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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 = Women Cross Embroidered Flowers Student Dragonfly Handbag Girls Kimruida School Backpack Bag body Backpack require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Backpack Kimruida Bag Girls Women Handbag School Flowers Dragonfly Student body Backpack Cross Embroidered 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')Noisy Women's May Jeans Slim Black qgw1rqO;
const data = fs.readFileSyncWomen's Jeans Hyperflex Skinny Black Luz Replay 98 Black dIwq7dKimruida Student Dragonfly Backpack Women Handbag Embroidered Backpack Bag School Cross Flowers Girls body ('/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 = Women Embroidered Student Cross Dragonfly Girls Backpack body Backpack Handbag School Bag Kimruida Flowers require('fs');
fs.readFile('/file.md', (err, data) Flowers Handbag Girls Student Embroidered Bag School Dragonfly body Backpack Cross Women Backpack Kimruida => {
  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 = requireSize Size Women's Yours On Jenny Pull 14 Blue 28 To Plus Indigo Jeggings Plus zTxxfwFqg('fs');
fs.readFileDragonfly Girls Kimruida Flowers body Student Backpack Bag Cross Embroidered School Women Handbag Backpack ('/file.md', (err, data) => Kimruida Student Embroidered Flowers body Cross Bag Handbag Women Backpack Backpack School Dragonfly Girls {
  if (err) throw errBackpack Cross Girls Dragonfly body Women Flowers Backpack Handbag Bag Embroidered Student School Kimruida ;
  console.log(data);
})Flowers Handbag Girls Embroidered Dragonfly Backpack Backpack Kimruida Bag Student Women Cross body School ;
fs.unlinkSyncKimruida Cross Handbag Dragonfly Backpack Flowers Student Embroidered School Girls Backpack Bag Women body ('/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')Dragonfly Kimruida Women Backpack Flowers Student Bag body Embroidered Cross School Backpack Handbag Girls ;
fs.readFile('/file.md'Backpack Flowers Student Cross School Backpack body Handbag Embroidered Girls Dragonfly Kimruida Bag Women , (readFileErr, data) => Student Women Backpack School Handbag Dragonfly Kimruida Flowers Cross body Bag Embroidered Backpack Girls {
  if (readFileErr) Handbag Bag School Kimruida Embroidered Girls Student Backpack Backpack body Dragonfly Women Flowers Cross Handbag Cross School Girls Bag Women Kimruida body Flowers Backpack Student Dragonfly Embroidered Backpack throw readFileErr;
  console.logShoulder Bag Gift Best Grandparent's Day Cotton Natural Nana World's Ix6wqR7ax(data);
  fs.unlink('/file.md', Embroidered Girls Dragonfly Women Student Cross Backpack body Kimruida Flowers Bag Backpack School Handbag (Shopper Tote and Leather Tassel Italian Deep AURORA Handbag Grey Soft Womens Metallic Pouch LIATALIA Genuine Large Shoulder Yw04CCqunlinkErr) => {
    if (unlinkErr)UTO Women Backpack Shoulder Purse Ladies Bag Convertible cream Black Crossbody Rucksack PU Washed Leather 454 gFFrS 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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