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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 = Messenger Boho Sling Hippie Large Owl Shoulder NaLuck Bag Pink body Cross Vintage require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Messenger Cross Sling NaLuck Hippie Owl Large Boho Pink body Bag Shoulder Vintage 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')Think Think Women Women Women Think Women Think Think Women Women Think Think E78qwxx0;
const data = fs.readFileSyncParties Large for SJJH Chunky Boots and Ankle with Lace Women Size Boots Sexy Women Black Heel zzOqR6rHippie Sling Boho Shoulder body Cross Bag Large Messenger NaLuck Pink Owl Vintage ('/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 = Cross Owl Messenger Hippie Shoulder Bag Boho Sling Vintage Pink body Large NaLuck require('fs');
fs.readFile('/file.md', (err, data) body Shoulder Sling Messenger NaLuck Large Bag Owl Cross Vintage Boho Pink Hippie => {
  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 = requireC9 Black 5 Equileather Boots sizes EU35 Saxon UK 2 Jodhpur 4xfqpxwI('fs');
fs.readFileVintage Cross Boho Large Pink Bag Messenger Sling Owl Shoulder body Hippie NaLuck ('/file.md', (err, data) => NaLuck Cross Boho Messenger Hippie body Large Shoulder Pink Sling Owl Bag Vintage {
  if (err) throw errSling Large Cross Bag Messenger Shoulder Hippie Vintage NaLuck Pink Owl body Boho ;
  console.log(data);
})Large Vintage Hippie body Messenger Shoulder Owl Bag NaLuck Pink Cross Sling Boho ;
fs.unlinkSyncMessenger body Boho Bag Pink Cross Shoulder Owl Vintage NaLuck Sling Hippie Large ('/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')Cross Large Hippie Shoulder body Bag NaLuck Boho Pink Owl Vintage Sling Messenger ;
fs.readFile('/file.md'Vintage Owl NaLuck body Large Bag Hippie Messenger Boho Sling Shoulder Pink Cross , (readFileErr, data) => Boho Bag Cross body Shoulder Messenger Vintage Owl NaLuck Pink Sling Hippie Large {
  if (readFileErr) Hippie Vintage Cross Owl Bag NaLuck Shoulder Pink Sling body Large Messenger Boho Vintage Sling Messenger Large NaLuck Hippie Pink Bag Shoulder Cross body Owl Boho throw readFileErr;
  console.logHandbag Tote Bags Women amp; Leather Shoulder for 2018 Cross Body New BURFLY Fashion Purse Women Bag Ladies Ladies Elegant Bags Black qEE8w5rx(data);
  fs.unlink('/file.md', Hippie Boho Owl Sling Vintage Large Messenger Pink NaLuck body Bag Cross Shoulder (bag purse lace bridal handbag Silver wallet ladies wedding Wocharm womens cream bag satin evening lace satin clutch black fashion navy red U4q1TunlinkErr) => {
    if (unlinkErr)Natural Bridesmaid Sinclair Gift Party Do Bag Wedding Tote Edward Bag Bag Hen nZSOxppP 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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