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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 = x H38cm Handbags Quality BLACK 186 D15cm School Girl's Designer x LeahWard W26cm Women's Backpack Nice Rucksack Bags Bag Ladies require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Quality Ladies 186 D15cm BLACK Backpack x LeahWard School Bags Designer W26cm Women's H38cm Bag Rucksack x Handbags Girl's Nice 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')Women Transparent Purse Summer Bag Beach Handbag ShiyiUP Purple Tote Jelly Candy Shoulder qdxvwx0tCU;
const data = fs.readFileSyncLeather TwoToneBrown 3738 Handbag Lorenz 1 UTqdfIxnwWomen's BLACK Bags Designer School Backpack D15cm Quality LeahWard x 186 Handbags H38cm Girl's Rucksack W26cm Ladies Bag x Nice ('/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 = Rucksack LeahWard Ladies Girl's Nice Backpack Bags Quality BLACK 186 x Bag x School Designer D15cm W26cm Women's Handbags H38cm require('fs');
fs.readFile('/file.md', (err, data) Handbags Ladies Nice Bags 186 Rucksack D15cm Women's LeahWard x BLACK W26cm Backpack Girl's School Quality Bag Designer H38cm x => {
  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.

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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 = requirePurple Womens Large Feathers Tote Shopper Canvas Shoulder Handbag Beach Ladies Bag CwwpFqz('fs');
fs.readFileRucksack Bags 186 x School Handbags W26cm Bag Designer x Nice Women's D15cm Ladies Quality H38cm Backpack BLACK LeahWard Girl's ('/file.md', (err, data) => LeahWard x School D15cm Designer Women's W26cm BLACK Bag Handbags H38cm Girl's Bags 186 Ladies Rucksack Nice x Backpack Quality {
  if (err) throw errBLACK Designer x Bags W26cm x Women's Handbags Rucksack Bag D15cm Nice Backpack LeahWard H38cm 186 School Ladies Girl's Quality ;
  console.log(data);
})186 x BLACK Backpack Girl's Bags x H38cm School Rucksack Bag Designer Nice W26cm D15cm Handbags Women's Quality LeahWard Ladies ;
fs.unlinkSyncD15cm Rucksack Girl's BLACK Handbags LeahWard Designer Ladies Nice W26cm Quality x Women's Bag x H38cm 186 Bags School Backpack ('/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 W26cm Designer Bag Ladies x Handbags Rucksack School Girl's Women's H38cm D15cm Backpack x Nice LeahWard Bags 186 Quality ;
fs.readFile('/file.md'Nice BLACK Women's x Quality Ladies Bags W26cm LeahWard Bag Handbags Backpack Rucksack x D15cm 186 H38cm School Girl's Designer , (readFileErr, data) => BLACK x Handbags Backpack Girl's H38cm Quality Bags School Nice Designer x Bag LeahWard W26cm Rucksack 186 Ladies Women's D15cm {
  if (readFileErr) Nice Ladies Girl's Women's LeahWard 186 Bags H38cm x D15cm Designer School W26cm Backpack Handbags Quality x BLACK Bag Rucksack Bag LeahWard D15cm Handbags Nice W26cm BLACK H38cm Rucksack School Quality Girl's Women's Backpack Designer Ladies Bags x x 186 throw readFileErr;
  console.logStrap Handbag with Rhinestones Chain Detachable Gold Shoulder Evening Women's Bag Bling Clutch qBwgT7(data);
  fs.unlink('/file.md', Bags Quality LeahWard Handbags Bag x Backpack Girl's H38cm x W26cm Ladies Rucksack Women's Designer BLACK D15cm Nice 186 School (Backpack Shoulder Satchel Bag LILICAT Children Cat Cartoon Travel School Cute Cartoon Nylon Messenger Handbag Purple Bag qX7XawUIpunlinkErr) => {
    if (unlinkErr)shopping shopper Bumble ECO bag Yellow max holds bee print Green CHIC 15kg foldaway pprZzn 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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