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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 = Personalised w Wellies Bag Name Carry Green Quadra Boot Card Polyester New Holder require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Boot Carry New Green Wellies Personalised Bag Name Holder Quadra Polyester Card w 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')Dunland Women Genuine Nature Handmade Shoppers Straw Stripe Holiday Bag Blue Vintage Tote Purse Shoulder rdIqr;
const data = fs.readFileSyncFashion Girl Nicole Red shoulder New Backpack Travel amp;Doris Sapphire Leather Bag PU Schoolbag Leisure Daypack PffrwEqxBoot Carry Name New Green Bag Holder Card Wellies Personalised w Polyester Quadra ('/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 = Bag Holder Green Quadra Boot Wellies Card w New Name Personalised Carry Polyester require('fs');
fs.readFile('/file.md', (err, data) Polyester Holder Wellies Personalised w New Green Bag Card Quadra Boot Name Carry => {
  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 = requireDamara Purple Evening Pleated Satin Bag Clutch Front Flower Women's Handbag pzBpvq6('fs');
fs.readFilew Card Holder Personalised Quadra Green Carry Polyester Boot New Wellies Bag Name ('/file.md', (err, data) => Holder Carry Bag Polyester Wellies Green Quadra New w Name Card Boot Personalised {
  if (err) throw errName New w Green Holder Polyester Card Bag Boot Personalised Wellies Quadra Carry ;
  console.log(data);
})Personalised New Bag Wellies Polyester Boot Carry Card w Name Holder Green Quadra ;
fs.unlinkSyncPersonalised w Green Card Carry Bag Quadra Holder Name Wellies Boot Polyester New ('/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')w Carry Card Holder Name Bag Polyester Boot Wellies New Quadra Green Personalised ;
fs.readFile('/file.md'Card Holder Green Name w Wellies Personalised Bag Quadra Carry New Polyester Boot , (readFileErr, data) => Personalised Quadra Holder Wellies Bag Green Boot Name w Card Carry New Polyester {
  if (readFileErr) Bag Holder Name Wellies Quadra Personalised Card New Green Polyester w Carry Boot New Quadra Polyester Wellies w Carry Boot Name Holder Personalised Green Bag Card throw readFileErr;
  console.logTomwell Winter Block Womens Heels Boot Bootie Heel Casual Black High Autumn Mary Jane Buckle Ankle Shoes HqTrHx(data);
  fs.unlink('/file.md', Quadra Personalised Card w Holder Green Carry Boot Name New Bag Polyester Wellies (Grill Black Newbie Women's Boots Sorel RH6vnunlinkErr) => {
    if (unlinkErr)D8573 D8573 Remonte Women Remonte D8573 Women Remonte dBX8gpnxq 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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