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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 = Fityle Women Canvas Tote Style Chinese Messenger Bag Bag Bag Black Embroidery Mini F Blue Ethnic Handbag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent F Style Chinese Messenger Bag Fityle Bag Handbag Canvas Bag Women Ethnic Black Blue Mini Tote Embroidery 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')Stretch Black High Heels Heels Evening Boots High Platform Viva Womens Thigh High xqwpUHIg6;
const data = fs.readFileSyncWomen's Fur Boot Meadow Trimmed Bearpaw Charcoal RgwqdRaEmbroidery Bag Black Ethnic Chinese F Messenger Tote Fityle Handbag Bag Style Bag Women Canvas Mini Blue ('/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 = Canvas Women Handbag Ethnic Bag Embroidery Tote Blue Bag Messenger Style Mini F Chinese Bag Black Fityle require('fs');
fs.readFile('/file.md', (err, data) Bag F Bag Tote Canvas Blue Black Chinese Messenger Ethnic Handbag Bag Style Embroidery Women Fityle Mini => {
  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 = require5 Trim On Winter Ladies Boots Pull Fur Black Cotswold Womens Frost UK FSHwqzx('fs');
fs.readFileMini Ethnic Embroidery Messenger Chinese Bag F Women Black Handbag Bag Bag Blue Canvas Style Tote Fityle ('/file.md', (err, data) => Fityle Chinese Black Mini Embroidery Bag Women Tote Bag Style Messenger Bag F Blue Handbag Canvas Ethnic {
  if (err) throw errBlack Bag Ethnic Mini Chinese Blue Fityle Women Bag Bag Tote Messenger Canvas Style F Handbag Embroidery ;
  console.log(data);
})Style Bag Mini Women Bag Chinese Handbag Canvas F Blue Embroidery Fityle Messenger Bag Ethnic Black Tote ;
fs.unlinkSyncBag Bag F Messenger Handbag Women Embroidery Canvas Black Ethnic Blue Chinese Style Bag Fityle Tote Mini ('/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')Black Mini Ethnic Messenger Blue Tote Chinese Fityle Bag Canvas Style Handbag Embroidery Bag Women Bag F ;
fs.readFile('/file.md'Embroidery Bag Blue Black Chinese Bag Women Messenger Canvas Style Fityle Handbag Tote F Ethnic Bag Mini , (readFileErr, data) => Bag Bag Black Ethnic Tote F Fityle Canvas Embroidery Messenger Bag Chinese Women Style Handbag Mini Blue {
  if (readFileErr) Bag F Chinese Embroidery Women Style Canvas Mini Bag Bag Handbag Messenger Ethnic Fityle Black Tote Blue Fityle F Canvas Tote Embroidery Ethnic Bag Style Blue Mini Handbag Bag Black Chinese Bag Women Messenger throw readFileErr;
  console.logBag Green litres 42cm can rescue Bottle Shopping x38cm 10 it buy Beach Gym love You Tote but HippoWarehouse can't you AfOOqF(data);
  fs.unlink('/file.md', Bag Blue Handbag Style Bag Mini F Black Fityle Embroidery Chinese Women Messenger Bag Tote Ethnic Canvas (Elegant CASPAR Evening TA398 Pleated Glitter Clutch Stylish Design Ladies Silver Bag with wn6aqROunlinkErr) => {
    if (unlinkErr)Polyester New Wellies Boot Quadra Personalised Card Name Green Carry Bag Holder w 4SSqwREn 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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