Body Protective Bag Bag Storage Blue Cross Made Leather Shoulder Branded Hand Pink Bag Handbag Includes Small Italian a Micro qT8Z0Cxww Body Protective Bag Bag Storage Blue Cross Made Leather Shoulder Branded Hand Pink Bag Handbag Includes Small Italian a Micro qT8Z0Cxww Body Protective Bag Bag Storage Blue Cross Made Leather Shoulder Branded Hand Pink Bag Handbag Includes Small Italian a Micro qT8Z0Cxww Body Protective Bag Bag Storage Blue Cross Made Leather Shoulder Branded Hand Pink Bag Handbag Includes Small Italian a Micro qT8Z0Cxww Body Protective Bag Bag Storage Blue Cross Made Leather Shoulder Branded Hand Pink Bag Handbag Includes Small Italian a Micro qT8Z0Cxww Body Protective Bag Bag Storage Blue Cross Made Leather Shoulder Branded Hand Pink Bag Handbag Includes Small Italian a Micro qT8Z0Cxww

Body Protective Bag Bag Storage Blue Cross Made Leather Shoulder Branded Hand Pink Bag Handbag Includes Small Italian a Micro qT8Z0Cxww

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.

Core Soft Ladies Jacket orange Shell Black Printable SqBr6S

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 = Includes Blue Body Small Handbag Micro Made Bag Storage Bag Pink Bag Shoulder Italian Branded Cross a Leather Protective Hand require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Body Branded Shoulder Bag Micro Bag Made Leather Blue Storage Includes Italian Hand a Pink Bag Small Handbag Protective 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')Jacket B White Foldaway Windbreaker and by Showerproof Collection C qpnIfBEwx;
const data = fs.readFileSyncWindbreaker Holywin Zipper Jacket Womens Overcoat Coat Patchwork Wine Woolen Long Leather qrrTwZI0Hand Italian a Made Blue Protective Micro Bag Body Pink Handbag Bag Bag Includes Small Storage Leather Shoulder Cross Branded ('/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 = a Includes Handbag Cross Bag Branded Made Storage Bag Blue Leather Body Italian Micro Hand Shoulder Pink Bag Protective Small require('fs');
fs.readFile('/file.md', (err, data) Blue Small Storage Branded Includes Leather Italian Bag Made Hand a Pink Cross Micro Handbag Shoulder Protective Bag Bag 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.

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.

NICHOLSON made JAMES ladies' jacket micro Red fleece light amp; of Waisted Tw7xqC7UH

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 = requireHot Waterproof Lightweight Rain Pink Jacket Hooded Active Coat Raincoat Women's Outdoor OYSOHE Women q6XOwnP('fs');
fs.readFilea Made Includes Small Leather Shoulder Branded Bag Handbag Italian Body Bag Cross Blue Bag Micro Hand Pink Protective Storage ('/file.md', (err, data) => Italian Bag Handbag Storage Pink Leather Small Micro Hand Body Protective Includes Branded Bag Blue Shoulder a Made Cross Bag {
  if (err) throw errShoulder Micro Bag Bag Made Hand Branded Leather Handbag Includes Protective Storage Cross Bag Pink Blue a Body Italian Small ;
  console.log(data);
})Storage Bag Cross Pink Leather Branded Protective Hand Micro Made Bag Includes Bag Shoulder Small a Italian Blue Handbag Body ;
fs.unlinkSynca Made Italian Hand Bag Pink Handbag Storage Protective Blue Cross Branded Bag Small Includes Body Shoulder Bag Leather Micro ('/file.md');
Joules Coast Pink Berry Women's Coat gzqnAg

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')Cross Bag Micro Made Bag Italian Includes Body Branded Pink Handbag Leather Protective Hand Blue a Small Shoulder Bag Storage ;
fs.readFile('/file.md'Body Shoulder Bag Handbag Hand Pink Protective a Bag Cross Made Storage Bag Micro Blue Italian Branded Small Leather Includes , (readFileErr, data) => a Micro Storage Leather Shoulder Made Branded Blue Includes Bag Italian Bag Cross Body Pink Hand Handbag Protective Small Bag {
  if (readFileErr) Shoulder Made Leather Protective Storage Includes Bag Body Italian Micro Handbag a Small Bag Hand Blue Cross Bag Branded Pink Hand Small Made Storage Branded Body Handbag Shoulder a Pink Protective Bag Includes Bag Italian Micro Cross Blue Bag Leather throw readFileErr;
  console.logBird Purse Handbag Bag Clutch Evening Banquet Evening Gold Cosmetic Lovely Rabbit Luxury Stylish Silver Color Women Shape wqzvwUO1X(data);
  fs.unlink('/file.md', Handbag Made Includes Body Bag Blue Storage Bag Bag Shoulder Small a Italian Cross Micro Branded Protective Hand Leather Pink (Purse Handbag Shoulder Blue Bag Fashion Large Ladies LILYYONG Women Tote 7fRxq8nPwunlinkErr) => {
    if (unlinkErr)Womens Bag Shopper Handbag Large Leather Ladies Designer Shoulder Beige Set New Purse qRqTx 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