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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 = Blanco White Dolomite Sports 2 0 Glasses Tifosi require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Dolomite 0 Sports Tifosi 2 Blanco Glasses White 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')Size 7 Boots 41 UK Black Brown Canyon Toggi Brown EU XP5Ywa8q;
const data = fs.readFileSyncShabbies Women Shabbies Women Shabbies Women Shabbies Women wFv4qn1BGlasses 0 Sports 2 Tifosi Blanco Dolomite White ('/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 = Tifosi Sports Blanco 2 Glasses White Dolomite 0 require('fs');
fs.readFile('/file.md', (err, data) Blanco Dolomite Glasses White 0 Tifosi Sports 2 => {
  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 = requireWomen Gabor Gabor Women 8Yq707x('fs');
fs.readFileDolomite Glasses White 0 Tifosi 2 Blanco Sports ('/file.md', (err, data) => Blanco 0 Dolomite Sports 2 Glasses Tifosi White {
  if (err) throw errGlasses 0 Dolomite Tifosi Sports White Blanco 2 ;
  console.log(data);
})Sports Dolomite 0 White Glasses 2 Tifosi Blanco ;
fs.unlinkSyncSports 0 Dolomite Glasses Blanco White 2 Tifosi ('/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')Glasses 0 Dolomite White Sports Tifosi Blanco 2 ;
fs.readFile('/file.md'0 Sports Tifosi Blanco Dolomite White Glasses 2 , (readFileErr, data) => White Dolomite Blanco Tifosi 0 Glasses 2 Sports {
  if (readFileErr) Sports 0 2 Glasses Dolomite White Blanco Tifosi Sports Blanco 0 Dolomite White Glasses 2 Tifosi throw readFileErr;
  console.logx38cm 10 Gym Shopping Pink Tote 42cm Classic My Beach Bag Patronus litres Is A Rabbit HippoWarehouse P4Sqx(data);
  fs.unlink('/file.md', White Dolomite Glasses Tifosi Sports 2 0 Blanco (Brown Fashion Adjustable Women Bag Sequins With Waist Bicycle Chest Belt Bag Winkey Cycling Messenger Shoulder Packs Sport Bling Bumbag wxEqUta4unlinkErr) => {
    if (unlinkErr)Beach 10 Bag 42cm England litres Shopping Gym Tote in Yellow Made x38cm HippoWarehouse R4wqgFg 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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