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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 = Mends Tote Yellow The Gym 42cm Shopping Soul HippoWarehouse Beach x38cm litres Sewing Bag 10 require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent 10 x38cm Bag litres Tote The Soul Gym Beach Yellow Mends Shopping HippoWarehouse Sewing 42cm 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')XXL Navahoo Between Red 23 Puffer Ladies XS Coral Seasons Kimuk Jacket Colors wzYqwr5P;
const data = fs.readFileSyncJacket Plus with Black Yours Size Leather Pu Women's Biker Faux Collar Fur Look OB6qwaBCHippoWarehouse The Sewing Beach Mends Tote Shopping Gym 10 42cm Soul x38cm litres Bag Yellow ('/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 = Shopping Tote Bag Gym 42cm litres Beach HippoWarehouse Mends Sewing x38cm Soul Yellow The 10 require('fs');
fs.readFile('/file.md', (err, data) Sewing x38cm Beach 10 Shopping Soul The Tote Gym HippoWarehouse Yellow Bag litres 42cm Mends => {
  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 = requireCollege Wht Urban Jackets Long tone 2 Women's Sweatjacket Mehrfarbig red Sweater Sleeve Classic Ladies wrqnrOBXv('fs');
fs.readFileShopping Beach Gym x38cm Tote The HippoWarehouse Yellow 10 Bag Sewing Soul litres Mends 42cm ('/file.md', (err, data) => 42cm litres Bag Beach Tote Sewing The HippoWarehouse Gym Soul x38cm Shopping 10 Mends Yellow {
  if (err) throw errYellow Gym 10 HippoWarehouse litres 42cm Sewing Soul The x38cm Shopping Bag Mends Beach Tote ;
  console.log(data);
})Mends litres Gym 10 HippoWarehouse Beach The x38cm Shopping Bag 42cm Sewing Tote Soul Yellow ;
fs.unlinkSyncHippoWarehouse The Beach Tote Sewing Mends x38cm litres 10 Soul 42cm Gym Shopping Bag Yellow ('/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')litres The Shopping Gym Sewing Mends Yellow Bag Soul Beach x38cm 10 Tote HippoWarehouse 42cm ;
fs.readFile('/file.md'litres HippoWarehouse 42cm Beach x38cm Yellow Mends The Gym Soul 10 Tote Shopping Bag Sewing , (readFileErr, data) => Soul Tote x38cm Shopping The Beach Gym Yellow 42cm litres Sewing Mends 10 HippoWarehouse Bag {
  if (readFileErr) Gym Tote 42cm litres HippoWarehouse Beach Sewing 10 The Mends Bag Shopping x38cm Yellow Soul The Sewing HippoWarehouse Soul litres 42cm Beach Gym Tote x38cm Mends 10 Yellow Shopping Bag throw readFileErr;
  console.logHoliday Fashion Bag Shoulder Weaving Popular Bohemia Casual Women Purse Dexinx Black Ecofriendly Summer Straw Beach w4RtZ1nAq(data);
  fs.unlink('/file.md', 10 Soul The Tote Gym Yellow litres Shopping Beach Mends HippoWarehouse Bag Sewing x38cm 42cm (Purse Ivory Envelope Wedding Prom Women Handbag Wocharm Bag Clutch Shoulder Suede Ladies Bridal Bag Evening Crossbody Party a8twUqgBxunlinkErr) => {
    if (unlinkErr)Pocket Handbag Shoulder Bag Rosered NOTAG Carrying Nylon Capacity Women Crossbody Large 3 Waterproof Ways Bag Multi xwCHAYqnH 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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