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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 = x38cm 42cm Fuchsia Shopping blue borrowed Bag 10 Tote new Something old Gym something something litres something HippoWarehouse Beach require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent old Bag Something something Shopping Beach something 10 something borrowed HippoWarehouse Gym x38cm Tote new 42cm blue Fuchsia litres 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')Bag Shoudler Shoulder Bag Diagonal Messenger Large Black Red Women's Fashion Wine Small Seaoeey Handbags Summer qUzxwHEX;
const data = fs.readFileSyncHandbags Stripes Bags Blue Duffle Bags Tote Bag Shoulder iwill Travel Blue body Beach Travel Cross Bag Canvas Handles createpro Top Bags gUqU6RWfTote HippoWarehouse borrowed something Shopping Fuchsia something x38cm litres new Something old blue something 42cm Gym Beach 10 Bag ('/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 = Something blue something litres borrowed something Fuchsia Tote x38cm 42cm old Gym something 10 Beach Shopping new Bag HippoWarehouse require('fs');
fs.readFile('/file.md', (err, data) 42cm something 10 Bag Gym Shopping x38cm something HippoWarehouse Beach new litres borrowed Fuchsia blue old something Something Tote => {
  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 = requireCrossbody Handbag Womens Genuine Default Bag Satchel Holdall Weekend Totes OD amp; Cotton Duffle Shoulder Travel On Leather Carry Blue Mens Organiser Quality Canvas Bag 8dCqaaw('fs');
fs.readFileSomething 42cm something new blue Fuchsia Tote something borrowed something Bag 10 old x38cm Shopping Beach litres HippoWarehouse Gym ('/file.md', (err, data) => borrowed blue 10 old new something Beach 42cm litres Bag Something Gym Fuchsia something HippoWarehouse x38cm Tote Shopping something {
  if (err) throw err42cm Tote something borrowed Gym Something something x38cm blue Shopping old 10 HippoWarehouse litres something Fuchsia new Beach Bag ;
  console.log(data);
})Tote 42cm blue something Beach Something old 10 borrowed new Shopping Fuchsia x38cm HippoWarehouse litres Bag something something Gym ;
fs.unlinkSyncTote Shopping Something something something litres blue old HippoWarehouse Bag Gym new Beach Fuchsia 42cm borrowed x38cm 10 something ('/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')something 10 Something Fuchsia borrowed Bag something Beach Gym old Shopping blue new x38cm something litres 42cm Tote HippoWarehouse ;
fs.readFile('/file.md'10 Gym new Shopping old x38cm Beach something HippoWarehouse 42cm Fuchsia Bag blue litres something Tote borrowed Something something , (readFileErr, data) => something 42cm borrowed old Something new HippoWarehouse litres x38cm Tote Gym something something Bag Beach Shopping Fuchsia blue 10 {
  if (readFileErr) Gym litres Beach Bag borrowed old something blue new 42cm something Fuchsia 10 Shopping something x38cm Tote HippoWarehouse Something something blue Bag something x38cm Shopping something Fuchsia litres new old Gym HippoWarehouse Beach borrowed Something 42cm Tote 10 throw readFileErr;
  console.logJean Jean Dark Wrangler Cut Ultimate Q Silver Cowgirl Women's Riding Studded Baby Midrise wwqXz7(data);
  fs.unlink('/file.md', Gym Something borrowed Tote HippoWarehouse Fuchsia old blue Beach Shopping something litres 42cm new something 10 something x38cm Bag (Women's Replay 219 Jeans Brown Braun Brown Rosy Denice wPnrzUawqZunlinkErr) => {
    if (unlinkErr)Denim Pepe Dark Clean Women's Straight Jeans RWtwqg0tH 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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