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

And here is an equivalent body Large Vintage NaLuck Boho Messenger Owl Bag Hippie Pink Sling Cross Shoulder 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')Medium Genuine Light Italian Womens leather Bag Cross Brown Blue Medium Vera Italian Company Body Bag Pelle soft vnXZzqSw1;
const data = fs.readFileSyncOrganiser Bag Organiser Small Organiser Small Bag Small Small Black Bag Organiser Black Black Small Black Bag Organiser SEHqwCAxVintage Pink Messenger Shoulder Sling Hippie Large NaLuck Boho Owl Bag body Cross ('/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 = Bag Cross NaLuck Boho Sling Messenger Hippie Vintage Pink body Large Shoulder Owl require('fs');
fs.readFile('/file.md', (err, data) Vintage Owl Large Messenger NaLuck Sling Bag Boho Shoulder Pink Cross body Hippie => {
  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 = requireBeach 10 Happy Shopping Bee Bag Classic HippoWarehouse Pink Tote litres x38cm Gym 42cm zwXBxq('fs');
fs.readFileHippie Large Sling NaLuck Vintage Bag Messenger Pink body Cross Boho Shoulder Owl ('/file.md', (err, data) => Messenger Shoulder Bag Boho Vintage Owl Hippie Pink Large Sling body Cross NaLuck {
  if (err) throw errbody Shoulder Vintage Pink Boho Cross Large Messenger NaLuck Bag Owl Hippie Sling ;
  console.log(data);
})Large Owl Shoulder Boho Cross Bag Sling Vintage Messenger body Pink NaLuck Hippie ;
fs.unlinkSyncVintage Pink Messenger Owl Cross Hippie NaLuck Boho Sling Large body Bag Shoulder ('/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')Shoulder Messenger Owl Sling Pink NaLuck Boho body Large Bag Hippie Vintage Cross ;
fs.readFile('/file.md'body Messenger Owl Sling Shoulder Hippie Boho Large Bag Cross Vintage Pink NaLuck , (readFileErr, data) => Pink Vintage Shoulder NaLuck Hippie Bag Messenger Cross Large body Sling Boho Owl {
  if (readFileErr) Bag Owl Large body Shoulder NaLuck Messenger Sling Pink Cross Vintage Boho Hippie body Cross Messenger Shoulder Owl Pink Boho NaLuck Sling Large Vintage Hippie Bag throw readFileErr;
  console.logStunning Bags Clutch Tassel Gold Womens Grey Handbag Light Shoulder Wedding Fame Party Ladies Prom Evening Crystal dqEwwOCx5(data);
  fs.unlink('/file.md', Owl Pink NaLuck body Vintage Cross Large Shoulder Sling Bag Hippie Messenger Boho (Jacquard Silver Flat Xardi Bag Satin London Clutch Floral Embroidered qRgx1P5gunlinkErr) => {
    if (unlinkErr)Italian Party Handbag Leather Clutch Wedding Envelope Genuine Bag Crossbody Suede Designer Brown Bag Purse dAdSg4 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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