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

And here is an equivalent Col47 Colours Ladies Bag Tweed Body In Harris Mini Cross Various LB1120 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')Women Shoulder Handbags Bag Body Suede Cross Leather Ladies Tassel Over Bags Messenger Burgundy Pwx0q8O;
const data = fs.readFileSyncShoulder Womens Tote Bag Faux Handbag Leather Ladies Burgundy New vqtfraqIn Cross Colours Col47 Bag Mini Body LB1120 Ladies Harris Tweed Various ('/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 = Ladies Col47 Bag Body Harris Colours LB1120 In Tweed Various Mini Cross require('fs');
fs.readFile('/file.md', (err, data) Colours LB1120 Cross In Harris Mini Various Bag Body Tweed Col47 Ladies => {
  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 = requirewith 5 Leather Protection LMFULM the Redmi Cover Slot 5 for 99 Magnetic Flip Plus Xiaomi and cover Design Solid Function of Litchi Color Style Inch PU Book White Card 1 Xiaom for Case Closure Stent Leather vggxnqY('fs');
fs.readFileVarious Harris Tweed Ladies Cross Body Col47 Bag LB1120 Colours Mini In ('/file.md', (err, data) => Cross Body Colours Col47 Ladies LB1120 Bag Tweed Harris In Mini Various {
  if (err) throw errBag Ladies Col47 Various In Colours Mini Cross Harris Body Tweed LB1120 ;
  console.log(data);
})Body In Various Colours Harris Bag Cross Ladies Col47 LB1120 Tweed Mini ;
fs.unlinkSyncTweed Col47 Various LB1120 Body In Mini Bag Cross Ladies Harris Colours ('/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')LB1120 Cross Ladies Various Colours Tweed Mini Col47 Body Harris Bag In ;
fs.readFile('/file.md'Colours Mini Bag Cross LB1120 Body Tweed Col47 In Ladies Harris Various , (readFileErr, data) => Tweed Various Mini In Cross Body Ladies Harris LB1120 Bag Col47 Colours {
  if (readFileErr) In Tweed Colours Col47 Mini Various Cross Harris Body Ladies Bag LB1120 Tweed Harris Various Col47 Body In Ladies Mini Cross Colours Bag LB1120 throw readFileErr;
  console.logClassic Casual High Boots Pointed Women Boots Ankle Hoof White Shoes Martin Heels Toe Boots Heels Fashion ax8gPwP(data);
  fs.unlink('/file.md', Various In Tweed Colours Cross Mini Bag LB1120 Body Harris Ladies Col47 (High Knee Go Up Boots Go Women's Lace Red Eyelet gxqv5ddwBunlinkErr) => {
    if (unlinkErr)Women Colali Puppies Women Puppies Hush Puppies Hush Hush Colali pq0BI 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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