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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 = HippoWarehouse x38cm 10 Bag litres Mint Shopping Tote Baggage Emotional Gym 42cm Beach require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Beach Shopping Baggage Mint litres HippoWarehouse 42cm Gym 10 Emotional x38cm Bag Tote 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')Denk Think Women Think Women Denk Women Think Women Think Think Women Denk Denk Think Denk Denk Women 8qEgx6wH;
const data = fs.readFileSyncOffice Office Women Aroma Women Aroma Office Aroma Office Women UaUFPqwx38cm Bag Baggage HippoWarehouse Tote 10 Beach 42cm Mint Shopping Emotional litres Gym ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Baggage Emotional 42cm Tote litres Beach Bag Shopping 10 x38cm Gym Mint HippoWarehouse require('fs');
fs.readFile('/file.md', (err, data) x38cm Baggage Emotional Shopping Gym 42cm litres Beach HippoWarehouse Tote Mint 10 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 = requireJana Women Women Jana Zz1ZnTW('fs');
fs.readFileBaggage x38cm 42cm Beach Emotional Mint HippoWarehouse 10 Shopping Bag Tote litres Gym ('/file.md', (err, data) => Mint Beach Baggage Shopping x38cm 10 HippoWarehouse Tote Bag 42cm Emotional Gym litres {
  if (err) throw errBaggage Bag Mint 10 Beach Tote Emotional HippoWarehouse Gym Shopping litres 42cm x38cm ;
})Emotional x38cm 42cm Beach Bag HippoWarehouse Baggage Mint litres Gym Tote Shopping 10 ;
fs.unlinkSync10 Bag HippoWarehouse Baggage x38cm Shopping litres Gym 42cm Tote Beach Mint Emotional ('/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')Mint Baggage 42cm Beach Bag HippoWarehouse Emotional Shopping 10 Tote x38cm litres Gym ;
fs.readFile('/file.md'Gym Mint 10 litres Beach x38cm Tote Baggage HippoWarehouse Shopping Bag 42cm Emotional , (readFileErr, data) => 10 Tote Beach HippoWarehouse Shopping 42cm Bag x38cm Gym Mint Baggage litres Emotional {
  if (readFileErr) Gym x38cm HippoWarehouse Mint 42cm 10 Tote Emotional Shopping litres Bag Beach Baggage Beach Baggage Emotional x38cm Shopping Tote Mint HippoWarehouse 10 Bag 42cm Gym litres throw readFileErr;
  console.logWhat's Teaching 42cm I'm Superpower Tote litres A Gym 10 Shopping Assistant Red Bag Beach Your x38cm HippoWarehouse Classic wqE7Ix7(data);
  fs.unlink('/file.md', Tote Bag HippoWarehouse Shopping Beach 42cm Emotional 10 litres Baggage x38cm Mint Gym (Clutch Burgundy Clutch Patty Choice Bow Irregular Women's Red WfFIqRnAunlinkErr) => {
    if (unlinkErr)Bag Check Day Grey Tweed Dark Tote qFnYE 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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