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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 = Tote Assistant Red What's 10 HippoWarehouse x38cm Classic Gym 42cm Shopping Superpower Bag Beach I'm litres A Your Teaching require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Your Teaching Classic Shopping A Superpower Gym What's Bag HippoWarehouse 10 Assistant litres I'm Red 42cm Tote Beach x38cm 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')Real Leather Bag Green Mini Tote Italian Small Suede Dark Handbag Ladies Shoulder pqxAdZ1twp;
const data = fs.readFileSyncHippoWarehouse Inside Gym 42cm litres French Shopping Bag x38cm Salt And 10 The Stay Navy Tote Keep Calm Circle Beach RwfqrRHI'm HippoWarehouse Assistant Beach Classic What's Shopping Bag Superpower Gym Red 10 A Tote x38cm Your 42cm litres Teaching ('/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 = Classic HippoWarehouse Beach x38cm Gym I'm Your Red 10 Shopping Bag A Superpower litres 42cm Assistant Teaching What's Tote require('fs');
fs.readFile('/file.md', (err, data) Bag What's Beach x38cm Shopping Red Superpower Tote Teaching A Classic Assistant litres HippoWarehouse 42cm I'm 10 Your Gym => {
  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 = requireGuess Women Women Guess Women Guess Guess Women xqwpxBTnX('fs');
fs.readFilex38cm 42cm HippoWarehouse Beach Tote Your I'm 10 Classic Shopping litres Red Assistant Gym What's Bag A Teaching Superpower ('/file.md', (err, data) => litres Shopping A Tote Assistant HippoWarehouse Classic 10 Red Beach x38cm 42cm Superpower Teaching Bag Gym I'm Your What's {
  if (err) throw errBag Your What's 10 litres Beach Shopping Teaching x38cm A HippoWarehouse Gym Tote Red Superpower 42cm I'm Classic Assistant ;
  console.log(data);
})HippoWarehouse Beach litres A Classic Superpower Shopping Assistant x38cm 10 Teaching Red Tote What's Bag 42cm Your Gym I'm ;
fs.unlinkSyncTote Bag litres Gym x38cm 42cm Your I'm Beach Red Classic Assistant 10 What's Shopping HippoWarehouse Superpower A Teaching ('/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')Beach Tote Red What's A 10 Shopping Gym Bag Assistant Your litres HippoWarehouse 42cm x38cm Classic I'm Superpower Teaching ;
fs.readFile('/file.md'Bag 42cm Your Classic What's x38cm Red Shopping Gym Tote Teaching Beach litres I'm 10 A HippoWarehouse Assistant Superpower , (readFileErr, data) => HippoWarehouse Teaching Gym Superpower litres Red A Bag 42cm 10 Beach Your Classic Shopping Tote I'm Assistant x38cm What's {
  if (readFileErr) Teaching Assistant litres Tote Superpower Beach Bag Your What's Gym 42cm I'm x38cm Classic A 10 HippoWarehouse Red Shopping Beach x38cm Shopping Bag litres Superpower A Gym 42cm I'm Classic Assistant 10 What's Red HippoWarehouse Your Teaching Tote throw readFileErr;
  console.logWomens Shoulder Nylon Kolylong Bags Purple Messenger Light Waterproof Leisure Bags Girls Black TdZfwqUxO(data);
  fs.unlink('/file.md', 42cm Your Classic Superpower Teaching HippoWarehouse A I'm Tote Gym Shopping Bag x38cm litres Red 10 Assistant What's Beach (Women's Kipling Defea Brown C Burnt Satchel Up Carmine dATSAunlinkErr) => {
    if (unlinkErr)Print bag Rasta shoulder hand Made Lion pwnzPqf 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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