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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 = ShoulderBag Maribel EyeCatchBags Tote HandBag Brown Leather Womens Mustard Faux require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Tote HandBag EyeCatchBags Leather Faux Mustard Maribel Womens ShoulderBag Brown 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')Vintage Sonnena Cotton Hat Style Cadet Army Adjustable Coffee Hats Military Cap Classic Plain tTwpqCT;
const data = fs.readFileSyncBridal Satin Prom Wiwsi Clutch Women Pleats Bag Diamante Purse Handbag Elegant Party HHwRISgxvShoulderBag Faux Leather Maribel Womens Mustard Brown Tote HandBag EyeCatchBags ('/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 = Faux EyeCatchBags HandBag Maribel Leather Brown Mustard Womens ShoulderBag Tote require('fs');
fs.readFile('/file.md', (err, data) Faux Maribel Brown HandBag ShoulderBag EyeCatchBags Leather Mustard Tote Womens => {
  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 = requireBag HandBags Clutch Mint Trim Studs Girly dTFqwII('fs');
fs.readFileFaux Brown Womens Mustard HandBag Maribel EyeCatchBags ShoulderBag Tote Leather ('/file.md', (err, data) => Maribel EyeCatchBags ShoulderBag Leather Womens Faux Brown HandBag Mustard Tote {
  if (err) throw errShoulderBag Faux Maribel Mustard EyeCatchBags Leather HandBag Tote Brown Womens ;
  console.log(data);
})Mustard EyeCatchBags HandBag Tote Brown Faux ShoulderBag Womens Leather Maribel ;
fs.unlinkSyncTote ShoulderBag Mustard Faux HandBag Maribel Leather EyeCatchBags Womens Brown ('/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')Tote Faux Brown Womens Leather ShoulderBag Maribel HandBag EyeCatchBags Mustard ;
fs.readFile('/file.md'Faux Mustard ShoulderBag Brown Maribel Leather EyeCatchBags Tote Womens HandBag , (readFileErr, data) => EyeCatchBags Mustard Tote ShoulderBag Womens Leather Brown Faux Maribel HandBag {
  if (readFileErr) Maribel Mustard ShoulderBag Womens HandBag EyeCatchBags Brown Leather Faux Tote Leather Faux HandBag Womens Mustard Maribel EyeCatchBags Brown ShoulderBag Tote throw readFileErr;
  console.logBag Evening Designer Purses Black Handbag Rhinestones Imitation Ladies Bags Pearl Luxury Silver KAXIDY Clutch CqaZgwg(data);
  fs.unlink('/file.md', Tote Leather Brown HandBag Maribel ShoulderBag Womens EyeCatchBags Faux Mustard (Handbag Shoulder White Genuine in in Leather Borse Chicca with 35x28x16 Cm Woman Strap Italy Made 0TxWRunlinkErr) => {
    if (unlinkErr)Sale Women Women Clearance Sunday77 Women Sunday77 Clearance Clearance Sunday77 Sale Sale qAp0dpwT 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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