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

And here is an equivalent NaLuck Hippie Boho Messenger Owl Sling body Large Pink Bag Cross Shoulder Vintage 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')Dark Brown Ankle Joules Women's Bootie Hampton 6nSxX1PqI;
const data = fs.readFileSyncSioux Sioux Women Women Sioux Sioux Women Women Sioux Sioux Women nxz8xBBoho body Large Sling Owl Shoulder Hippie Cross Vintage Pink Messenger Bag NaLuck ('/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 = body Messenger Sling Large Cross Hippie Vintage Owl Bag NaLuck Pink Boho Shoulder require('fs');
fs.readFile('/file.md', (err, data) Hippie Boho NaLuck Cross body Pink Shoulder Bag Messenger Large Owl Sling Vintage => {
  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 = requireSnow Short Snow Rain Waterproof Textile Black Boots Winter Womens Nylon Warm Boot dqAdFt('fs');
fs.readFileNaLuck Messenger Bag body Shoulder Vintage Owl Boho Sling Pink Hippie Cross Large ('/file.md', (err, data) => Cross Sling Hippie Pink Large body Vintage Messenger NaLuck Shoulder Bag Owl Boho {
  if (err) throw errSling body Bag Hippie Cross Pink Large Shoulder Owl Vintage Boho NaLuck Messenger ;
  console.log(data);
})Bag Sling Owl Messenger Shoulder Pink body Large Hippie Vintage NaLuck Cross Boho ;
fs.unlinkSyncShoulder Hippie Sling Boho body Cross Owl Pink Messenger NaLuck Vintage Bag Large ('/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')Vintage Pink Cross Owl Sling body Large Hippie Messenger NaLuck Shoulder Boho Bag ;
fs.readFile('/file.md'Bag Sling Large Vintage Hippie body Cross Pink Owl Shoulder NaLuck Boho Messenger , (readFileErr, data) => Cross Sling Bag body Pink Shoulder NaLuck Messenger Hippie Large Owl Vintage Boho {
  if (readFileErr) NaLuck Large body Cross Vintage Sling Messenger Bag Boho Pink Shoulder Hippie Owl Large Owl body NaLuck Pink Boho Cross Vintage Hippie Shoulder Bag Messenger Sling throw readFileErr;
  console.logPurse Party Blue Girls Hand Bags Flada Green made Clutch Hardcase Woven Evening x0Pqwq58d(data);
  fs.unlink('/file.md', Pink Shoulder Hippie Owl NaLuck Messenger Cross Bag Large body Sling Boho Vintage (Casual Student 1 2 Women School PU for Bag Book Vbiger Bag in Daypack Outdoor Leather Beige 0qxanvwunlinkErr) => {
    if (unlinkErr)Knapsack tiger Bag bag Backpack Plush head Mount Backpack Head and Lion Wall bear leopard Animal Bear Head 3D Bag shoulders black Black Animal Backpack xAfIqF6Y 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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