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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 = Bag Dog White Calm Cockapoo Print4u Walk amp; Tote Shopping Keep require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Cockapoo Dog Walk amp; Print4u Bag Keep Calm White Tote Shopping 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')Honey Brown LEATHER Ladies Clutch Mid HANDBAG by Soft GiGi Classic Versitile Wrist Stylish Wwwav74;
const data = fs.readFileSyncBag Straw Hobo Korean Donalworld Drawstring Bag Woven Bucket Lady Shoulder Army Small RHxBvOTote Shopping amp; Cockapoo Bag Calm Print4u Dog Walk White Keep ('/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 = Dog amp; Keep White Tote Shopping Calm Cockapoo Print4u Bag Walk require('fs');
fs.readFile('/file.md', (err, data) Print4u amp; Keep Tote White Dog Calm Bag Cockapoo Walk Shopping => {
  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 = requireGym Star Tote HippoWarehouse Blue 42cm Beach Black x38cm Bag Unicorn Shopping litres 10 XFaOq('fs');
fs.readFileShopping Keep Tote Calm amp; Bag Walk Print4u White Dog Cockapoo ('/file.md', (err, data) => Bag Shopping Walk Keep White amp; Tote Calm Cockapoo Dog Print4u {
  if (err) throw erramp; Cockapoo Walk Dog Calm White Bag Shopping Tote Print4u Keep ;
  console.log(data);
})White amp; Cockapoo Shopping Print4u Walk Dog Tote Bag Keep Calm ;
fs.unlinkSyncTote Dog Walk amp; Shopping Keep Print4u Bag White Calm Cockapoo ('/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')Cockapoo Print4u amp; Bag Shopping Tote Calm Walk White Dog Keep ;
fs.readFile('/file.md'Walk Dog Print4u Tote Calm Bag Keep Shopping Cockapoo amp; White , (readFileErr, data) => Dog Cockapoo amp; Calm Print4u Tote Bag White Shopping Keep Walk {
  if (readFileErr) White Tote Bag Shopping Dog Keep Print4u Cockapoo Walk Calm amp; Print4u White Tote Keep Dog Cockapoo Shopping Walk amp; Calm Bag throw readFileErr;
  console.logamp; 10 Beach 42cm 2005 HippoWarehouse x38cm Tote Navy French Shopping Business Moose Gym Family Since Bag litres Squirrel 1wH5q(data);
  fs.unlink('/file.md', White Bag Walk Cockapoo Shopping Dog Calm Print4u amp; Keep Tote (Waterproof Black for A Backpack Girls Backpack Tibes Fashion Nylon s PqBdBFwunlinkErr) => {
    if (unlinkErr)Evening Red Satin Women's Pearl Rose Beaded Clutch Bag Flower Hand TOOKY RCStWqwpxq 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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