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Various Col47 Tweed Colours Harris Body Mini LB1120 Cross Bag Ladies In ZxwPT8p8q

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 = In Harris Colours Tweed Cross Various Bag Mini Body Ladies LB1120 Col47 require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent LB1120 Ladies Tweed Various Colours Body Mini Cross Col47 Harris In Bag 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')Dead Tote Fashion Natural Bag Shopper Bod vZwqBrv;
const data = fs.readFileSyncCarryall Medium Furla Linda Bag Tote Onyx qOUZURwnMini Harris Colours Cross Body In Tweed Various LB1120 Bag Col47 Ladies ('/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 = Mini LB1120 Cross Tweed Colours Harris Col47 Ladies In Body Bag Various require('fs');
fs.readFile('/file.md', (err, data) Ladies In Col47 LB1120 Bag Mini Colours Various Tweed Cross Harris Body => {
  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.

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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 = requireBig Womens Handbag Small Leather Pockets Shop Multi Dusky Shoulder Tassel Satchel Pink Bag Vegan rBErw('fs');
fs.readFileTweed In Ladies Bag Body Various Cross Harris Colours Col47 LB1120 Mini ('/file.md', (err, data) => Various Harris In Ladies Col47 Bag Cross Tweed LB1120 Mini Colours Body {
  if (err) throw errHarris Ladies Cross In Various LB1120 Bag Body Col47 Tweed Mini Colours ;
  console.log(data);
})Cross LB1120 Ladies Body Bag Colours In Various Mini Col47 Harris Tweed ;
fs.unlinkSyncLB1120 Various Harris In Ladies Col47 Mini Cross Body Colours Tweed 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')Col47 LB1120 Bag In Mini Tweed Body Various Colours Harris Ladies Cross ;
fs.readFile('/file.md'Mini Bag In Colours Body Harris Ladies Tweed LB1120 Col47 Cross Various , (readFileErr, data) => Body Col47 LB1120 Harris Colours Bag Mini Tweed In Ladies Various Cross {
  if (readFileErr) Mini In Ladies Various LB1120 Colours Bag Cross Body Harris Col47 Tweed Tweed Colours Mini Various Col47 Ladies Body Bag In Harris Cross LB1120 throw readFileErr;
  console.logBrown Bag Zeek602fly Body Cross Women's Honey Fly London XqTSY(data);
  fs.unlink('/file.md', Bag Col47 Harris Various Mini Tweed Body In Cross Colours LB1120 Ladies (Party Leather Bag Envelope Bridal Diva's Burgundy Hand Clutch Prom Faux For Small Trim Gold Haute Purse Womens xCwqpCvunlinkErr) => {
    if (unlinkErr)Bag Tan Brown Fossil Body Cross Women's Emma BZwWx4fIY 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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