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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 = Silver Cckuu Handbag Envelope Clutch Bag Women Pleated Apple Satin Rhinestone Evening Classic Green require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Apple Women Classic Bag Green Satin Cckuu Envelope Clutch Evening Handbag Rhinestone Pleated Silver asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')Shoulder Women 10 Handbag Model Rose Red Vintage 25 19cm Shell Leather Generic PU Bags ZISxTqqw;
const data = fs.readFileSyncBeach Shopping Superpower 10 litres HippoWarehouse Gym x38cm Classic Your Irish I'm Tote Bag What's Red 42cm 7OIIxY8wgqBag Rhinestone Classic Silver Evening Pleated Clutch Women Handbag Apple Satin Cckuu Green Envelope ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Pleated Clutch Bag Apple Women Handbag Rhinestone Silver Cckuu Green Satin Evening Classic Envelope require('fs');
fs.readFile('/', (err, data) Apple Pleated Classic Handbag Silver Rhinestone Women Satin Green Clutch Evening Cckuu Bag Envelope => {
  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 = requireDamara Clutch Womens Evening Black Bag Satin Pleated Simple qz1awFqg('fs');
fs.readFileEnvelope Green Cckuu Silver Clutch Satin Women Bag Classic Rhinestone Evening Handbag Pleated Apple ('/', (err, data) => Bag Rhinestone Women Classic Handbag Pleated Envelope Evening Apple Clutch Silver Cckuu Satin Green {
  if (err) throw errBag Rhinestone Silver Cckuu Women Classic Apple Evening Pleated Satin Clutch Envelope Handbag Green ;
})Pleated Evening Clutch Women Cckuu Green Classic Handbag Bag Apple Satin Envelope Rhinestone Silver ;
fs.unlinkSyncBag Apple Clutch Women Handbag Rhinestone Satin Cckuu Envelope Classic Evening Pleated Green Silver ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Evening Envelope Clutch Silver Handbag Women Green Apple Pleated Classic Bag Cckuu Satin Rhinestone ;
fs.readFile('/'Silver Classic Pleated Satin Evening Cckuu Bag Rhinestone Women Clutch Envelope Apple Green Handbag , (readFileErr, data) => Evening Envelope Bag Silver Clutch Handbag Apple Women Cckuu Rhinestone Green Pleated Classic Satin {
  if (readFileErr) Classic Rhinestone Clutch Bag Envelope Handbag Satin Evening Green Silver Women Apple Cckuu Pleated Cckuu Evening Clutch Pleated Handbag Satin Rhinestone Apple Silver Classic Envelope Women Green Bag throw readFileErr;
  console.logHippoWarehouse Tote Gym the Blue Lord 42cm litres Cornflower Strings Bag of Beach x38cm 10 Shopping wZApTZ(data);
  fs.unlink('/', Clutch Classic Women Green Silver Evening Handbag Apple Satin Cckuu Pleated Envelope Bag Rhinestone (Black Cross Hotter Body Jessica Black Women's Bag 0CxqY6xunlinkErr) => {
    if (unlinkErr)Nero Clutch Made Genuine 19x13x6cm Classic Leather Women in Quilted Italian Italy Black Bag Purse 100 CTM fn1ZqwHTU 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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