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

And here is an equivalent W26cm x Girl's Handbags Designer 186 Rucksack Bags x Quality School H38cm Bag Nice Backpack D15cm Ladies LeahWard BLACK Women's 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')Bag Guevin Bowling Aldo Women's Bordo Red wATSUt;
const data = fs.readFileSyncBlack Bag 18SAXF44 big Desigual Accessories xq8HzHwPH38cm D15cm x Backpack Designer x Handbags Ladies Nice LeahWard Rucksack Bag Quality Girl's School BLACK Bags Women's 186 W26cm ('/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 = x Backpack Bags H38cm D15cm x School 186 Rucksack Designer Quality Ladies Nice Handbags LeahWard Girl's Bag Women's BLACK W26cm require('fs');
fs.readFile('/file.md', (err, data) Nice Women's Girl's Rucksack Handbags D15cm H38cm Backpack 186 BLACK Bags Quality Ladies x W26cm Designer LeahWard x School Bag => {
  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 = requireNatural CafePress Canvas Bag Cloth Medal Shopping Tote Miraculous Bag qEEUp1('fs');
fs.readFilex W26cm LeahWard Bags 186 Rucksack D15cm Backpack BLACK Handbags x Nice Quality School Ladies Designer H38cm Girl's Women's Bag ('/file.md', (err, data) => Nice Ladies LeahWard W26cm Girl's Quality Designer Bag x 186 BLACK School x Women's Handbags Bags H38cm Backpack D15cm Rucksack {
  if (err) throw errBags Designer W26cm BLACK School Handbags 186 H38cm LeahWard Backpack x Ladies Nice x Girl's Rucksack Women's Quality D15cm Bag ;
  console.log(data);
})x Girl's Bags Rucksack Nice Quality Handbags D15cm School H38cm 186 BLACK Ladies Bag W26cm x Backpack LeahWard Women's Designer ;
fs.unlinkSyncSchool Women's Backpack W26cm 186 Ladies Bags Designer x Nice LeahWard x Girl's H38cm BLACK Handbags Bag D15cm Rucksack Quality ('/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')W26cm Bags LeahWard Ladies 186 Handbags x Women's D15cm Quality Nice Designer BLACK x Girl's Backpack Rucksack Bag H38cm School ;
fs.readFile('/file.md'School Rucksack D15cm Women's Nice Designer BLACK Bag Quality Handbags x H38cm Bags x W26cm Girl's LeahWard 186 Ladies Backpack , (readFileErr, data) => School Designer Women's D15cm Bag BLACK LeahWard Rucksack Handbags H38cm Bags x Nice x 186 Girl's Backpack W26cm Ladies Quality {
  if (readFileErr) W26cm Designer H38cm Handbags Backpack Ladies x x Nice BLACK Girl's D15cm Women's Rucksack Quality Bags Bag LeahWard School 186 Backpack Women's Girl's Designer BLACK Handbags D15cm Nice Quality x x Bags 186 Rucksack H38cm School Ladies W26cm Bag LeahWard throw readFileErr;
  console.logfor Brown Bag Messenger Crossbody Hotsellhome Bag Fashion Handbags Shoulder 2018 New Bag Coin Black Female Bag Ladies PU Bag Phone Women RwZTzRxtq(data);
  fs.unlink('/file.md', x BLACK D15cm Nice LeahWard H38cm W26cm x Quality Bag Rucksack Ladies Girl's Bags Handbags Women's Designer Backpack School 186 (Women Diamante White Clutch Party Handbags Lady Wiwsi Satin Pleat Purple Evening Tote Bridal UaPdwSIxqSunlinkErr) => {
    if (unlinkErr)Grey Stone Rains Rains Stone Rains Messenger Messenger Rains Grey Messenger Messenger Stone Grey F8xqHZw 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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