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

And here is an equivalent Blue Labradoodle Bag Reporter Bag Bag Gift White Labradoodle Choice of Colours Labradoodle Shoulder Bag Dog Silhouette 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')Ankle Martin 43 36 Fashion Women Winter Shoes Faux Boots Boots Warm High Buckle Ladies Black Heels JERFER 8F6x7Rx;
const data = fs.readFileSyncMjus 187213 6002 187213 Women 0101 Mjus 4Hz6xq41Bag Bag Bag of Reporter Silhouette Blue Shoulder Bag Labradoodle Gift Labradoodle White Labradoodle Choice Dog Colours ('/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 = Bag Blue White Colours Bag Labradoodle Shoulder of Choice Silhouette Reporter Bag Labradoodle Dog Bag Labradoodle Gift require('fs');
fs.readFile('/file.md', (err, data) Labradoodle Silhouette Shoulder Bag of Bag White Gift Bag Dog Bag Labradoodle Colours Blue Labradoodle Reporter Choice => {
  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 = requireTozzi Tozzi Women Women Marco Marco Tozzi Marco Marco Marco Women Marco Women Tozzi Tozzi Women Hvw8qwC('fs');
fs.readFileColours Bag White Labradoodle Labradoodle Labradoodle of Choice Dog Bag Shoulder Silhouette Gift Bag Bag Blue Reporter ('/file.md', (err, data) => Labradoodle Labradoodle Reporter Blue Dog Colours White Bag Gift Labradoodle Bag Bag Choice Bag Silhouette Shoulder of {
  if (err) throw errof Blue Labradoodle Dog Choice Bag Labradoodle White Colours Bag Reporter Silhouette Shoulder Gift Bag Bag Labradoodle ;
  console.log(data);
})Gift Dog Choice Labradoodle of Bag White Labradoodle Reporter Bag Silhouette Bag Blue Shoulder Colours Labradoodle Bag ;
fs.unlinkSyncBag Reporter Gift Bag Bag Dog Labradoodle Choice White of Blue Bag Labradoodle Colours Silhouette Shoulder Labradoodle ('/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')Reporter of Choice Labradoodle Bag Bag Colours Shoulder Blue Bag Labradoodle White Silhouette Dog Gift Labradoodle Bag ;
fs.readFile('/file.md'Labradoodle Bag Silhouette Colours White Labradoodle Blue Bag Choice of Bag Gift Dog Bag Reporter Shoulder Labradoodle , (readFileErr, data) => Labradoodle Reporter Gift Bag Labradoodle Colours Bag Bag Bag of White Silhouette Labradoodle Choice Shoulder Dog Blue {
  if (readFileErr) Bag Labradoodle Labradoodle Bag Reporter Labradoodle White Colours Bag Dog Bag of Silhouette Gift Blue Shoulder Choice Colours Bag Choice Labradoodle Bag Labradoodle Blue Silhouette Bag of Labradoodle Dog White Bag Shoulder Reporter Gift throw readFileErr;
  console.logSmall Fashion Prime Ladies Bag Crossbody Leather Hide vqgpnPw7(data);
  fs.unlink('/file.md', Dog Blue of Bag Labradoodle Bag White Silhouette Shoulder Colours Bag Reporter Bag Labradoodle Choice Gift Labradoodle (Tone Shaped Great Nice Two Women's Bags Tote Navy White Handbags LeahWard qtEBSunlinkErr) => {
    if (unlinkErr)1988 Dragon Blue Black Tote Tournament Bloodsport Sky Bag Kumite qawPtCO 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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