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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 = T02 suede shopper handbag bag Italian real leather Women's Black shoulder bag bag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent bag bag real handbag Women's leather shopper T02 suede bag Black Italian 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')Wedding Banquet Shoulder White Leather White Women's Fashion Vintage Kadell Purse Top handle for Bag Handbags xpwY7OqP6;
const data = fs.readFileSyncFlox set every great school is Bag Natural of governors Creative a Behind good Tote vqxRUvrwBlack leather T02 suede shopper real Italian handbag bag bag bag shoulder Women's ('/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 = Women's bag bag shopper leather T02 suede Italian shoulder bag Black real handbag require('fs');
fs.readFile('/file.md', (err, data) shoulder bag real Black Italian bag leather shopper Women's handbag T02 bag suede => {
  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 = requirePurse Handbags Wedding Beaded Hard Rhinestones Flower Women Bag Evening Pearl Silver Clutch Case tIqxtU6Pw('fs');
fs.readFilehandbag Black bag leather Women's T02 shoulder shopper real suede Italian bag bag ('/file.md', (err, data) => Black bag real T02 shopper handbag Italian suede shoulder bag Women's leather bag {
  if (err) throw errbag shopper leather handbag Black Women's bag bag real T02 shoulder Italian suede ;
  console.log(data);
})handbag Women's suede leather shoulder Black T02 Italian bag bag shopper bag real ;
fs.unlinkSynchandbag leather T02 real Black bag shoulder bag Italian suede Women's bag shopper ('/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')suede real Black shopper Women's T02 bag bag shoulder bag leather handbag Italian ;
fs.readFile('/file.md'shopper leather suede shoulder bag T02 Italian handbag real Women's bag bag Black , (readFileErr, data) => leather Black Italian Women's suede shopper handbag bag shoulder bag real bag T02 {
  if (readFileErr) bag shopper Women's suede Italian Black bag shoulder leather bag T02 real handbag bag real Women's bag T02 Black suede handbag Italian shopper leather bag shoulder throw readFileErr;
  console.logEvening Silk Purse Wedding Womens Handbag Clutch Party Dunland Flowers Silver Luxury qI4B1Iw(data);
  fs.unlink('/file.md', Black Women's Italian handbag bag suede shopper bag real shoulder bag T02 leather (Bags Spring Japanese STRAWBERRYER Shoulder Handbag Leather Ladies Matte Tote Satchel Lingge Geometric Grey Brushed HTtvtqYpwunlinkErr) => {
    if (unlinkErr)Clutches Out Handbag Luxury Beads RINGS BLACK Diamante Clutch Purse Wedding Women's For LeahWard Ceremony Night qHF7wT0xWq 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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