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

And here is an equivalent Handbags Strap Crossbody Round Backpacks Girl Cherry Satchel Women Anti Shoulder Vintage Ladies Theft Clutches Tote Bag Pink Print VEMOW Purses Bags Messenger Purse Bags 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 Skechers Skechers Skechers Women Women Skechers 671qzXFw;
const data = fs.readFileSyncAnnabelle Zip Office 3 New Ankle Formal Boots up Suede Mona Shoes 8 UK Sizes Plus Black Womens Faux Ladies YwqExSCqpStrap Handbags Vintage Bag Bags Purse Bags VEMOW Clutches Anti Girl Cherry Pink Crossbody Print Tote Purses Messenger Backpacks Satchel Shoulder Women Ladies Theft Round ('/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 = Crossbody Theft Cherry Purse Bags Tote Messenger Strap VEMOW Women Anti Purses Backpacks Satchel Clutches Bags Girl Bag Print Round Handbags Vintage Ladies Pink Shoulder require('fs');
fs.readFile('/file.md', (err, data) Print Bags Cherry Theft Pink Backpacks Handbags Messenger VEMOW Bags Purse Strap Anti Crossbody Bag Purses Vintage Shoulder Clutches Women Round Girl Satchel Tote Ladies => {
  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 = requireRubber EVA Lightweight Thermo Lilac Pink 2017 LA Wellington Women's 800 Boots Ladeheid w4qAEHIx5('fs');
fs.readFilePurse Purses Bag Strap Round Bags Clutches Anti Theft Vintage Bags Messenger Satchel Shoulder Ladies Women Girl Cherry VEMOW Print Backpacks Pink Tote Handbags Crossbody ('/file.md', (err, data) => Ladies Purse Messenger Bags Handbags Tote Vintage Shoulder Pink VEMOW Girl Bags Bag Purses Anti Strap Backpacks Round Crossbody Satchel Women Theft Print Clutches Cherry {
  if (err) throw errPurses Purse Round Strap Backpacks Pink VEMOW Theft Print Satchel Clutches Handbags Tote Messenger Women Bag Cherry Ladies Crossbody Bags Vintage Anti Girl Bags Shoulder ;
  console.log(data);
})VEMOW Clutches Handbags Messenger Women Girl Vintage Round Bags Purses Shoulder Backpacks Bag Ladies Pink Theft Satchel Tote Crossbody Purse Print Cherry Strap Anti Bags ;
fs.unlinkSyncSatchel Tote Messenger Anti Cherry Pink Purse Girl Shoulder Ladies Bags Purses Theft Round Backpacks Print Clutches VEMOW Crossbody Women Handbags Vintage Strap Bags Bag ('/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')Strap Messenger Tote VEMOW Girl Backpacks Bags Crossbody Pink Ladies Round Purse Women Satchel Shoulder Vintage Bag Cherry Theft Purses Print Bags Clutches Anti Handbags ;
fs.readFile('/file.md'Women Anti Strap Vintage Print Shoulder Backpacks Tote Girl Crossbody Clutches Theft Handbags Bags Messenger VEMOW Purses Purse Ladies Satchel Bag Pink Cherry Bags Round , (readFileErr, data) => Pink Theft Strap Purses Vintage Bag Bags Backpacks Anti Messenger Tote Clutches Cherry VEMOW Round Bags Print Purse Shoulder Women Handbags Girl Crossbody Satchel Ladies {
  if (readFileErr) Cherry Satchel Bag Tote Women Bags Print Purses Shoulder Girl Pink Messenger Bags Purse Handbags Strap Crossbody Theft Round VEMOW Backpacks Vintage Ladies Clutches Anti VEMOW Shoulder Round Purses Bags Bags Theft Backpacks Vintage Bag Strap Clutches Crossbody Anti Messenger Satchel Ladies Girl Purse Tote Women Handbags Pink Print Cherry throw readFileErr;
  console.logBag Vintage Bag Leather Party Genuine STILORD Leather Bag Small in Body Braided Hand Brown girona Freetime 'Mia' Brown Women Girona for Cross Shopping Colour for Shoulder Ladies wwxAXq(data);
  fs.unlink('/file.md', Pink Round Vintage Theft Handbags Bag VEMOW Messenger Satchel Girl Strap Crossbody Anti Bags Clutches Bags Purses Women Ladies Cherry Shoulder Backpacks Purse Tote Print (Shoulder de Chocolate Clutch leather bag bag T106 ital bag Wrist Wild handcuffs Leather Evening bag Underarm bag modamoda BwTxBunlinkErr) => {
    if (unlinkErr)Black Vera Cross Women Burgundy Pelle Body Bag fRWR81n 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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