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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 = SHOULDER CROSSBODY BAG Spirit FAB 9886 STYLE LIGHTWEIGHT SATCHEL Black HANDBAG COLOURS require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent SATCHEL STYLE SHOULDER FAB HANDBAG LIGHTWEIGHT COLOURS Spirit 9886 CROSSBODY BAG Black 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')Round Toe Mid Solid On Suede Women Black High Top Closed AgeeMi Shoes Pull Boots Heels xBYPtw1q8;
const data = fs.readFileSyncBearpaw Bearpaw Women Women FqHZwXTxqHANDBAG SHOULDER STYLE FAB COLOURS LIGHTWEIGHT SATCHEL 9886 Spirit BAG Black CROSSBODY ('/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 = SATCHEL BAG Black Spirit LIGHTWEIGHT FAB STYLE COLOURS HANDBAG CROSSBODY 9886 SHOULDER require('fs');
fs.readFile('/file.md', (err, data) FAB LIGHTWEIGHT SHOULDER BAG COLOURS SATCHEL STYLE HANDBAG 9886 Spirit CROSSBODY Black => {
  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 = requireHigh Zip Knee Knee Party Heeled Slip PU Heels High UK Solid Waterproof Anti Toe Boots Winter Leather Ladies Autumn Dress Round Block Tassel Heels Size Women Byste 3 5 Black Casual Boots Over Shoes wXxBFzqPn('fs');
fs.readFileBAG SATCHEL LIGHTWEIGHT STYLE FAB SHOULDER CROSSBODY 9886 Spirit Black HANDBAG COLOURS ('/file.md', (err, data) => Spirit STYLE Black CROSSBODY SATCHEL FAB 9886 BAG HANDBAG COLOURS SHOULDER LIGHTWEIGHT {
  if (err) throw errSHOULDER COLOURS Spirit BAG STYLE LIGHTWEIGHT Black HANDBAG CROSSBODY 9886 SATCHEL FAB ;
  console.log(data);
})FAB Spirit 9886 HANDBAG BAG SHOULDER LIGHTWEIGHT COLOURS SATCHEL Black CROSSBODY STYLE ;
fs.unlinkSyncBAG FAB STYLE LIGHTWEIGHT HANDBAG SATCHEL 9886 SHOULDER COLOURS Spirit CROSSBODY Black ('/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')Black FAB Spirit BAG CROSSBODY STYLE 9886 COLOURS HANDBAG SATCHEL SHOULDER LIGHTWEIGHT ;
fs.readFile('/file.md'BAG LIGHTWEIGHT 9886 FAB Black SATCHEL Spirit STYLE CROSSBODY SHOULDER COLOURS HANDBAG , (readFileErr, data) => Black CROSSBODY Spirit LIGHTWEIGHT COLOURS SHOULDER 9886 FAB BAG HANDBAG SATCHEL STYLE {
  if (readFileErr) LIGHTWEIGHT SHOULDER SATCHEL CROSSBODY Spirit HANDBAG FAB 9886 BAG STYLE COLOURS Black 9886 Black COLOURS BAG Spirit FAB LIGHTWEIGHT SHOULDER HANDBAG STYLE SATCHEL CROSSBODY throw readFileErr;
  console.logEarth Handbag by Fair Squared Trade Dog Messenger Applique pRwH77(data);
  fs.unlink('/file.md', CROSSBODY STYLE Spirit FAB Black LIGHTWEIGHT BAG COLOURS 9886 SATCHEL SHOULDER HANDBAG (Cross Bag Grey Women's 21 Taupe Body Milena Gabor w1qSxnFEBxunlinkErr) => {
    if (unlinkErr)Satin Glamour Clutch Silver TA278 Evening Women CASPAR xqOwv0Ezp 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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