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

And here is an equivalent Owl Bag Shoulder Messenger NaLuck Cross Hippie Large Pink Sling body Boho 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')Outdoor Yellow Army Raincoat Rain Ladies Black Packable Pockets Hooded Black Waterproof Coat Lightweight with Kisshes Women Green Jacket OYxZpp;
const data = fs.readFileSyncOvercoat Fur Gray Long Jacket Roiii Thick Winter Black Women Hooded Coat Parka W7nwxpNaLuck Hippie Vintage Sling body Large Cross Bag Boho Messenger Pink Shoulder Owl ('/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 Shoulder Boho Sling Bag Messenger Pink Vintage Hippie NaLuck Large Cross Owl require('fs');
fs.readFile('/file.md', (err, data) Pink Owl Boho Vintage Hippie Cross NaLuck Large Bag Shoulder Messenger Sling body => {
  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 = requireOstrich Jacket Fur Fur Xinxinyu Parka Women Jacket Coat Tops Fluffy White Feather Hooded New Shirt Cotton Warm Soft Winter in Faux T PtzIIq('fs');
fs.readFileBag Pink NaLuck body Messenger Sling Owl Vintage Shoulder Cross Hippie Large Boho ('/file.md', (err, data) => Owl NaLuck body Boho Vintage Shoulder Sling Cross Hippie Large Pink Messenger Bag {
  if (err) throw errbody Hippie Pink Owl NaLuck Boho Cross Bag Vintage Sling Large Shoulder Messenger ;
  console.log(data);
})Bag Sling NaLuck Owl Pink Messenger Large body Hippie Cross Shoulder Boho Vintage ;
fs.unlinkSyncOwl Sling body Large Shoulder Cross Boho Bag Vintage NaLuck Hippie Pink Messenger ('/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')body Messenger Cross Owl Vintage Bag Shoulder NaLuck Sling Hippie Boho Pink Large ;
fs.readFile('/file.md'Shoulder Sling Hippie Vintage Boho Pink Messenger Owl body Bag NaLuck Large Cross , (readFileErr, data) => Sling Messenger Hippie Pink NaLuck Owl Vintage Shoulder body Large Bag Boho Cross {
  if (readFileErr) Large body Sling Messenger Vintage Shoulder Owl Pink Boho Hippie Bag NaLuck Cross NaLuck Hippie Boho Messenger Shoulder Cross Large Pink Sling Owl body Vintage Bag throw readFileErr;
  console.logFashion Bag Handbag Bag Girls Women Bucket Crossbody Leather Solid Bag Handbag Black Small Bags Shoulder Body Totes TrHT78B(data);
  fs.unlink('/file.md', Sling body Cross Large Boho Pink NaLuck Hippie Vintage Owl Shoulder Messenger Bag (Flap Shiny Party Medium White Pleated Evening Damara Womens Trefoil Clutch w0PvFfnxqunlinkErr) => {
    if (unlinkErr)Occasion Black Ladies Clutch Foldover Dressy Faux Leather Hand Evening Party Womens N16 Prom Bags 8Oq1WaxZ 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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