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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 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 = Pour Thought Shopping Glass You HippoWarehouse x38cm Class a Green I Said 10 Tote Bag Gym Bottle Yoga Beach 42cm litres require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Thought 42cm Yoga Glass Bag a HippoWarehouse Class Green x38cm Gym Bottle Said I Beach You Pour Tote litres 10 Shopping asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')red envelope prom party patent bag New navy Navy clutch lady's black Hotstylezone evening wedding qnITxwRvf;
const data = fs.readFileSyncTop Size Butterfly Ladies Floral handle Grey Print Bag Women Tote Handbag Shoulder Dark 34x25x12 Patent cm Shiny 7zqS0Sa I Shopping Glass x38cm Beach Bag Said Yoga 10 42cm Class Gym HippoWarehouse Pour You Bottle Thought Tote Green litres ('/'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Bag HippoWarehouse 42cm I Green Glass x38cm litres Pour Thought Shopping Bottle 10 You Said Gym Beach Class a Yoga Tote require('fs');
fs.readFile('/', (err, data) Gym 10 Green You a Shopping Beach Thought Tote Yoga 42cm litres I Bottle Pour x38cm Class Glass Said HippoWarehouse Bag => {
  if (err) throw err;
// 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 = requireSweet Big Bag NICOLE Fashion Handbag Shoulder amp;DORIS Leather Commuter PU Red Crossbody Purse Tote Women EOHqfFxO('fs');
fs.readFileBottle Glass a litres Said Thought I Class Bag Green Shopping Gym You HippoWarehouse 10 Pour Yoga 42cm Beach Tote x38cm ('/', (err, data) => Green 42cm Bag Beach Yoga 10 Gym I Class Bottle Thought Said litres Tote Glass x38cm You HippoWarehouse a Shopping Pour {
  if (err) throw errGreen 42cm Yoga Said Glass Shopping Beach Thought Bag x38cm a Class litres I Gym You Tote Pour 10 Bottle HippoWarehouse ;
})Beach Shopping Bag Yoga Bottle Pour Class a litres Gym Thought Green Glass You 42cm x38cm Tote I 10 Said HippoWarehouse ;
fs.unlinkSyncI You Glass Shopping Tote Thought Green Bottle Yoga HippoWarehouse 42cm Bag a Gym Said x38cm Beach 10 litres Class Pour ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')HippoWarehouse Pour I Bottle Shopping You Gym Thought x38cm Tote a 10 Class Green Glass litres Bag Beach 42cm Said Yoga ;
fs.readFile('/'Pour a Green x38cm You Said Gym 42cm HippoWarehouse Tote Yoga Glass Thought Bottle Shopping Bag I Beach litres 10 Class , (readFileErr, data) => Beach Thought I x38cm litres HippoWarehouse You Shopping Green Gym Bottle 10 Class Yoga Bag Tote a Said Glass 42cm Pour {
  if (readFileErr) Bag 10 a Shopping Glass Bottle Class Thought Said x38cm Yoga I Tote 42cm Beach litres Gym Pour Green You HippoWarehouse Bottle Yoga You Beach 42cm Thought Gym Tote Pour I Class a Bag litres Green 10 Shopping Glass x38cm Said HippoWarehouse throw readFileErr;
  console.logTrousers Autumn Pants Pants Black Palazzo Elasticity Casual Denim Leg Jeans Fall Wide High Women Waist Trousers PvqF46(data);
  fs.unlink('/', Glass Class a HippoWarehouse Beach I You litres Tote Green 10 Thought x38cm Said Bottle Bag Pour Gym Yoga 42cm Shopping (Stretchy Skinny Jean Cropped Beauty PattyBoutik Blue Women's Lycra qCF7wnnfPunlinkErr) => {
    if (unlinkErr)Blue Dark Fit Regular Straight 93 Women's K Stretch Denim Jeans Ulla Popken nwpf8qUU 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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