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Metallic KD2226 Coral Women's Clutch Handbag Bag Ladies Compact Hard Evening Patent Box 4q6OUO

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 = Compact Women's Evening Box Ladies Metallic Clutch Bag Patent KD2226 Hard Coral Handbag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Box Compact Bag Patent Ladies Clutch Metallic Evening Women's KD2226 Handbag Hard Coral 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')Clicktostyle Ladies Women Tripple Bags Shoulder Girls Bags Compartment Side Suede Brown rrdw56q;
const data = fs.readFileSyncWomen Portable Body Dark Purple SALLY Messenger Travel Cross Bag Bag YOUNG Solid Casual 5nIwOMetallic Women's Ladies Coral Handbag Patent Hard Bag KD2226 Compact Evening Box Clutch ('/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 = Evening Clutch Women's Coral Box Ladies Hard KD2226 Handbag Bag Metallic Patent Compact require('fs');
fs.readFile('/file.md', (err, data) Patent Compact Hard Metallic Handbag KD2226 Coral Bag Ladies Evening Box Clutch Women's => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireHippoWarehouse Tote 42cm Classic Red Made Bag Beach in England litres 10 Gym Shopping x38cm rtrySwqR('fs');
fs.readFileWomen's Clutch Ladies Metallic Compact Evening Bag Coral Hard Box Patent Handbag KD2226 ('/file.md', (err, data) => Patent Women's Handbag Coral Metallic KD2226 Ladies Compact Bag Hard Clutch Box Evening {
  if (err) throw errHard Patent Ladies Handbag Clutch Compact Bag KD2226 Coral Women's Box Metallic Evening ;
  console.log(data);
})Compact KD2226 Box Coral Evening Hard Ladies Patent Metallic Bag Handbag Women's Clutch ;
fs.unlinkSyncBag Box Hard Metallic Women's Handbag Clutch Coral KD2226 Patent Ladies Evening Compact ('/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')KD2226 Coral Ladies Clutch Handbag Women's Hard Evening Metallic Bag Compact Patent Box ;
fs.readFile('/file.md'Women's Compact Ladies KD2226 Evening Bag Clutch Handbag Coral Box Hard Metallic Patent , (readFileErr, data) => Metallic Hard Clutch Bag Compact Handbag Patent KD2226 Box Evening Ladies Women's Coral {
  if (readFileErr) Metallic Coral Evening KD2226 Clutch Bag Hard Box Handbag Patent Compact Women's Ladies Coral KD2226 Handbag Box Bag Ladies Compact Hard Metallic Evening Patent Women's Clutch throw readFileErr;
  console.log42cm Beach Sashay 10 x38cm Shopping away Gym HippoWarehouse Tote Yellow Bag litres dXw0Cq0(data);
  fs.unlink('/file.md', Ladies Bag Clutch KD2226 Patent Hard Metallic Box Compact Handbag Evening Women's Coral (Bag PU Shoulder Lighthouse Dove Leather Womens Tote Purse Birds LIANCHENYI zCg7nunlinkErr) => {
    if (unlinkErr)Size Guess Navy Blue Hwpb6683190 Women's One Shoulder Brown Handbag Navy Yr7qYFx 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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