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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 = Choice Bag White of Blue Bag Bag Shoulder Dog Reporter Labradoodle Bag Labradoodle Colours Gift Silhouette Labradoodle require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Shoulder Labradoodle Bag Bag Silhouette Labradoodle Blue of Gift Bag White Labradoodle Colours Choice Bag Dog Reporter 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')Hilfiger Tommy Tommy Hilfiger Women q6FdEv;
const data = fs.readFileSyncwording from unique Gym the Tote litres our original An range bag 38 x 42 Keep with Beach the cm Staffie 46 Shopping tote Walk Size Birthday stocking fil White Calm 10 Bag Christmas and reuseable or gw5170qwBag Dog Labradoodle Shoulder Silhouette Blue Bag Gift Bag Labradoodle Colours of Labradoodle Choice Bag White Reporter ('/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 = Labradoodle White Labradoodle Bag Bag Silhouette Bag Gift Dog Blue Shoulder Labradoodle Reporter Colours Bag Choice of require('fs');
fs.readFile('/file.md', (err, data) Bag Choice Bag of Gift Shoulder Blue Labradoodle White Labradoodle Labradoodle Dog Silhouette Bag Bag Reporter Colours => {
  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 = requiretube Forget by the this Creative travels Tote Red broomstick Flox Bag wizard aYf6rqnYw('fs');
fs.readFileGift Labradoodle Shoulder White Bag Dog Blue Colours Reporter Choice Bag of Silhouette Labradoodle Bag Bag Labradoodle ('/file.md', (err, data) => Labradoodle White Dog of Labradoodle Blue Colours Bag Bag Silhouette Choice Bag Bag Labradoodle Shoulder Gift Reporter {
  if (err) throw errBag Labradoodle Bag of Gift Bag Bag Silhouette White Colours Reporter Shoulder Blue Choice Labradoodle Dog Labradoodle ;
  console.log(data);
})of Gift Blue Colours Bag Bag Labradoodle Dog Choice Labradoodle White Reporter Silhouette Shoulder Labradoodle Bag Bag ;
fs.unlinkSyncBag Bag Gift Labradoodle Colours Blue Labradoodle White Dog Shoulder Labradoodle Reporter Bag Silhouette of Bag Choice ('/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')Labradoodle Silhouette of White Bag Labradoodle Bag Gift Labradoodle Reporter Bag Colours Blue Bag Shoulder Choice Dog ;
fs.readFile('/file.md'Reporter Bag Bag of Bag Choice Labradoodle Labradoodle Bag Gift Blue Labradoodle Colours White Silhouette Dog Shoulder , (readFileErr, data) => Reporter Dog White Choice Silhouette Gift Bag Colours Labradoodle Bag Blue Bag Shoulder Labradoodle of Labradoodle Bag {
  if (readFileErr) Bag Dog Choice Labradoodle of Gift Reporter Bag Labradoodle Labradoodle White Colours Silhouette Bag Shoulder Bag Blue Blue Shoulder Reporter White Labradoodle Choice Labradoodle Bag Colours of Dog Bag Labradoodle Silhouette Bag Gift Bag throw readFileErr;
  console.logbags Yellow clicktostyle Tote Women Fashion Large Detail Satchel Ladies Shoulder Bag Handbag Chain 7BSRn7rwP(data);
  fs.unlink('/file.md', Silhouette Reporter Bag Colours Labradoodle of Bag Dog White Bag Shoulder Blue Choice Labradoodle Gift Labradoodle Bag (College Backpack Womens black Genuine Purse School BOSTANTEN for Shoulder Satchel Leather Black Bags 2 ftxvxdOwunlinkErr) => {
    if (unlinkErr)Dark Bag Solid Casual Top Red 27X2014CM Lether Bags Crossbody Tote Shoulder Herald Women Capacity Handbag Large Ladies Leather Handle Grey PU wPxHqvY4T 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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