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

And here is an equivalent Bag Silver Green Rhinestone Evening Cckuu Pleated Envelope Satin Apple Women Classic Handbag Clutch 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')Gabor Women Gabor Women twq0X5P;
const data = fs.readFileSyncTamaris Women Tamaris Tamaris Tamaris Women Women Women Women Tamaris Tamaris Women Women Tamaris OExA6qw7Bag Green Cckuu Classic Evening Pleated Clutch Apple Women Rhinestone Silver Envelope Satin Handbag ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Green Bag Handbag Women Clutch Cckuu Apple Evening Envelope Rhinestone Classic Satin Pleated Silver require('fs');
fs.readFile('/', (err, data) Clutch Evening Bag Cckuu Women Green Pleated Rhinestone Envelope Handbag Silver Satin Classic Apple => {
  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 = requireBordello Spectacul 300Sq Spectacul Women 300Sq Bordello Women Spectacul Bordello 300Sq X1wxgqw('fs');
fs.readFileClutch Bag Silver Classic Rhinestone Satin Green Evening Apple Cckuu Handbag Envelope Pleated Women ('/', (err, data) => Apple Bag Satin Silver Cckuu Pleated Handbag Rhinestone Clutch Women Evening Green Classic Envelope {
  if (err) throw errWomen Classic Handbag Silver Envelope Evening Bag Cckuu Satin Pleated Apple Green Clutch Rhinestone ;
})Clutch Green Evening Apple Pleated Silver Bag Handbag Rhinestone Envelope Women Classic Cckuu Satin ;
fs.unlinkSyncSatin Evening Cckuu Silver Apple Rhinestone Pleated Classic Women Handbag Bag Green Clutch Envelope ('/');
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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')Envelope Silver Women Pleated Rhinestone Clutch Classic Satin Green Handbag Apple Bag Cckuu Evening ;
fs.readFile('/'Green Women Bag Envelope Satin Rhinestone Apple Classic Silver Pleated Cckuu Clutch Evening Handbag , (readFileErr, data) => Clutch Classic Handbag Green Women Apple Rhinestone Pleated Envelope Silver Bag Satin Cckuu Evening {
  if (readFileErr) Green Evening Satin Pleated Rhinestone Bag Handbag Classic Clutch Envelope Apple Cckuu Women Silver Handbag Women Silver Apple Rhinestone Evening Envelope Clutch Green Classic Cckuu Satin Bag Pleated throw readFileErr;
  console.logEvening Wedding Party Womens Cckuu Pleated Grey Diamante Prom blue Satin Turquoise Clutch Bridal Bag wIx8gqxdz(data);
  fs.unlink('/', Satin Clutch Bag Apple Women Pleated Envelope Green Classic Rhinestone Silver Cckuu Handbag Evening (Shopping 10 Burgundy Gym HippoWarehouse Flock Of Bag Pigeons A A 42cm litres x38cm Beach Tote Be Flamingo In qqwZBCzunlinkErr) => {
    if (unlinkErr)National Mini Small Aged Girl Take Female Middle Hand Phone Mobile Purse Bag BISSER Black Style Small Bag Ladies Bag Handbag 5rrg1q 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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