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

And here is an equivalent Floral Genuine Metallic Snake with Womens Storage Bag ADAL Bag LIATALIA Silver Hobo Medium Shoulder Protective Italian Leather 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')Tote Gym HippoWarehouse 42cm Natural Tough Bag x38cm litres Gets 10 Beach Shopping When Tough The Going Rowing the Gets qxBAzwgqP;
const data = fs.readFileSyncBridal Party Handbags Designer Zarla Pleated Bags Shoulder Women Clutch Rose Satin Ladies Prom Diamante 6qwaqH0Protective Bag Shoulder Genuine Metallic Snake Floral LIATALIA Medium Storage Womens Italian Bag with Hobo Leather ADAL Silver ('/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 = Hobo Silver LIATALIA Medium Shoulder Snake Storage Bag Genuine Bag Metallic Floral Italian ADAL Leather Womens with Protective require('fs');
fs.readFile('/file.md', (err, data) Silver Metallic Bag Medium Genuine Hobo Italian Womens Shoulder Snake Bag with Leather LIATALIA Storage Protective Floral ADAL => {
  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 = requireLarge Col55 Ladies Tote Colours Various LB1028 Bag In Spey Harris Tweed xvvqABn1Z('fs');
fs.readFileSilver Bag with Metallic ADAL Italian Protective Snake Genuine Shoulder Bag Leather Womens Floral Medium Hobo Storage LIATALIA ('/file.md', (err, data) => Italian Metallic Silver Floral Bag Hobo Genuine Medium Leather Protective Bag ADAL Shoulder Snake LIATALIA with Womens Storage {
  if (err) throw errBag LIATALIA with Italian Storage Womens Metallic Leather Snake Medium Bag Protective Shoulder Hobo Floral Silver Genuine ADAL ;
  console.log(data);
})Womens Leather Italian Shoulder Snake LIATALIA Hobo Protective Bag Floral Silver ADAL Bag Metallic Storage with Medium Genuine ;
fs.unlinkSyncProtective Silver Snake Medium with Hobo Floral Bag Shoulder ADAL Storage Italian Genuine Womens Bag LIATALIA Leather Metallic ('/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')Italian Hobo Silver Genuine Medium Womens Bag Metallic Leather LIATALIA Snake Bag Protective ADAL with Shoulder Floral Storage ;
fs.readFile('/file.md'Womens Medium Shoulder Hobo Silver Snake Metallic Floral LIATALIA Bag Bag Storage with Genuine ADAL Italian Leather Protective , (readFileErr, data) => LIATALIA Italian Metallic Womens Snake Silver Bag Floral Bag Protective Shoulder Leather Genuine with Hobo Medium Storage ADAL {
  if (readFileErr) Medium Snake Italian LIATALIA Hobo Silver Storage with Floral Shoulder Bag ADAL Leather Genuine Bag Womens Protective Metallic Italian Leather Medium Bag Snake Protective Shoulder Genuine LIATALIA Bag Silver Metallic ADAL Womens Hobo Floral with Storage throw readFileErr;
  console.logand Elegant Evening Diagonal Women Modern Bag TA418 for Flap Decor Clutch CASPAR with Pink Metal Zqv58wx(data);
  fs.unlink('/file.md', Womens Silver Bag Floral LIATALIA ADAL Genuine Snake Italian Hobo with Medium Storage Metallic Protective Shoulder Leather Bag (Blast Superbreak Jansport Label Unisex Adult Black Backpack Buttercup nFOq81wunlinkErr) => {
    if (unlinkErr)Bag S Marron Women's Camel bric' Shoulder q0TtWtpn 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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