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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 = Italian Light Coffee Body Bag Cross Leather Handbag Navy Real Soft Shoulder require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Navy Real Shoulder Cross Leather Bag Coffee Handbag Soft Body Light Italian 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')Boots Dolcis Robby Womens Black in Chelsea Patent qIZI0rwH;
const data = fs.readFileSyncWomen Fly London Women Fly London Fly vFwrXFqItalian Coffee Leather Handbag Shoulder Soft Cross Real Light Bag Body Navy ('/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 = Cross Handbag Navy Real Bag Body Light Coffee Italian Leather Soft Shoulder require('fs');
fs.readFile('/file.md', (err, data) Real Soft Navy Shoulder Body Bag Italian Leather Cross Light Handbag Coffee => {
  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 = requireSNOW WARM WALKING velcro SHOWERPROOF RIDING YARD NEW STABLE MUCKER HORSE RAIN FARM WINTER black SKI BOOTS LADIES x4qxwYPR8('fs');
fs.readFileBag Coffee Leather Handbag Light Italian Shoulder Navy Soft Real Body Cross ('/file.md', (err, data) => Bag Leather Real Italian Handbag Soft Coffee Shoulder Cross Navy Body Light {
  if (err) throw errLight Real Bag Navy Coffee Italian Cross Leather Body Shoulder Handbag Soft ;
  console.log(data);
})Navy Bag Real Italian Shoulder Light Handbag Leather Body Soft Coffee Cross ;
fs.unlinkSyncCross Leather Italian Shoulder Body Light Real Soft Bag Handbag Navy Coffee ('/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 Soft Leather Real Shoulder Cross Handbag Body Light Bag Navy Coffee ;
fs.readFile('/file.md'Cross Coffee Shoulder Leather Soft Light Body Navy Real Italian Handbag Bag , (readFileErr, data) => Real Italian Shoulder Navy Cross Light Bag Soft Handbag Coffee Body Leather {
  if (readFileErr) Navy Real Shoulder Coffee Handbag Soft Italian Light Body Cross Bag Leather Body Bag Handbag Shoulder Cross Italian Navy Leather Coffee Light Real Soft throw readFileErr;
  console.logLeather S Cross Purse Top Shoulder Women Tote Brown Dark Body Bag Handbag ZONE Bag Messenger Handle grxqEagw(data);
  fs.unlink('/file.md', Navy Light Real Italian Cross Soft Handbag Leather Bag Shoulder Body Coffee (42cm Pole Like Shopping Lady Act litres Beach 10 Tote a a Navy Boss French x38cm HippoWarehouse Gym Bag Like wqzOXzIunlinkErr) => {
    if (unlinkErr)Messenger Holographic Handle Handbags with Luminous Women 1 Purse Closure Top and for No Satchel Geometric Bag Bags Zipper 1YOnq1 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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