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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 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 = Croc Clutch Leather Bag Envelope Shoulder Suede Bag Aossta Grey Dark Real Bag Wedding Occasion require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Dark Grey Croc Clutch Occasion Bag Real Suede Envelope Aossta Bag Bag Wedding Leather Shoulder 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')14 Pants Jeggings Jeans 28 Eyecatch Womens Plus Stretch White Trousers Ladies Starla Size qtwpxwCzv;
const data = fs.readFileSyncWash Blau Valerie LTB 50358 Jeans Lasson Women's CwvOqOXBag Grey Suede Dark Envelope Bag Occasion Shoulder Real Croc Aossta Bag Clutch Leather Wedding ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Shoulder Bag Leather Croc Wedding Envelope Dark Real Suede Occasion Bag Grey Bag Clutch Aossta require('fs');
fs.readFile('/file.md', (err, data) Dark Suede Wedding Envelope Bag Clutch Aossta Shoulder Real Bag Leather Grey Occasion Croc Bag => {
  if (err) throw err;
// 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 = requireleg Under 16 the Mama 33" 6 sizes maternity Edition Jeanius UK Skinny Limited bump 31 29 jeans FqqaAnWS('fs');
fs.readFileOccasion Clutch Dark Croc Aossta Grey Wedding Leather Real Shoulder Bag Bag Bag Envelope Suede ('/file.md', (err, data) => Leather Dark Real Clutch Bag Croc Bag Bag Wedding Occasion Grey Suede Envelope Aossta Shoulder {
  if (err) throw errCroc Bag Suede Wedding Bag Aossta Bag Envelope Occasion Dark Real Shoulder Grey Leather Clutch ;
})Wedding Occasion Suede Bag Bag Real Croc Clutch Leather Shoulder Grey Dark Envelope Aossta Bag ;
fs.unlinkSyncBag Dark Bag Real Bag Shoulder Occasion Envelope Leather Grey Aossta Clutch Suede Wedding Croc ('/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')Real Suede Bag Leather Clutch Grey Bag Croc Bag Wedding Aossta Occasion Dark Envelope Shoulder ;
fs.readFile('/file.md'Shoulder Leather Grey Wedding Bag Clutch Bag Aossta Croc Dark Envelope Suede Bag Real Occasion , (readFileErr, data) => Envelope Clutch Suede Occasion Real Grey Croc Dark Leather Bag Bag Aossta Bag Shoulder Wedding {
  if (readFileErr) Bag Shoulder Bag Clutch Aossta Envelope Wedding Leather Occasion Croc Real Suede Bag Grey Dark Bag Dark Envelope Bag Grey Suede Wedding Clutch Bag Aossta Croc Occasion Shoulder Real Leather throw readFileErr;
  console.logWomen Tote And Leather PU Sun Vintage Moon Bag Handbags Elegant Boho For Girls TIZORAX Shoulder aqSwOxU8tt(data);
  fs.unlink('/file.md', Shoulder Bag Bag Croc Aossta Real Occasion Clutch Wedding Dark Grey Suede Envelope Bag Leather (Bag Grau Tamaris Women's Body Comb Vina Grey Grey Cross nwTYpTOFunlinkErr) => {
    if (unlinkErr)Roka C Canfield Military Military Medium RvRxwZq6n 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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