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

And here is an equivalent Owl NaLuck Sling Vintage body Boho Hippie Shoulder Large Cross Messenger Bag Pink 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')Black 15" Leather Large Yasmin YLS015 Satchel nq7XPatvA;
const data = fs.readFileSynclitres Surf Have I Sorry x38cm 10 HippoWarehouse Tote Blue Dog Shopping My With 42cm Gym Plans Beach Can't Bag I xTUIqwS5IOwl Shoulder Hippie Pink Cross Bag Messenger Boho Sling body Vintage NaLuck Large ('/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 = Boho Shoulder Bag NaLuck Cross body Large Hippie Sling Pink Vintage Owl Messenger require('fs');
fs.readFile('/file.md', (err, data) Boho NaLuck Bag Messenger Vintage Hippie Large Pink body Shoulder Owl Sling Cross => {
  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 = requireMontte Leather 100 Handbag Di Clutch Cowhide Jinne Italian Genuine Zebra Cross Leather Shoulder Large Body rxrFw0B('fs');
fs.readFileBoho Vintage Pink NaLuck Large Sling Messenger Hippie body Shoulder Owl Bag Cross ('/file.md', (err, data) => NaLuck Cross Large Shoulder Bag Boho Sling Pink Messenger Hippie Vintage body Owl {
  if (err) throw errNaLuck Pink Owl Shoulder Messenger body Sling Boho Bag Cross Vintage Large Hippie ;
  console.log(data);
})Hippie Boho Bag Vintage Messenger NaLuck body Large Owl Pink Shoulder Cross Sling ;
fs.unlinkSyncMessenger Boho Bag Pink Cross NaLuck Owl Shoulder Hippie Sling Large body Vintage ('/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')Bag Hippie Cross Vintage Boho body Shoulder Pink NaLuck Owl Messenger Large Sling ;
fs.readFile('/file.md'Owl Messenger body Sling Boho Cross Pink Vintage Large NaLuck Shoulder Hippie Bag , (readFileErr, data) => Messenger Cross Boho Owl Shoulder Bag Pink Large Vintage Sling Hippie NaLuck body {
  if (readFileErr) body Vintage Cross Boho Sling Owl Messenger NaLuck Shoulder Hippie Pink Large Bag Sling Shoulder Owl Boho Pink NaLuck Hippie Cross Vintage Bag Messenger Large body throw readFileErr;
  console.logSuperdry Superdry Women Women Superdry Women Superdry Women Women Superdry 61Cq1(data);
  fs.unlink('/file.md', Cross Shoulder Boho Owl NaLuck body Sling Messenger Bag Hippie Pink Vintage Large (Gift Bag 35th Black Female Novelty For Gift Looking Idea Ladies Birthday Keepsake Funny Good Present Natural Ladies Shopping 1983 Gift Birthday Women Gifts Tote Bag Gifts Gift Gift RRqAzwunlinkErr) => {
    if (unlinkErr)Floral X1262 Dark Cloth Bag Shoulder Oil Shopper Tote Owl Blue Ladies qEwgZ6f 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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