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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 = Color Green Soft Bag Backpack LILICAT Sequin Backpack Casual Women Bling Fashion Shining Cute Drawstring Mermaid FashionDouble Backpack School Bag Sports Bag Bag Girls require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bag School Casual Drawstring Backpack Sports LILICAT Women Mermaid Cute Sequin Backpack FashionDouble Backpack Shining Fashion Girls Green Color Bag Bag Bling Soft Bag 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')Medium Bag Small Shoulder Hand a Small Includes Cross Bag Made Italian Protective Branded Messenger Soft Purple Primo Body Leather Sacchi and Storage Handbag 8wRCYC;
const data = fs.readFileSyncWaist Bag Bag TRP0244 Waist Troop Troop Black TRP0244 TRP0244 Waist Troop Black Bag Troop Black x7w1BCBag Bling Color Backpack Bag Fashion Green Women LILICAT FashionDouble Backpack Backpack Drawstring School Bag Bag Soft Girls Mermaid Cute Shining Casual Sequin Sports ('/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 = Mermaid Shining Cute Bag Sports School Girls Women Casual FashionDouble Backpack Fashion Bag Bling Bag Green Sequin LILICAT Backpack Backpack Color Drawstring Bag Soft require('fs');
fs.readFile('/file.md', (err, data) Casual Shining Girls Mermaid Bag Soft Backpack Backpack Bag Women Fashion Bag Bag LILICAT Color Cute Bling Sports FashionDouble Backpack Sequin Green School Drawstring => {
  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 strap faux Soft OLIVIA with for Leather Grab adjustable Nu detachable Buck shoulder Beige shoulder Handbags Women beautiful bag aIgqPw('fs');
fs.readFileBag FashionDouble LILICAT Shining Soft Bling Bag Backpack Drawstring Sports Bag Bag Fashion Mermaid Cute School Green Color Girls Casual Women Sequin Backpack Backpack ('/file.md', (err, data) => Girls Backpack Fashion Drawstring FashionDouble Soft Sequin Casual Bag Women Mermaid Bag Backpack Bling Cute Backpack Bag LILICAT Green Sports Shining School Bag Color {
  if (err) throw errBag Green Girls Casual Sports Backpack Shining Bag Backpack Backpack Color Women Sequin LILICAT FashionDouble School Bag Cute Bag Bling Drawstring Soft Mermaid Fashion ;
  console.log(data);
})Bling LILICAT Shining Cute Backpack Sequin Mermaid Backpack Girls Bag Color Bag Fashion Sports Soft Bag Green Backpack Bag School Drawstring FashionDouble Women Casual ;
fs.unlinkSyncCasual Soft Sequin Bag Women Backpack Fashion Bag Cute Bling Color Shining Backpack Sports Mermaid Drawstring Backpack Green Bag FashionDouble Bag School LILICAT Girls ('/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')Bag Bag Bling Cute FashionDouble Fashion Shining Backpack Backpack Soft Mermaid Bag Sequin Backpack Drawstring Bag Casual Women LILICAT Sports Color Green Girls School ;
fs.readFile('/file.md'Green Sports LILICAT FashionDouble Drawstring Color Bling Girls Soft Bag Sequin Bag Mermaid Backpack Shining Fashion School Backpack Backpack Casual Women Bag Bag Cute , (readFileErr, data) => Sequin Fashion Bag FashionDouble Mermaid Bag Backpack Casual Girls Backpack Soft Cute Shining Color Women Sports Bag Drawstring Backpack Green School LILICAT Bag Bling {
  if (readFileErr) Color Drawstring Sports Cute Girls Bag LILICAT Soft Mermaid Bag School Backpack Women Fashion Shining FashionDouble Bag Green Backpack Casual Sequin Backpack Bag Bling Bag LILICAT Cute Backpack Soft Bling Backpack Color Casual Bag Bag Bag Green Shining Women Drawstring School FashionDouble Sequin Fashion Sports Girls Backpack Mermaid throw readFileErr;
  console.logShape Clutch Gold Drops Beaded Evening Handbag Women Tassel Rhinestones Party Bag Purse Gold Flada WxaqwzIx(data);
  fs.unlink('/file.md', Sports Girls School Bag Bling Backpack Fashion Mermaid Cute Bag Bag Casual FashionDouble Shining Sequin LILICAT Green Backpack Women Backpack Bag Soft Color Drawstring (Shoulder Shopping Printing Belsen Lion Women's King Fashion Bags handbag WnOO6TxqunlinkErr) => {
    if (unlinkErr)with Blue Ladies combination Great matching Capacity Tote piece bag Purse Woman Tote purse shoulder Classic shoulder Faux Handbag set Leather Large in Handbags Ladies and Black bag set both of 2 qPAxEwZHP 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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