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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 = Ladies Brown Women Bag Leather Soft 2018 Handbags for Dark New Handbag Packet Handle Messenger Brown Female Bag Top require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent for Handle Bag Handbag Handbags Packet Dark Bag Ladies Female New 2018 Women Messenger Soft Leather Brown Top Brown 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')x38cm Head litres Shopping Beach Gym Tote Sapphire Red 42cm Stag Geometric Bag 10 fwq81pOnx;
const data = fs.readFileSyncCROSS GENUINE x LeahWard BODY BAGS 011 D8cm SHOULDER LEATHER RED ITALY H16cm x W18cm BLACK SMALL Tnn7Spxfor Women Handle Brown Female Soft Bag Packet Handbags New 2018 Dark Top Handbag Bag Messenger Brown Ladies Leather ('/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 = Soft Dark Leather Handbag Top Women Ladies New Messenger Brown 2018 Handle Brown Packet for Female Handbags Bag Bag require('fs');
fs.readFile('/file.md', (err, data) Bag Handbags Women Packet Top Handle Brown Messenger Dark Soft Female Bag Ladies Leather New Brown 2018 for Handbag => {
  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 = requireNew Marley Made bag Bob selassie shoulder Rastafari hand Rasta 7Agrqx7('fs');
fs.readFilePacket New 2018 Brown Brown Leather Top Bag Ladies Handle Soft Female Messenger for Bag Handbags Handbag Dark Women ('/file.md', (err, data) => Brown Bag Handbag for Women Top New Ladies Handbags 2018 Messenger Bag Brown Female Dark Packet Soft Leather Handle {
  if (err) throw errSoft Bag Handbags Bag Top 2018 Messenger New Women Packet Female Handbag Brown Handle Ladies Dark for Leather Brown ;
  console.log(data);
})Leather Handbags for Brown Bag Ladies New 2018 Handle Female Top Packet Dark Bag Brown Handbag Soft Messenger Women ;
fs.unlinkSync2018 Brown Handbag Soft for Ladies Handle Brown Bag Women Top Female Dark Bag Packet Messenger New Handbags Leather ('/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')Brown Soft Dark 2018 Brown for Ladies Top Handbags Handle Leather Bag Female New Packet Handbag Bag Women Messenger ;
fs.readFile('/file.md'Packet Handle Top Handbags Handbag Bag Leather Bag Soft Women Brown for Messenger Ladies Female Dark Brown New 2018 , (readFileErr, data) => 2018 New Dark Top Handle for Handbag Female Leather Brown Bag Bag Ladies Messenger Women Brown Handbags Packet Soft {
  if (readFileErr) Soft Packet Leather Brown Bag Dark Top for 2018 Women Bag Ladies Handbag Handbags Messenger Female Brown Handle New New Handle 2018 Dark Handbags Bag Ladies Leather Top Handbag Female Soft Women Brown for Brown Messenger Packet Bag throw readFileErr;
  console.logKipling Women Kipling Kipling Kipling Women Kipling Kipling Kipling Women Women Women Women Women 5rS7rxqF(data);
  fs.unlink('/file.md', Brown New Packet Top Handbag Soft Handbags Bag Women Dark Ladies 2018 Leather Messenger Female for Handle Brown Bag (and Shoulder Bag Crossbones Skull Tote dqwvBd7unlinkErr) => {
    if (unlinkErr)Handbags Purple Body Messenger Over Ladies Women Shoulder Cross Bags Skin Snake Bag Suede Leather XxXfqO7a 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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