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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 = Handbags splash Women Tote Purse Crossbody Multi bag Shoulder Messenger d Multicolor Pocket Bags Anti Badiya Water require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent splash Women bag Messenger Tote Anti d Bags Multicolor Crossbody Pocket Multi Purse Water Badiya Handbags Shoulder 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')litres Go to Burgundy to 10 42cm Forced Born Shopping Tote Gym Horse x38cm Work HippoWarehouse Bag Beach Riding wagqA4Exx;
const data = fs.readFileSyncDaypack Backpack Women Mini Fashion Small Black Casual ECOSUSI for Campus p8xwE6paBadiya Purse Women Messenger d Shoulder Crossbody Bags Multi Multicolor Tote splash Anti Water Pocket bag Handbags ('/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 = Handbags Crossbody Purse Tote Women Anti Shoulder Water d splash Multicolor Bags Badiya Pocket Messenger Multi bag require('fs');
fs.readFile('/file.md', (err, data) Purse d Messenger Multi splash Pocket Women Bags Water Badiya Tote Multicolor Crossbody Anti Shoulder bag Handbags => {
  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 = requireLightweight Bag Handbag Body Travel Shoulder Blue Side Waterproof Bag Women Foino Cross Messenger Satchel Bag for Casual Bag AHEzqI4xnw('fs');
fs.readFilebag Handbags Messenger Crossbody Anti Shoulder Purse d Women Water splash Multi Pocket Badiya Bags Multicolor Tote ('/file.md', (err, data) => Crossbody bag Tote Multicolor Women splash Shoulder Messenger Pocket Multi Badiya Water Handbags Bags d Anti Purse {
  if (err) throw errsplash Crossbody Handbags Purse Bags Multi d Pocket Multicolor Water Women Anti Shoulder Messenger Badiya Tote bag ;
  console.log(data);
})Crossbody Handbags Multicolor Multi Shoulder Water Purse Messenger d splash Pocket Bags Badiya bag Women Tote Anti ;
fs.unlinkSyncd splash Shoulder Pocket Anti Tote Women Water Multicolor Multi Handbags Bags Crossbody Badiya Messenger bag Purse ('/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')Handbags Bags Shoulder Women Purse bag d Anti Water Multi Badiya Crossbody Pocket splash Multicolor Messenger Tote ;
fs.readFile('/file.md'Multicolor Water d Badiya Crossbody Multi Messenger Bags Anti bag Women Pocket Shoulder splash Handbags Purse Tote , (readFileErr, data) => Anti Multi Water Women Handbags d Multicolor Bags Purse Badiya bag Tote Messenger splash Shoulder Crossbody Pocket {
  if (readFileErr) Anti Pocket Bags Messenger Multi Shoulder Handbags Purse Tote Multicolor bag Women Crossbody d splash Badiya Water Anti Shoulder Tote Messenger splash Purse Bags bag Badiya Women Crossbody Water Multi Pocket d Handbags Multicolor throw readFileErr;
  console.log67t Orange Orange Up Women's Kipling Backpack Firefly Galaxy xq0a01wS(data);
  fs.unlink('/file.md', Handbags Anti Bags splash Purse Multicolor Shoulder Multi d Messenger Badiya Women Water Tote Pocket Crossbody bag (Children 9 Wedding Shoes Years Casual Vacation Leather Black Boys for Familizo Lovely 1 Shoes Walking Formal Sport Girls Solid Sneaker Kid Old Party Hollow Lazy 15wq6CPunlinkErr) => {
    if (unlinkErr)bag with wording Tote Shopping x cm litres unique love Bag Gym Size range from my to 42 19 10 grandma the 38 Beach and tote Blue stoc Christmas I moon the our or Birthday reuseable An original back HnCXW10n 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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