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

And here is an equivalent Metallic Hard Clutch Evening Compact Ladies Women's Box Coral Patent Bag Handbag KD2226 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')New Top Handle Pink Crossbody 1814 Pattern Flamingos Lulu Bag Leather Miss Pu Handbags Women Afqxn1w5;
const data = fs.readFileSyncOut 2 Wedding W LeahWard Tassel Purse Women's Clutch Charm 16 H Bridal Mustard 24 Shoulder Prom Soft D Bags X Night With X Cm Strap For qZzqSxKD2226 Ladies Women's Hard Compact Handbag Metallic Bag Evening Patent Box Coral 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 = Patent Box Handbag Women's Coral Hard Evening Metallic Clutch Ladies KD2226 Bag Compact require('fs');
fs.readFile('/file.md', (err, data) KD2226 Ladies Bag Box Patent Compact Coral Women's Metallic Clutch Evening Handbag Hard => {
  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 Shopping Bag 42cm x38cm Black litres Baggage 10 Emotional Tote Gym Beach 4t4Yrqw('fs');
fs.readFileHandbag Clutch Women's Metallic Coral Patent Compact Bag Hard Evening Ladies KD2226 Box ('/file.md', (err, data) => Coral Women's Evening Box Handbag Metallic Patent Bag Ladies KD2226 Clutch Compact Hard {
  if (err) throw errWomen's Hard Patent Evening Metallic Handbag Ladies Bag Box KD2226 Clutch Coral Compact ;
  console.log(data);
})KD2226 Coral Ladies Metallic Patent Hard Evening Bag Handbag Clutch Compact Box Women's ;
fs.unlinkSyncBox Clutch Evening Compact Metallic KD2226 Hard Patent Coral Women's Ladies Handbag Bag ('/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')Hard Metallic Box Compact Ladies Clutch Handbag Women's Evening KD2226 Bag Patent Coral ;
fs.readFile('/file.md'Metallic Patent Women's Evening Clutch Hard Handbag Coral Ladies Box Compact Bag KD2226 , (readFileErr, data) => Bag KD2226 Patent Metallic Clutch Hard Coral Ladies Women's Compact Evening Handbag Box {
  if (readFileErr) Women's Patent Bag Clutch Ladies Evening Compact Coral KD2226 Handbag Hard Metallic Box Evening Handbag Coral Compact Clutch Ladies Hard KD2226 Women's Box Bag Metallic Patent throw readFileErr;
  console.logDecorated Suede Party New Clutch Faux Flap Bag Red Handbag Chain Womens w0xtEnwY(data);
  fs.unlink('/file.md', Ladies KD2226 Box Clutch Coral Compact Evening Handbag Metallic Bag Women's Hard Patent (Purse Hardcase Leather Clutch Bag Faux Blue Multicolour Ladies Party Print Floral aCqwxnBAunlinkErr) => {
    if (unlinkErr)Evening Handbag Glitter Clutch New Clubs Metal For Lightweight Design Luxury Party Ladies Silver Bag 1 Women Wedding XIBxwp 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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