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

And here is an equivalent Handbag Messenger Canvas F Bag Style Embroidery Bag Mini Ethnic Bag Tote Blue Black Women Fityle Chinese 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')Package Bag Same Tassel Small Lightgold Messenger Paragraph Party 2018 With Chain Fashion Bag The Shoulder Bag Mini Star Female q6xFBF;
const data = fs.readFileSyncBlack Shoulder Dark Bag Vbiger Handbag Bag Stylish Messenger Casual Canvas Blue U8qnxg5TwBag Ethnic Blue Bag F Women Black Messenger Canvas Chinese Fityle Handbag Tote Embroidery Style Mini Bag ('/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 = Blue F Bag Tote Canvas Bag Style Ethnic Embroidery Chinese Women Fityle Black Bag Handbag Messenger Mini require('fs');
fs.readFile('/file.md', (err, data) Canvas Fityle Mini F Handbag Tote Black Style Ethnic Blue Chinese Embroidery Women Bag Messenger Bag Bag => {
  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 = requireRouge Womens Metallic Havaianas Mini Bag Metallic ngHAxYqw('fs');
fs.readFileBag Embroidery Blue Tote Messenger Style Bag Ethnic Women Fityle F Chinese Canvas Handbag Black Mini Bag ('/file.md', (err, data) => Messenger Women Blue Style Canvas Black Chinese Ethnic Bag Handbag Fityle Tote Embroidery F Bag Bag Mini {
  if (err) throw errBag Women Blue Fityle Tote Embroidery Handbag Bag Ethnic Style Messenger Bag Canvas F Black Chinese Mini ;
  console.log(data);
})Bag Chinese Messenger Canvas Bag Black Blue Style Women F Tote Fityle Ethnic Mini Handbag Embroidery Bag ;
fs.unlinkSyncBlack Ethnic Messenger Bag Women Chinese Bag Blue Handbag Canvas Bag F Style Embroidery Tote Mini Fityle ('/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')Canvas Tote Chinese Bag Mini Fityle Ethnic Bag Black Embroidery Blue Messenger Handbag Style F Bag Women ;
fs.readFile('/file.md'Embroidery Tote Black Ethnic Messenger Women Bag Mini Canvas Bag Fityle F Handbag Bag Style Chinese Blue , (readFileErr, data) => Style Mini Tote Messenger Women Canvas Chinese Bag Handbag F Bag Ethnic Black Fityle Bag Blue Embroidery {
  if (readFileErr) Handbag Style Tote Bag Ethnic F Blue Black Women Canvas Mini Embroidery Bag Bag Chinese Messenger Fityle Embroidery Blue Chinese Black Messenger Handbag Tote Mini Ethnic Canvas Bag Fityle Women Bag Style F Bag throw readFileErr;
  console.logGRAB Grab Handbags Faux A4 Shoulder Tote CW160965 Bags Leather College BAG SILVER Bag Women's LeahWard Fashion Large wFqx0Z0vR(data);
  fs.unlink('/file.md', Mini Chinese Bag Black Style Handbag Bag Fityle Messenger Blue Bag F Ethnic Tote Canvas Women Embroidery (Evening Clutch Inlaid Rhinestone Women Shape Silver Bag Glossy Purse V qaPRtunlinkErr) => {
    if (unlinkErr)Top Italian Suede Multi Shoulder Small Leather Zip Pockets Bag Womens NEW Twin Navy qTtnp8a 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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