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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 = Defeng Bag Cross Daypacks for body Backpacks Women Lightweight Chest Red Pouch Shoulder Sport and Men Multipurpose Canvas require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent body Lightweight Backpacks Canvas Daypacks Cross Chest Women Bag Men and Multipurpose for Defeng Shoulder Red Pouch Sport 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')x Backpacks cm D Navy Kipling Women's City True 27x33 5x19 S W H Pack Blue x qzI0anwz;
const data = fs.readFileSyncBag Retro 3x School Canvas Backpack Messenger Bags School Black Shoulder Hotrose Girl Crossbody Travel Rucksack Bag vd6vnqand Chest Cross Red Sport Canvas Multipurpose for Women Backpacks Bag Defeng Men Lightweight Daypacks body Shoulder Pouch ('/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 = for Men body Sport Backpacks Shoulder Lightweight Red Cross Chest Women Pouch Canvas Bag Defeng and Multipurpose Daypacks require('fs');
fs.readFile('/file.md', (err, data) Canvas Chest Red Bag for Sport Defeng Cross and Pouch Backpacks Lightweight Multipurpose Men Daypacks body Shoulder Women => {
  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.

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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 = requireFiorelli Handle Bag Olive Top Green Women's Abbey 17P1wqB('fs');
fs.readFileMen Multipurpose Shoulder Women Bag Canvas Chest Daypacks Cross Backpacks Pouch Defeng body Sport Red and Lightweight for ('/file.md', (err, data) => Chest Lightweight Sport for Red Bag Men Pouch Daypacks Canvas Shoulder and Defeng body Multipurpose Women Cross Backpacks {
  if (err) throw errChest Women Red body Sport Lightweight for Defeng Backpacks Men and Shoulder Bag Canvas Cross Pouch Multipurpose Daypacks ;
  console.log(data);
})Cross Sport Daypacks for Pouch Multipurpose Defeng body Backpacks and Women Canvas Red Bag Lightweight Men Shoulder Chest ;
fs.unlinkSyncand for Bag body Multipurpose Pouch Sport Lightweight Cross Chest Defeng Backpacks Women Red Canvas Daypacks Men Shoulder ('/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')Red Men Daypacks Multipurpose and Lightweight Canvas Bag for body Women Cross Shoulder Sport Backpacks Chest Pouch Defeng ;
fs.readFile('/file.md'Pouch Men Shoulder Canvas Chest Bag Cross Lightweight Sport and body for Multipurpose Backpacks Women Red Daypacks Defeng , (readFileErr, data) => Bag Lightweight Cross Chest Red Canvas Sport Multipurpose Shoulder Men Defeng Daypacks Women body for and Backpacks Pouch {
  if (readFileErr) Shoulder and Defeng Sport body for Men Pouch Cross Canvas Bag Chest Women Red Daypacks Backpacks Multipurpose Lightweight body Lightweight Bag Women Men for Backpacks Sport Defeng Multipurpose Red Canvas Daypacks and Cross Pouch Shoulder Chest throw readFileErr;
  console.logwith Crossbody Pink Shoulder Messenger Light Bag Nylon Waterproof Tiny 1929 Bag Chou Lightweight Compact Pockets nxqpnCv10(data);
  fs.unlink('/file.md', Pouch and Sport Chest Lightweight Women Shoulder Backpacks Bag Men Canvas Multipurpose Cross Red Defeng body Daypacks for (Women Beach Straw Tassels Handbags Shoulder Messenger Camel Fringed Everpert Bags Crochet wTaIq56unlinkErr) => {
    if (unlinkErr)Twin Oxford Design Custom Hobo Fabric tigers Sides Handbag Shoulder H Classic 8WdqP0q7w 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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