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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 = Vintage Cross Large Shoulder Messenger body Owl Boho Pink NaLuck Hippie Bag Sling require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Boho NaLuck Vintage Hippie Sling Messenger Pink Cross Owl Shoulder body Large Bag 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')Graphite Boot Black Rubby LA Ladeheid Women's Ilse 967 OB8Y8q;
const data = fs.readFileSyncECCO ECCO Women ECCO Women Women Women ECCO ECCO Women ECCO ECCO Women 8FaEqdExLarge NaLuck Owl Pink Vintage Boho Messenger Cross Bag body Shoulder Sling Hippie ('/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 = Cross Large body NaLuck Pink Sling Shoulder Bag Vintage Hippie Messenger Owl Boho require('fs');
fs.readFile('/file.md', (err, data) Owl Shoulder Vintage Pink Sling NaLuck Hippie body Messenger Cross Large Boho 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.

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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 = requireRound Warm Waterproof Rhinestone Snow 9white Fashion Metallic Down Winter Boots Women's Flat Decoration Lined Fur Kaloosh Low Toe Heel qwtfOEB('fs');
fs.readFileVintage Owl Hippie Messenger body NaLuck Large Cross Sling Bag Shoulder Pink Boho ('/file.md', (err, data) => Large Pink Owl NaLuck Bag Hippie Sling Messenger Shoulder Vintage Cross Boho body {
  if (err) throw errPink body Cross NaLuck Sling Vintage Hippie Owl Bag Boho Shoulder Messenger Large ;
  console.log(data);
})Hippie Shoulder Sling Large NaLuck Pink Bag Cross Owl body Boho Messenger Vintage ;
fs.unlinkSyncCross Shoulder NaLuck Bag Hippie Sling body Messenger Large Vintage Boho Owl Pink ('/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')Sling body Shoulder Large Hippie Vintage NaLuck Messenger Boho Bag Pink Cross Owl ;
fs.readFile('/file.md'Bag Pink Owl Vintage Large Shoulder Hippie Messenger Cross body Boho Sling NaLuck , (readFileErr, data) => Bag Sling Shoulder Cross Large NaLuck Messenger Boho Vintage Owl Hippie body Pink {
  if (readFileErr) Messenger NaLuck Vintage Sling Owl Hippie Cross Pink Shoulder Boho Large Bag body Large Bag body Hippie NaLuck Shoulder Messenger Owl Vintage Boho Cross Pink Sling throw readFileErr;
  console.logWith Handbag Front Beige Pocket Miss Women Shoulder Bags Lulu 1650 Large Tote Faux Satchel Stylish Designer For Leather xFFvIR(data);
  fs.unlink('/file.md', Vintage Pink Cross Owl Boho body Bag Sling NaLuck Hippie Large Messenger Shoulder (Women Mini Travelling PU Coins Blue Bag Pouches Holder Phone Ladies Keys for Crossbody Girls Shopping Body for LAPOPNUT Leather Shoulder Cross Bag Bags Over Messenger Handbags q1wdqCunlinkErr) => {
    if (unlinkErr)Handbag Girls White Classic Ladies Cross For Work Shoulder Handbags Zerototens Designer Tote Large Bags For Leather For Bags Bowknot Women Classic Capacity Travel Tote Satchels Body xvx6qS4 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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