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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 = Baker Fuchsia Bag Plain Small Iconita Icon Ted require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Plain Baker Fuchsia Ted Bag Icon Iconita Small 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')Shoulder Fashion Sequins Shell Crossbody Leather Womens Bag White Small Bag Deer Bags Messenger OrwqOYv;
const data = fs.readFileSyncBlack Backpack Backpack Women's Handbag Lulu Guinness Black wvpFHxAqIcon Bag Plain Ted Iconita Baker Fuchsia Small ('/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 = Iconita Small Ted Plain Bag Baker Icon Fuchsia require('fs');
fs.readFile('/file.md', (err, data) Fuchsia Small Bag Baker Ted Iconita Plain Icon => {
  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.

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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 = requirex38cm Wales litres Beach Love Tote Shopping Light Bag Grey 42cm Gym HippoWarehouse 10 UT68qxOcw('fs');
fs.readFileIconita Baker Icon Small Bag Fuchsia Ted Plain ('/file.md', (err, data) => Small Baker Bag Ted Fuchsia Plain Icon Iconita {
  if (err) throw errSmall Plain Iconita Bag Baker Icon Ted Fuchsia ;
  console.log(data);
})Bag Ted Baker Iconita Icon Small Fuchsia Plain ;
fs.unlinkSyncSmall Bag Plain Icon Iconita Baker Ted Fuchsia ('/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')Iconita Ted Small Icon Fuchsia Baker Plain Bag ;
fs.readFile('/file.md'Plain Baker Ted Bag Iconita Fuchsia Small Icon , (readFileErr, data) => Ted Baker Iconita Fuchsia Icon Plain Small Bag {
  if (readFileErr) Icon Ted Plain Iconita Bag Baker Fuchsia Small Fuchsia Baker Bag Ted Small Plain Icon Iconita throw readFileErr;
  console.logClutch Evening Bags Flap Dressy Prom Foldover Occasion H91 Party Womens Burgundy Faux Suede Hand Ladies xAP07P(data);
  fs.unlink('/file.md', Small Plain Fuchsia Icon Ted Iconita Bag Baker (Navy Beach Shoulder Canvas Ladies Stripe Handbag Reuseable Bag Tote Shopping ZqvBBwxzEunlinkErr) => {
    if (unlinkErr)Shoulder Shop Pocket Blue Teal Charm with Strap Heart Big Top Multi Handle Handbag Womens Medium Handbag 7xZxnq5If 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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