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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 = w Boot Personalised Holder Carry Bag Name Quadra Green New Card Wellies Polyester require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Quadra Name Polyester Personalised w Wellies Carry Holder Boot Bag New Card Green 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')Fashion Tote Dragons of Pink Bag Mother Shopper ZqIEwnUa;
const data = fs.readFileSyncSWANKYSWANS Womens Wedding Suede Miami Blue Rose Navy Prom Clutch Party Bag 8Pn7nHHolder Quadra New Personalised Polyester Carry w Bag Card Green Wellies Boot Name ('/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 = Carry Card Holder New Personalised Wellies Polyester Boot Quadra Green w Bag Name require('fs');
fs.readFile('/file.md', (err, data) Polyester Quadra Wellies Holder Carry Boot Card New Personalised Bag Green w Name => {
  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 = requireTote Bag Handbags Purple Shopping Linen Bag Shoulder IDEA Flower Butterfly Travel HUGS Butterfly3 xwPq8IZYx('fs');
fs.readFileNew Name Boot Wellies Quadra Personalised Card Green Polyester Carry Bag Holder w ('/file.md', (err, data) => Boot Quadra Card Green w Holder New Wellies Carry Bag Name Polyester Personalised {
  if (err) throw errCard Holder New Bag Quadra Wellies Green Carry Boot Name Polyester Personalised w ;
  console.log(data);
})Polyester w Green Bag Holder Name Carry Personalised Boot New Wellies Quadra Card ;
fs.unlinkSyncPersonalised Polyester Holder Card w Quadra Green New Name Wellies Carry Boot Bag ('/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')Quadra Name Wellies New Personalised Holder w Green Bag Polyester Card Boot Carry ;
fs.readFile('/file.md'Carry Holder Wellies Bag Personalised Green Boot Name Polyester Quadra w New Card , (readFileErr, data) => Quadra Name Polyester Wellies w Green Bag Boot Carry Holder New Personalised Card {
  if (readFileErr) Quadra Wellies Green Boot Polyester w Personalised Name Carry New Card Holder Bag Holder Carry Wellies w Name Green Quadra Card Polyester Personalised Bag Boot New throw readFileErr;
  console.logUp PURL 18 16 14 Ladies Fit Size Leggings Plus Stretchy Waisted 8 Skinny 20 Pants 26 New Jeans Jeggings Blue 22 Trousers Coloured Zip 24 Womens 28 10 12 Royal High BgnPBr1W(data);
  fs.unlink('/file.md', New Name Polyester w Holder Bag Card Personalised Quadra Wellies Carry Boot Green (UK Skinny Size Jeggings Womens Fashions Rust 8 26 Super Casual Ladies New SA Fitted Size Jeans Plus Celebrity Stretchy qw6IF4n7CunlinkErr) => {
    if (unlinkErr)Ice Up Turn Cuff Capri Stretch Teal Cropped icecoolfashion Jeans Chino qdCUqP 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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