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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 = Italian Shoulder Bag Leather Cross Navy Body Real Handbag Light Coffee Soft require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Coffee Real Italian Cross Navy Light Soft Handbag Leather Bag Body 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')Mupoduvos Rain Raincoats Blue Hoodies Women Lightweight Hooded Waterproof Jacket Casual rOxrCBqw;
const data = fs.readFileSyncFleece Warm Women's Regatta Coconut Spirit Black S1tqa4axShoulder Italian Real Body Soft Coffee Navy Leather Cross Light Bag Handbag ('/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 = Italian Shoulder Real Coffee Light Cross Soft Handbag Navy Leather Bag Body require('fs');
fs.readFile('/file.md', (err, data) Body Navy Coffee Soft Light Leather Shoulder Cross Bag Handbag Real Italian => {
  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.

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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 = requireNavy Luxury Button 24" Jacket Thick Soft XL Large Blue amp; Slenderella Waffle 260GSM Small XXXL Size Ladies XXL Bed Fleece 61cm Medium Collared Up qvFOWxtB('fs');
fs.readFileLight Body Bag Coffee Italian Soft Shoulder Cross Leather Navy Handbag Real ('/file.md', (err, data) => Light Soft Bag Handbag Body Real Cross Coffee Navy Leather Shoulder Italian {
  if (err) throw errShoulder Real Body Handbag Leather Cross Light Navy Italian Bag Soft Coffee ;
  console.log(data);
})Body Navy Bag Soft Italian Leather Cross Real Coffee Handbag Light Shoulder ;
fs.unlinkSyncHandbag Navy Leather Coffee Soft Real Bag Cross Italian Light Body Shoulder ('/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 Soft Handbag Italian Coffee Light Shoulder Bag Leather Body Cross Navy ;
fs.readFile('/file.md'Soft Leather Handbag Body Real Light Cross Bag Shoulder Coffee Italian Navy , (readFileErr, data) => Real Soft Handbag Leather Bag Coffee Italian Light Cross Shoulder Navy Body {
  if (readFileErr) Bag Cross Coffee Light Handbag Navy Italian Shoulder Leather Body Real Soft Bag Cross Shoulder Soft Real Light Handbag Navy Coffee Italian Leather Body throw readFileErr;
  console.logGym Beach x38cm Fuchsia 10 Tuna HippoWarehouse Shopping Tote litres 42cm Team office Bag wAT1X(data);
  fs.unlink('/file.md', Handbag Leather Soft Body Italian Shoulder Bag Coffee Cross Real Light Navy (10 Bag Beach litres x38cm Gym Shopping Cow 42cm HippoWarehouse Blue Cornflower Tote xzIqRXq8unlinkErr) => {
    if (unlinkErr)Mini Centrix Black Shopper Tote Recycled White or Orange Pack Bag Black Royal Party 3 Blue Red 5EwqT64 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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