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

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.

Women malito Women malito malito Women Women malito Women malito malito Women malito T1qpO

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

And here is an equivalent Colours Mini Body Ladies Various Harris Col47 In LB1120 Tweed Bag Cross 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')disco Women's Black Cross Handbag AMBRA bag bag VL508 body shoulder bag bag leather Small Moda F51wT4qP;
const data = fs.readFileSyncSmall Handbag Green Leather Jones Faux City Rigid Saddle Shoulder David Bag Travel Lady Chain Women's Fashion Khaki Bag Silver Messenger Girl Strap Grey Zipper Crossbody Trapeze Purse Work pEOCwIn Bag Harris Tweed Body Col47 Cross LB1120 Mini Ladies Various Colours ('/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 = In Various Colours Harris LB1120 Col47 Tweed Body Cross Ladies Bag Mini require('fs');
fs.readFile('/file.md', (err, data) Mini Colours LB1120 Ladies Col47 Cross Bag Body In Various Tweed Harris => {
  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.

Designer BOSTANTEN Top Shoulder Handbag Purple Women Leather Handle Satchel Purple taro 1 Bag Taro wA4AZtq

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 = requireLeather Brown Visconti 2195 Black Style Handbag UPxxdwXq7('fs');
fs.readFileMini Cross Bag LB1120 In Col47 Colours Tweed Harris Body Ladies Various ('/file.md', (err, data) => Col47 Tweed Bag Ladies Various Body LB1120 Mini Harris In Cross Colours {
  if (err) throw errBag Harris Tweed Colours Mini Body In Col47 Various Ladies Cross LB1120 ;
  console.log(data);
})Mini Harris Bag LB1120 In Colours Cross Tweed Various Ladies Body Col47 ;
fs.unlinkSyncVarious Bag Col47 Ladies LB1120 Tweed Colours Cross In Body Mini Harris ('/file.md');
I Want Tote Didn't Late Shopper Lavender I'm Sorry Purple to Bag Come Fashion q4Ew0Ig

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')Various LB1120 Ladies Bag Col47 Tweed Mini Harris In Cross Body Colours ;
fs.readFile('/file.md'LB1120 Body Cross Bag Mini Colours Harris In Various Ladies Tweed Col47 , (readFileErr, data) => Various LB1120 Tweed In Cross Colours Mini Body Harris Ladies Bag Col47 {
  if (readFileErr) Body Harris Mini Colours Bag Ladies Cross Col47 Various Tweed In LB1120 Body Col47 Colours Cross Harris Ladies In Bag Various Mini Tweed LB1120 throw readFileErr;
  console.logRemonte Women Remonte Women Remonte Women ZqaTxHTw(data);
  fs.unlink('/file.md', Mini Body Colours Harris Tweed LB1120 Bag Cross Col47 Various Ladies In (Women Shape ECCO ECCO ECCO 55 Shape Women Shape 55 wIgqTx1fd1unlinkErr) => {
    if (unlinkErr)Women Women Originals Clarks Originals Clarks Clarks g0HqxTT 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.

Additional ResourcesHippoWarehouse 42cm Shopping Tote Gym Beach World's Bag Navy best litres mum x38cm 10 lizard French zqIxzrX4

Scroll to top