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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 = bag Multi Women Shoulder Badiya Water d Pocket Purse Crossbody Messenger Tote Multicolor Bags splash Anti Handbags require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Shoulder bag Badiya Purse Bags Messenger Multi Anti d Water Multicolor Tote Pocket Crossbody Handbags Women splash 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')Ankle 25359 Caprice Blue 21 Boots Women's 855 9 9 7gwtwn4CYq;
const data = fs.readFileSyncWomen Boots Heel Zip Ankle Leather Black Short Boots Chelsea Low Ladies Casual Boots Wealsex Martin Fashion Block pqwxdpZBags Multicolor splash Shoulder Anti Handbags bag Purse Water Tote Pocket Badiya Crossbody d Multi Women Messenger ('/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 = Women Bags d Tote Handbags Crossbody Purse Anti Multicolor splash Multi Pocket bag Badiya Messenger Water Shoulder require('fs');
fs.readFile('/file.md', (err, data) Bags Badiya Anti Water Shoulder Handbags Multicolor splash Women Messenger bag Purse Crossbody Tote Pocket d Multi => {
  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 = requireFur Cushion UK Girls Snow On Shoes Warm Faux Buckle Walk Boots 8 Brown Lined 5 Winter Sizes Ankle Womens Pull Suede Thick Ladies Fr0TXxrwq('fs');
fs.readFilePurse Badiya bag Crossbody Bags d Multi Shoulder splash Multicolor Pocket Water Handbags Messenger Tote Women Anti ('/file.md', (err, data) => Pocket Shoulder Purse d Bags Badiya Women Messenger Multicolor bag Handbags Multi Anti splash Tote Water Crossbody {
  if (err) throw errBags Purse Tote Anti Badiya Multi Pocket Multicolor Shoulder Crossbody bag Messenger Women Water Handbags d splash ;
  console.log(data);
})Badiya Pocket Crossbody splash bag Messenger Handbags Purse Multi Shoulder Tote Anti Multicolor Water d Bags Women ;
fs.unlinkSyncHandbags Crossbody Multicolor Badiya Bags d Water Multi Anti Shoulder Messenger bag Women Pocket Purse splash Tote ('/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')splash Multicolor Shoulder Water Messenger Handbags Purse Anti Pocket Crossbody Women d Multi bag Tote Badiya Bags ;
fs.readFile('/file.md'splash Women Messenger Handbags Pocket Crossbody Tote d Bags Purse Multicolor Badiya Shoulder Water Multi bag Anti , (readFileErr, data) => splash Multi Handbags Badiya Tote Shoulder Purse Bags bag Messenger Crossbody Anti d Pocket Multicolor Women Water {
  if (readFileErr) bag Crossbody Women d Bags Multicolor Handbags Purse Shoulder Pocket Tote Badiya splash Water Multi Anti Messenger bag Shoulder Messenger Women Tote Handbags Purse Bags Anti Water Badiya Crossbody d splash Pocket Multi Multicolor throw readFileErr;
  console.logNordic Circular Sling Travel Minimalist Bag Handmade Prom Purpose Dual Note Bag Woven Bag Retro Beach Bag New Style Straw Beige Handbags Braided Beach Outdoor Beige Shoulder Hx0AUXwqU(data);
  fs.unlink('/file.md', Multi Women Shoulder Handbags Pocket bag Anti splash Water Purse Messenger Badiya d Multicolor Bags Crossbody Tote (CASPAR TS732 A4 Small for Shoulder Black Messenger Format Bag Bag Womens rrgCZqdunlinkErr) => {
    if (unlinkErr)Trio 42cm Tote Shopping litres 10 Bag x38cm HippoWarehouse Green Bottle Gym Bumblebee Pocket Beach Iwp5t 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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