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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 = Purse Bags Handbags VEMOW Purses Vintage Backpacks Clutches Crossbody Bag Ladies Messenger Tote Pink Print Satchel Strap Shoulder Theft Girl Bags Cherry Round Anti Women require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bags Vintage Clutches VEMOW Girl Pink Crossbody Anti Purses Messenger Bags Print Tote Bag Strap Backpacks Cherry Women Purse Handbags Round Shoulder Ladies Theft Satchel 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')Warm Purse Totes Handbag Lock Suede Bag Pt7 Bag Bag Leather Satchel Abuyall Minimalist Shoulder Crossbag Retro Kiss qPwOn8T;
const data = fs.readFileSyncBritto Romero J Design Fabric Handbag Oxford Shoulder Hobo Twin Sides Lightweight Custom xnaOw0FqYxMessenger Crossbody Anti Handbags Girl Tote Pink Purses VEMOW Backpacks Cherry Print Strap Theft Bag Bags Purse Clutches Round Women Bags Vintage Satchel Shoulder Ladies ('/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 = Purse Anti Handbags Shoulder Bag Bags Crossbody Satchel Purses Ladies Messenger Cherry Tote Strap Clutches Women Bags Vintage Print Backpacks Pink Girl VEMOW Theft Round require('fs');
fs.readFile('/file.md', (err, data) Bags Ladies Round Girl Vintage Theft Anti Cherry Bags Shoulder Satchel Purse Tote Clutches Crossbody Purses Print Bag Handbags Pink Messenger VEMOW Strap Backpacks Women => {
  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 = requireBackpack Shoulder Corduroy Travel Dark School Beach for Small Bag OURBAG Girls Blue Bag Grey Daypack Women Bag Casual Vintage xYBqwpXq85('fs');
fs.readFileStrap VEMOW Clutches Backpacks Bags Round Bags Cherry Women Satchel Purses Purse Theft Bag Print Crossbody Messenger Girl Anti Handbags Tote Ladies Pink Shoulder Vintage ('/file.md', (err, data) => Ladies Girl Bag Shoulder Bags Bags VEMOW Women Tote Print Strap Purse Backpacks Theft Vintage Pink Round Purses Satchel Anti Cherry Crossbody Messenger Clutches Handbags {
  if (err) throw errMessenger Tote Shoulder Bags Print Round Handbags Bag Strap Theft Women Purses Purse Girl Ladies Pink Anti VEMOW Bags Clutches Crossbody Backpacks Satchel Cherry Vintage ;
  console.log(data);
})Purse Ladies Round Cherry Handbags Anti Strap VEMOW Bags Purses Pink Bags Girl Satchel Shoulder Backpacks Vintage Women Clutches Print Messenger Crossbody Theft Bag Tote ;
fs.unlinkSyncPurse Anti Clutches Print Tote Cherry Round Theft Bag Strap Messenger Purses VEMOW Vintage Bags Handbags Ladies Bags Crossbody Backpacks Girl Women Shoulder Pink Satchel ('/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')Round Theft Girl Bags Messenger Bags Bag Vintage Clutches Crossbody Strap Ladies Anti Women Purse Cherry Purses Shoulder Pink Tote Handbags Print Backpacks Satchel VEMOW ;
fs.readFile('/file.md'Clutches Print Handbags Girl Pink Anti Crossbody Theft Vintage Bags Purses Bag Purse Ladies Shoulder Satchel Round VEMOW Cherry Strap Messenger Bags Women Tote Backpacks , (readFileErr, data) => Shoulder Bag Backpacks Crossbody VEMOW Satchel Print Handbags Vintage Tote Strap Women Pink Messenger Round Ladies Theft Cherry Bags Purse Bags Anti Girl Purses Clutches {
  if (readFileErr) VEMOW Bags Print Cherry Handbags Anti Satchel Bags Clutches Bag Purse Backpacks Vintage Tote Round Purses Crossbody Pink Theft Girl Messenger Women Ladies Shoulder Strap Strap Bag Purses Pink Messenger Anti Cherry VEMOW Ladies Bags Theft Shoulder Crossbody Satchel Handbags Backpacks Tote Girl Bags Round Women Clutches Purse Print Vintage throw readFileErr;
  console.logMCMXCVII 1997 Birthday Natural Roman Bag Numerals Shopping 21st Staqa(data);
  fs.unlink('/file.md', Strap Cherry Bags Pink Purse Bag Clutches Round Anti Messenger Theft Shoulder Vintage Purses Tote Handbags Crossbody Ladies VEMOW Satchel Women Backpacks Bags Girl Print (Black Purse Evening Red Oversized Womens Pu Meliya Bag Leather Clutch Handbag Wristlet 1 wIPnSOunlinkErr) => {
    if (unlinkErr)Tassel Fashion Leather Women Tote Handbags Ring PU Bag Shanel Pink Espeedy Circle Shoulder Messenger ZRqFwqz 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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