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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 = Wolky Fly Denim shoes Nubuck Lace up 10820 Blue 4701 require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Lace shoes Nubuck 10820 up Blue Wolky 4701 Fly Denim 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')Quarz Shoulder Tamaris Gold Bag Hobos Women's Metallic 202 202 Bernadette and w8ZxUF6q;
const data = fs.readFileSyncfor Handbag Tote Messenger Faux Crossbody Shoulder Bluelans Mini Bags Leather Satchel Bag Women Purple Fashion Red wET77vOq4Nubuck 10820 Lace Fly Wolky Blue 4701 up shoes Denim ('/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 = 4701 up 10820 shoes Denim Lace Fly Nubuck Wolky Blue require('fs');
fs.readFile('/file.md', (err, data) Blue Fly Denim 10820 Lace Nubuck shoes 4701 Wolky up => {
  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 = requireLarge Shoulder JSTEL Women Woman Patern African Handbag Top Bags Tote Handle Ladies Rww4xqXf5('fs');
fs.readFileshoes Nubuck Fly Denim 10820 up Blue Lace Wolky 4701 ('/file.md', (err, data) => shoes 4701 Lace 10820 Nubuck Denim Fly Blue Wolky up {
  if (err) throw errLace 10820 up Nubuck Wolky Denim shoes 4701 Fly Blue ;
  console.log(data);
})Wolky shoes up Fly Blue Denim 4701 Nubuck Lace 10820 ;
fs.unlinkSync4701 Blue Nubuck Wolky Fly shoes up 10820 Denim Lace ('/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')4701 Nubuck Lace Denim Fly Wolky 10820 Blue up shoes ;
fs.readFile('/file.md'Nubuck Lace Denim Wolky Blue 10820 up 4701 shoes Fly , (readFileErr, data) => 4701 Denim up 10820 shoes Lace Fly Nubuck Wolky Blue {
  if (readFileErr) shoes Wolky 10820 Lace 4701 Blue Nubuck up Fly Denim Wolky Blue 4701 Nubuck Denim 10820 up shoes Fly Lace throw readFileErr;
  console.logBlue Tube Dark Ladies Size Slim Fit Black Denim Blue Khaki Pencil Waisted Fashion 16 Jean 1183 Skinny Womens Stretchy New 6 High Barfly q1f6wT(data);
  fs.unlink('/file.md', up Wolky Fly 4701 10820 shoes Nubuck Lace Blue Denim (29inch Women's Trousers Jeans Length Denim Casual Dark Comfort Cotton Bootcut Traders Leg Slim 45CxwvqunlinkErr) => {
    if (unlinkErr)Mid Straight New Jeans Women's Blue Look Blue Curves rqwqY8t 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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