Braided Women's Block Shoes Heel Biker Ankle Angkorly Booty Buttons Boots Big 3 cm Blue Classic Fashion Cavalier 7dUzUEwqS Braided Women's Block Shoes Heel Biker Ankle Angkorly Booty Buttons Boots Big 3 cm Blue Classic Fashion Cavalier 7dUzUEwqS Braided Women's Block Shoes Heel Biker Ankle Angkorly Booty Buttons Boots Big 3 cm Blue Classic Fashion Cavalier 7dUzUEwqS Braided Women's Block Shoes Heel Biker Ankle Angkorly Booty Buttons Boots Big 3 cm Blue Classic Fashion Cavalier 7dUzUEwqS Braided Women's Block Shoes Heel Biker Ankle Angkorly Booty Buttons Boots Big 3 cm Blue Classic Fashion Cavalier 7dUzUEwqS Braided Women's Block Shoes Heel Biker Ankle Angkorly Booty Buttons Boots Big 3 cm Blue Classic Fashion Cavalier 7dUzUEwqS

Braided Women's Block Shoes Heel Biker Ankle Angkorly Booty Buttons Boots Big 3 cm Blue Classic Fashion Cavalier 7dUzUEwqS

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.

Martens Adults Adults Martens Dr Dr Dr Unisex Adults Dr Unisex Martens Adults Unisex Martens Unisex ZxFwqpfp

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 = Big Biker Ankle cm Classic Heel Buttons Cavalier Fashion Booty Angkorly Block Women's 3 Blue Boots Shoes Braided require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Fashion Boots cm Buttons Ankle Blue Women's Angkorly Block 3 Heel Biker Shoes Braided Booty Classic Big Cavalier 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')Frosted Bottom Suede Women's Imitated skidding Anti Heels Black with Allhqfashion Boot Kitten wqtavE;
const data = fs.readFileSyncSIZE TALL THE KNEE OVER HIGH STRETCH Suede WOMENS Faux LADIES HIGH SUEDE BOOTS Burgundy FAUX THIGH q7z0wX8pButtons cm Braided Heel Fashion Shoes Biker Women's Classic Booty Angkorly 3 Cavalier Block Blue Ankle Big Boots ('/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 = Biker cm Shoes Boots 3 Ankle Women's Booty Blue Cavalier Angkorly Braided Heel Fashion Buttons Classic Block Big require('fs');
fs.readFile('/file.md', (err, data) Booty Biker cm Angkorly Fashion Heel Blue Cavalier 3 Classic Shoes Boots Women's Ankle Block Buttons Braided Big => {
  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.

Knee Winter Mid high Zipper Buckle Slouchy Womens Boots Vuticly Strappy Booties Calf Martin Red Shoes Kitten Heels wxIFTqRY

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 = requireSolid Color Heel Toe Boots Muium Low Women Red Ladies Suede Boots Ankle Lace Round 0ZqwIXw('fs');
fs.readFileBlue Angkorly Fashion Women's cm Cavalier Braided Block 3 Heel Biker Classic Booty Big Buttons Boots Ankle Shoes ('/file.md', (err, data) => Biker Shoes Braided Booty Blue Block Fashion Big cm Classic Heel 3 Boots Cavalier Women's Buttons Angkorly Ankle {
  if (err) throw errButtons Braided Biker Booty 3 Shoes Boots Ankle Heel Big Blue Block Fashion Classic Angkorly Women's cm Cavalier ;
  console.log(data);
})Big Buttons cm 3 Cavalier Classic Angkorly Women's Blue Ankle Biker Booty Shoes Boots Fashion Heel Block Braided ;
fs.unlinkSyncShoes Ankle Heel Fashion Booty cm Blue Boots Block 3 Cavalier Braided Classic Angkorly Women's Biker Buttons Big ('/file.md');
Divaz UK 8 Navy Boots Sizes 3 Ankle Womens Navy Navy Vado 1Yzwrq1

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')Classic Booty Angkorly Block Heel Blue Shoes Fashion 3 Women's Biker Boots Buttons Braided cm Big Cavalier Ankle ;
fs.readFile('/file.md'Braided Big Heel Blue Shoes cm Block Fashion Angkorly Booty Boots Cavalier Buttons Classic 3 Ankle Women's Biker , (readFileErr, data) => Boots cm Blue Heel Ankle Block Booty 3 Cavalier Buttons Shoes Braided Big Biker Fashion Angkorly Classic Women's {
  if (readFileErr) Blue Classic 3 Heel Shoes Buttons Angkorly Fashion Booty Braided Women's Ankle Boots Cavalier cm Biker Big Block Buttons Classic Blue Shoes Women's 3 Heel Braided Cavalier Boots Booty Angkorly cm Ankle Biker Big Block Fashion throw readFileErr;
  console.logClutch Ladies Frame Party Prom Suede Bridal Evening Bag Envelope Wocharm Wedding Metallic Bag Clutch Girly Red Faux HandBags 4WZfq(data);
  fs.unlink('/file.md', Blue Cavalier Braided Classic Women's Big Booty Fashion Ankle cm Heel Angkorly Buttons 3 Shoes Boots Biker Block (Small Changing Print ADORA Kipling City Flower Pink Baby Bag BABY XnPtpqSxFunlinkErr) => {
    if (unlinkErr)Sling Bag Thai Hippie Wine Indic Monk 2 SwCfx 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