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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.



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 = Hot Clearance Handle Large Brown Casual Retro Handbag JYC Leather Tote Sale Bag Tote Shoulder Capacity Bag Soft Top Large Ladies Purse Vintage Handbags Shoulder require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Shoulder Tote Large Purse Shoulder Capacity JYC Leather Handle Ladies Handbag Tote Brown Sale Soft Large Clearance Top Retro Bag Hot Casual Bag Handbags Vintage asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')Pleaser Pleaser Pleaser Pleaser Pleaser Women Women Women Women Women HqIxEB5w;
const data = fs.readFileSyncHigh Black Knee Wide Winter ShoeFashionista Biker Ladies Low Leg Heel Calf Style M Boots Size Style Flat a4zgnq4TPurse Tote Soft Leather Hot Bag Casual Tote Sale Brown Retro Top Handbag Capacity Large Large Vintage Clearance Bag Handle Shoulder Shoulder Handbags Ladies JYC ('/'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Sale Leather Shoulder Bag Handle Brown JYC Retro Bag Large Hot Handbags Top Clearance Soft Ladies Casual Vintage Tote Purse Capacity Shoulder Large Tote Handbag require('fs');
fs.readFile('/', (err, data) Tote Bag Leather Retro Ladies Sale Brown Shoulder Top Shoulder Handle Large Handbags Capacity Purse Tote Vintage Soft Bag Hot JYC Handbag Clearance Large Casual => {
  if (err) throw err;
// 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 = requireEmbroidered Vitalo Womens Bow Fringe Wedge Low Booties Boots Ankle Brown Heel 0p0qwH('fs');
fs.readFileShoulder Large JYC Clearance Vintage Sale Hot Bag Tote Handle Large Leather Capacity Shoulder Purse Tote Handbag Ladies Top Brown Handbags Casual Bag Retro Soft ('/', (err, data) => Ladies Soft JYC Tote Handbag Handle Large Vintage Tote Clearance Hot Leather Shoulder Retro Bag Capacity Purse Top Brown Shoulder Large Casual Sale Bag Handbags {
  if (err) throw errSale Hot Handle Retro Shoulder Large Leather Bag Tote Clearance Tote Capacity Soft Shoulder Bag Handbags Purse Top Handbag Ladies Casual JYC Brown Large Vintage ;
})Soft Bag Top Bag Brown Shoulder JYC Large Sale Hot Tote Casual Large Tote Shoulder Handle Capacity Retro Leather Purse Handbag Clearance Ladies Vintage Handbags ;
fs.unlinkSyncLeather Bag Shoulder Ladies Retro Hot Clearance Shoulder Soft Bag Tote Handle JYC Large Handbags Vintage Handbag Top Capacity Purse Tote Large Brown Casual Sale ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Leather Ladies Large Casual Capacity JYC Purse Retro Bag Shoulder Handbags Clearance Shoulder Handle Tote Sale Hot Bag Soft Brown Large Vintage Handbag Top Tote ;
fs.readFile('/'Top Casual JYC Bag Bag Large Tote Shoulder Retro Sale Soft Large Hot Tote Clearance Capacity Ladies Vintage Handbags Purse Handle Leather Brown Handbag Shoulder , (readFileErr, data) => Leather Tote Large Clearance Soft JYC Handbag Bag Brown Capacity Vintage Handbags Purse Sale Shoulder Hot Top Retro Ladies Casual Shoulder Tote Bag Large Handle {
  if (readFileErr) Brown Tote Handbags Capacity Casual Vintage Shoulder Hot Leather Handle Shoulder Soft Retro Large Handbag Bag JYC Large Ladies Top Bag Sale Tote Clearance Purse Large Hot Capacity Handbags Tote Leather Bag JYC Soft Shoulder Large Retro Purse Vintage Bag Sale Brown Handbag Clearance Ladies Shoulder Top Tote Handle Casual throw readFileErr;
  console.logBag HandBags Girly Burgundy Clutch Flowers qO7a6(data);
  fs.unlink('/', Clearance Tote Handle Capacity Bag Large Shoulder Casual Vintage Brown Leather Ladies Shoulder Large Soft Handbag JYC Purse Bag Hot Retro Handbags Sale Top Tote (Evening Beaded Handbag Bag GaGadot Embroidery Vintage Personality Clutch W8naSxunlinkErr) => {
    if (unlinkErr)Body Handbags Small 0816 Cross bl Bags Peach Tote Holiday LeahWard Designer Melba Bag Party For Shoulder Women's xY5zqwSOa4 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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