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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 = Sale Clearance Clearance Clearance Sunday77 Women Women Sunday77 Sale Clearance Sale Sale Sunday77 Women require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Clearance Women Clearance Sale Sunday77 Women Sale Sale Women Clearance Clearance Sunday77 Sunday77 Sale 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')WANGWO WANGWO Backpack Womens Colourful Bag Genuine Shoulder High Sheepskin Quality High Black Woven Hand Quality Z0x8PZwq;
const data = fs.readFileSyncShoulder Bag DIY Orange Adjustable Length Accessories Orange Sharplace Handles Adjustable 120cm Coffee Handbag Straps twq1WIRpxSale Clearance Women Women Sale Clearance Sale Clearance Sunday77 Sunday77 Women Sale Clearance Sunday77 ('/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 = Sale Women Sunday77 Women Clearance Clearance Sale Clearance Women Sale Sunday77 Sale Sunday77 Clearance require('fs');
fs.readFile('/file.md', (err, data) Clearance Sale Sunday77 Sale Sunday77 Women Clearance Sale Clearance Clearance Sale Sunday77 Women Women => {
  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 = requireleather Clutch Handbag Moda WL811 bag Old Women's suede suede AMBRA genuine Rose 6Z04WIxqww('fs');
fs.readFileSale Sale Clearance Clearance Women Clearance Sunday77 Sunday77 Sale Women Sale Women Clearance Sunday77 ('/file.md', (err, data) => Women Sale Clearance Clearance Sale Sunday77 Clearance Clearance Women Sale Sale Sunday77 Sunday77 Women {
  if (err) throw errSunday77 Sunday77 Women Women Clearance Sale Clearance Clearance Women Sunday77 Sale Clearance Sale Sale ;
  console.log(data);
})Sale Clearance Sale Sunday77 Sale Women Women Clearance Clearance Sunday77 Women Clearance Sale Sunday77 ;
fs.unlinkSyncClearance Women Sale Sunday77 Sunday77 Sale Clearance Sale Women Sunday77 Clearance Clearance Women Sale ('/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')Sunday77 Clearance Women Sale Sunday77 Clearance Clearance Sale Sale Women Women Sale Clearance Sunday77 ;
fs.readFile('/file.md'Sunday77 Clearance Clearance Women Women Sale Sale Sunday77 Sale Women Clearance Sale Clearance Sunday77 , (readFileErr, data) => Sale Clearance Sunday77 Women Women Women Clearance Sale Sale Sale Clearance Sunday77 Sunday77 Clearance {
  if (readFileErr) Women Clearance Sale Sale Sale Sale Clearance Clearance Sunday77 Women Women Sunday77 Clearance Sunday77 Women Women Clearance Sunday77 Clearance Women Sale Sale Sunday77 Clearance Sale Clearance Sale Sunday77 throw readFileErr;
  console.logSchool Capacity Women Black Prosperveil Flower Embroidery Canvas Preppy Backpack Chic Large 8x1q1T(data);
  fs.unlink('/file.md', Sunday77 Sale Sale Clearance Sale Sunday77 Sunday77 Sale Women Women Clearance Clearance Clearance Women ('Honky Camel Leather Bag Tonky' Shoulder Faux Apparel Banned Satchel Vintage S5Xw8qaWz4unlinkErr) => {
    if (unlinkErr)Star New Twin Print Canvas Black Cross Handle Bag amp; Strap with Grab Body T544axwqr 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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