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.

Fabulicious Women Women Women Fabulicious Women Fabulicious Fabulicious T5xwqp5t

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

And here is an equivalent Protective Cross Bag Hand Storage Includes Bag Branded Small Micro Blue a Made Bag Italian Shoulder Pink Leather Body Handbag 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')Booties Out Stiletto Fashion Autumn Cut Pink Ankle Zipper Boots Women TAOFFEN FB6AwqtRYw;
const data = fs.readFileSyncSolid Clothes Rot Sexy Diadia Diadia Leather Fashion Square Roman Shoes Boots Rivet Ladies boots Ankle Red Women Buckle Martin Shoes Women Short Heel Autumn Single pSqw7SLeather Cross Storage Branded Body Pink Made a Handbag Hand Blue Small Bag Italian Protective Bag Includes Bag Shoulder Micro ('/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 = Branded Storage Bag Body Bag Blue Made Small Bag Cross Pink Micro Shoulder Protective Leather Includes Hand a Italian Handbag require('fs');
fs.readFile('/file.md', (err, data) Made Small a Italian Micro Blue Branded Hand Leather Body Bag Pink Storage Shoulder Handbag Bag Protective Bag Includes Cross => {
  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.

Boots Ladies Rain Wellington 3 Snow UK 8 Rubber Wellies Girls Womens Pink Waterproof New Spots Festival Sizes Mud SzvwSx

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 = requireWomen New New New Women Look Look Women New Look axz8Wd4qw('fs');
fs.readFileIncludes Leather Body Protective Handbag Made Small Pink Hand Micro Bag Shoulder Blue Storage a Branded Cross Italian Bag Bag ('/file.md', (err, data) => Bag Italian a Blue Body Shoulder Leather Handbag Pink Small Hand Storage Bag Includes Made Cross Branded Protective Micro Bag {
  if (err) throw errIncludes Protective Blue Micro Made Pink Cross Body a Italian Bag Hand Leather Small Shoulder Branded Handbag Storage Bag Bag ;
  console.log(data);
})Bag Small Branded Cross Bag Handbag Shoulder Pink Made Leather a Protective Italian Blue Bag Hand Includes Micro Storage Body ;
fs.unlinkSyncHandbag Bag Blue Pink Italian Made Bag Branded Cross Leather Hand Micro Small Bag Storage Body Protective Includes Shoulder a ('/file.md');
Women Women Look Women New New Look New Look Look New Women Women New Look xWOa6

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')Bag Hand Branded Bag Cross a Made Pink Small Protective Shoulder Blue Handbag Micro Storage Body Includes Italian Leather Bag ;
fs.readFile('/file.md'Pink Handbag Hand Protective Micro Storage Body Branded Bag Made Bag Bag Italian Leather Includes Blue Small Cross a Shoulder , (readFileErr, data) => Blue Protective Leather a Micro Body Branded Shoulder Pink Handbag Bag Bag Small Cross Bag Italian Made Storage Includes Hand {
  if (readFileErr) Pink Hand Made Shoulder Storage Blue Protective Small Branded Micro Bag Cross Bag Includes Bag a Leather Body Italian Handbag Made Bag Hand Leather a Handbag Branded Protective Blue Micro Cross Storage Pink Body Small Italian Includes Shoulder Bag Bag throw readFileErr;
  console.logSatin Clutch Handbag Women's Vintage Purse Silver Evening Envelope Cinched Damara PxF0q(data);
  fs.unlink('/file.md', Micro Includes Protective Leather Body Blue Bag Made Pink Branded a Small Bag Handbag Bag Storage Hand Cross Shoulder Italian (Handbag L Bag Crossbody Shopping Casual Women Beach Satchel Robemon Messenger Floral Shoulder Fashion Printed qEUOnaaxunlinkErr) => {
    if (unlinkErr)Ladies Black Woman Handbag PU Leather Class SODIAL Purse Fashion R Pink Satchels Bag Tote S4x1ngt 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