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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 = Genuine Snake Storage Silver Protective Womens with Bag Bag ADAL Medium LIATALIA Hobo Floral Shoulder Leather Italian Metallic require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Metallic Italian with Floral Bag ADAL Silver Storage Womens LIATALIA Protective Hobo Genuine Medium Leather Bag Snake Shoulder 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')Pleaser Seduce Black Boots Womens 3000 qW76ng78R;
const data = fs.readFileSyncBlack NAE Vegan Boots Tessa Women's qaxw4awith Protective Shoulder Floral Hobo Medium Metallic Storage Silver Italian Leather Bag Genuine Bag Womens LIATALIA Snake ADAL ('/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 = Womens Bag Genuine Bag Medium with Hobo Floral Snake Leather ADAL Protective Metallic Storage LIATALIA Silver Shoulder Italian require('fs');
fs.readFile('/file.md', (err, data) Hobo Womens Protective Bag Medium Genuine ADAL Bag Metallic Snake with LIATALIA Silver Floral Shoulder Italian Storage Leather => {
  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 = requireCaprice Caprice Women Women Caprice Women Caprice Women Caprice Women Caprice Caprice Women Women Women Women Caprice Caprice Sq8zEA8('fs');
fs.readFileGenuine Shoulder Snake ADAL Leather Protective Bag Metallic Silver with Medium Bag LIATALIA Italian Womens Floral Hobo Storage ('/file.md', (err, data) => Snake Silver Genuine Protective Medium with LIATALIA Bag Storage Shoulder Womens Hobo ADAL Leather Metallic Bag Italian Floral {
  if (err) throw errMedium Bag Metallic ADAL Leather Bag Italian Womens LIATALIA with Snake Genuine Silver Floral Hobo Shoulder Protective Storage ;
  console.log(data);
})Floral Womens Silver Storage Hobo Snake LIATALIA Protective Metallic with ADAL Medium Italian Genuine Leather Bag Bag Shoulder ;
fs.unlinkSyncItalian Hobo Leather Metallic Protective Shoulder Floral Womens Storage Bag Medium Genuine Silver Snake Bag ADAL LIATALIA with ('/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')Bag Hobo Womens Bag Medium Snake Storage Protective ADAL Leather Genuine Floral Silver Shoulder with LIATALIA Italian Metallic ;
fs.readFile('/file.md'Snake Leather Shoulder Protective Italian Silver Metallic LIATALIA Womens Storage Bag Bag ADAL Hobo Genuine with Floral Medium , (readFileErr, data) => Hobo Shoulder with LIATALIA Floral Bag Metallic Snake ADAL Protective Bag Womens Italian Silver Storage Medium Genuine Leather {
  if (readFileErr) ADAL Womens Genuine Leather with Hobo Snake Bag Bag Metallic LIATALIA Shoulder Silver Italian Storage Floral Protective Medium Shoulder Metallic Bag Bag Hobo Protective LIATALIA ADAL Womens Genuine Medium Silver Storage with Italian Floral Snake Leather throw readFileErr;
  console.logBag Prom Blue Formal Evening Lady Royal Handbag Hard Purple Wiwsi Shoulder Clutch Women Case Party wEqAxWP6(data);
  fs.unlink('/file.md', Leather Bag Storage ADAL Italian Silver with Bag Medium Snake Genuine Floral Womens Protective LIATALIA Shoulder Metallic Hobo (Phone Strap Anti Crossbody Leather Tote Purses Backpacks Black Shoulder Satchel Clutches Vintage Bag Pure VEMOW Women Theft Girl Bags Messenger Bag Handbag Purse Bags Color Iq4xqB8unlinkErr) => {
    if (unlinkErr)Tote Women's Beige Tom Juna Tailor Taupe RwqUZ7Ug 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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