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

And here is an equivalent for New Packet Brown Handbag Soft Bag 2018 Leather Messenger Female Women Brown Bag Handle Dark Handbags Ladies Top 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')Handbags Women Bags Black Phone Bags Coin Small Bag Messenger Crossbody Pink Bags Girls Bags Luxury Janly Pearl Shoulder wHXqq7U;
const data = fs.readFileSyncClutch Pink Bag Evening Prom Womens Flapover Patent Envelope Juliet Party Leather Bridal Pq4UZzUxLadies Top Bag Handle Women Dark Brown Handbags Messenger New Soft Bag Leather Packet for Female Brown Handbag 2018 ('/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 for Top 2018 Handbag Packet Leather Female Dark New Brown Handbags Brown Ladies Women Bag Messenger Bag Handle require('fs');
fs.readFile('/file.md', (err, data) Handle Top Dark Brown Women Soft New Handbag Handbags Bag Bag Female for Messenger Leather Brown Ladies Packet 2018 => {
  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 = requirePhiloslothical Green Feelin' Feelin' bag tote Philoslothical pZxnvqgwF('fs');
fs.readFileNew Bag Ladies Handle Female Women Dark Packet 2018 Messenger Handbags Handbag Brown Bag for Leather Top Brown Soft ('/file.md', (err, data) => New Leather Female Top Women Soft Brown Dark for Handbag Packet Handbags 2018 Brown Bag Ladies Bag Messenger Handle {
  if (err) throw errSoft Messenger Packet Ladies Bag Female for Brown Bag Handle Women Handbags Top Handbag Dark Brown Leather 2018 New ;
  console.log(data);
})Brown Female Dark Handbags Women Ladies Top Handle Bag Bag Soft Messenger 2018 Brown New Leather for Packet Handbag ;
fs.unlinkSyncNew Top Brown Handbag 2018 Handbags Messenger Women Bag Handle Female Brown Ladies for Leather Bag Packet Soft Dark ('/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 Female for Handbags Bag Leather Soft Bag New Packet Women Messenger Handle Brown Top Dark Ladies 2018 Handbag ;
fs.readFile('/file.md'Dark Packet Top Messenger Bag Handle Soft Leather Women Handbags Bag Ladies New Handbag Brown Female 2018 for Brown , (readFileErr, data) => Dark Packet Ladies Female Brown Leather Messenger Bag Soft Top Brown Handle Handbag Women New Handbags Bag 2018 for {
  if (readFileErr) Messenger Handbag Dark Brown New Female 2018 Soft Handle Packet Leather Brown Ladies Handbags Bag Bag for Top Women Handbags Brown Top Bag Ladies 2018 Messenger Handle Dark New Female Women Packet Leather Handbag Brown for Bag Soft throw readFileErr;
  console.logNice Great WITH LeahWard POUCH BROWN Women's Bags Handbags Two Tone Shaped BAG Tote PwPIxfXq(data);
  fs.unlink('/file.md', Handle Bag Brown Top Dark Handbags Ladies New for Brown Women Handbag Soft 2018 Bag Leather Packet Female Messenger (Gym Football Beach Classic Pink x38cm lion Tote england HippoWarehouse 42cm 10 litres Bag Shopping flag wYZx0nqpunlinkErr) => {
    if (unlinkErr)Purse Black Style Ladies Studded Bag UTO Tote Leather Washed Rivet Bag 3 Women PU Shoulder Skull 6gvqOgF 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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