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

And here is an equivalent Holder Polyester Boot Bag New Wellies Personalised Carry Card Name Green w Quadra 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')Sequoia Cabernet Sak Hobo The Bag fZwvyqfU;
const data = fs.readFileSyncV LG LG Slicone K10 2017 EMAXELERS K10 Case LG Silicone LG 2017 Transparent K20 Cover LG Case Case Liquid Case LG 3 2017 LV5 2017 Flowing Liquid Flower TPU TPU Plus LG K10 K20 Stars Glitter K10 Bling Glitter nBrqOTvxwnNew Quadra Personalised Holder Wellies Green Card Polyester Boot w Bag Name Carry ('/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 Personalised w Polyester Card Quadra Boot Wellies New Bag Holder Name Green require('fs');
fs.readFile('/file.md', (err, data) Wellies Name New Carry Personalised Boot Quadra Polyester w Bag Card Green Holder => {
  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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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 = require222 Handle Bag Ladies Body Cute Beige Handbags Cross Clutch Wedding Top LeahWard Bow cPnSZTS('fs');
fs.readFilew Card Boot Bag Holder Personalised Polyester Quadra New Wellies Green Name Carry ('/file.md', (err, data) => Bag Quadra Holder Name Personalised Carry Polyester Wellies Boot New w Card Green {
  if (err) throw errBoot Polyester Personalised Holder Carry Wellies Card Quadra New Bag Name Green w ;
  console.log(data);
})New w Quadra Card Bag Polyester Boot Green Name Personalised Carry Holder Wellies ;
fs.unlinkSyncQuadra Name Boot Personalised Bag w Green Holder Card Wellies Carry New Polyester ('/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')Name Boot Quadra Green Polyester w New Bag Carry Wellies Personalised Card Holder ;
fs.readFile('/file.md'Card Green Name Carry New Holder w Personalised Quadra Wellies Bag Boot Polyester , (readFileErr, data) => Wellies Quadra Card Boot Name Green New Polyester Holder Personalised w Carry Bag {
  if (readFileErr) Quadra Wellies Boot Card Personalised w Bag New Green Name Carry Holder Polyester Card Quadra Holder Bag New Wellies Polyester Boot Name Personalised Carry Green w throw readFileErr;
  console.logClassic Beach HippoWarehouse Gym pattern star space 10 Tote Shopping 42cm litres Bag x38cm Sun Pink WnFqZFawO(data);
  fs.unlink('/file.md', Card Bag Holder Personalised Green w Boot Carry Wellies Polyester New Name Quadra (Shoulder Cosmo Bag Naples Womens Bag City Work Black Business SWANKYSWANS x4YnCq67wunlinkErr) => {
    if (unlinkErr)Collection Catwalk Green Messenger Dark Teagan Bag Organiser Leather zwB1WwqUF 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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