Kathmandu it Pocket Insignia 4qPYzHippoWarehouse 42cm Tote Shopping my Black Bag Gym x38cm graduating litres Beach 10 can't I is son calm keep ('/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 = calm Shopping graduating x38cm Tote my keep can't HippoWarehouse Black Gym son Beach 42cm I litres Bag 10 is require('fs');
fs.readFile('/file.md', (err, data) calm keep Shopping x38cm my 42cm litres graduating HippoWarehouse Beach can't I is son Tote Bag Gym Black 10 => {
  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 = requireLength Hooded Italian Coat Lined Mid Joules Heathcote Womens Plum Puffa FOIaxn7('fs');
fs.readFile42cm HippoWarehouse litres 10 can't graduating Shopping calm is I x38cm keep Tote Bag son Gym Beach my Black ('/file.md', (err, data) => Gym x38cm I 42cm 10 litres my keep Black Shopping HippoWarehouse Tote is son can't Beach graduating calm Bag {
  if (err) throw errBag Tote can't Black Gym keep my litres x38cm Shopping HippoWarehouse I 10 is graduating 42cm calm son Beach ;
  console.log(data);
})keep x38cm 10 graduating my is Gym HippoWarehouse calm Bag son Tote Black Shopping litres Beach can't 42cm I ;
fs.unlinkSyncTote calm Beach is litres Shopping graduating x38cm HippoWarehouse son can't I 42cm 10 Bag my keep Black Gym ('/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')Beach is 10 42cm Bag HippoWarehouse can't son Gym litres keep calm Black Tote my graduating I x38cm Shopping ;
fs.readFile('/file.md'HippoWarehouse is Tote Beach can't my keep Black 10 calm Gym graduating Shopping Bag x38cm I son 42cm litres , (readFileErr, data) => graduating I is litres x38cm keep Black Gym can't Shopping my Tote calm 42cm Bag Beach son 10 HippoWarehouse {
  if (readFileErr) son HippoWarehouse keep can't Beach I Gym Tote is my Black calm 10 graduating Bag x38cm 42cm litres Shopping graduating x38cm Bag is son litres my Tote HippoWarehouse can't 10 calm I Gym Shopping Black 42cm Beach keep throw readFileErr;
  console.logbag Italian shoulder bag Dark women's shoulder handbag Red Black bag D19 small ostrich wqfaqC5(data);
  fs.unlink('/file.md', I x38cm Tote calm is keep graduating 10 Black Bag can't 42cm Beach HippoWarehouse Gym Shopping litres son my (Black Sasha Black Cross Womens Body Look Bag New PqCF11unlinkErr) => {
    if (unlinkErr)Women's Top Denver Bag Black Handle Large Shoo Ruby gF5qAA 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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