The islands architecture recap

2022/09/21 Techfeed Experts Night #4

About me 👋

Yuji Sugiura

  • PixelGrid Inc.
    • Senior Engineer
  • TechFeed Expert No.52

りぃ / @leader22

Today's theme

Islands architecture 🏝


  • Page consists of static ocean🌊 and dynamic islands🏝
    • Static: Layouts, texts, images, etc...
    • Dynamic(w/ JS): Carousel, Popup, VideoPlayer, etc...
  • Page is (pre-)rendered on the server
    • !== SPA
  • Dynamic islands are become interactive on the client
    • Hydrated with rendered HTML
    • Multiple entries, small apps

Why? 🤔

Static >= Dynamic

  • Most web pages consist of static contents and dynamic contents(requires JS)
  • Generally the static contents are more than you think

Of course, it depends on the situation. You may (NOT) need SPA 😌

Concerns about modern frameworks

  • Recent frameworks(SPA in mind) treat everything as dynamic by nature
    • Single entry, large app
  • This means that a large amount of JS must be downloaded, parsed, and executed 💨
    • Includes framework itself, templates already rendered, hydration things, etc...
    • Even if pre-rendering(SSG mode)
    • Even if page is 100% static 🙄

It leads slow page load, waste bandwidth, and also slow TTI.

(Full) Hydration is pure overhead

  • Hydration is top-down process with no priority
    • Carousel at the top should become interactive immediately
    • But, why should we load (and wait) the video player at the bottom?
  • We don't need useless JS in the first place!
    • You may not need a SPA, but even with a MPA, extra JS should be avoided
  • 🤖 Always imagine poor mobile device on slow network

I wish I could use minimum JS when the time I need it?

Islands architecture
Partial hydration

Partial hydration to the rescue 🚒

Instead of full(page-level), why not perform partial(component🏝-level) hydration?

  • Minimum size of JS are always used
    • 0-JS by default 👌
  • Non-blocking, isolated hydration in parallel 🔀
  • Bonus: Lazy loading + hydration ✨
    • With requestIdleCallback()
    • When visible w/ IntersectionObserver
    • On mouseenter, touchstart
    • etc...

Various islands architecture oriented frameworks implement these features!

Modern progressive enhancement?

  <!-- Server rendered HTML here -->
    $("#js-carousel").carousel({ speed: 500 });

At that time, it took a lot of effort to cope with. 🤣

  • HTML was handwritten, server and client templates are doubled
  • Manual execution timing management
  • Mount multiple Apps with complicated build steps
  • Web Components had SSR issues

But now in 2022, sophisticated frameworks do it better with nice DX.

Try it out!

Choose your starter!

Maybe more to come! 🦄

Example code w/ Astro

import Layout from "../components/layout.astro";
// ...
import VideoPlayer from "../components/video-player.jsx";

  <Header />
  <Carousel speed={500} client:load />
    <Popup client:idle />
  <!-- ... -->
    <VideoPlayer src="https://..." client:visible />
  <Footer />

Looks familiar to you?

User's voice w/ Astro

Rebuilt my (next.js) blog using @astrodotbuild out of curiosity...holy shit the difference in bundle size.

Home route: 138kb -> 7.6kb
"All posts": 570kb -> 100kb (85kb was images)

Seriously considering moving my static sites to this

Thank you!

Go 1 step further?


  • No hydration, resumable
  • Lazy loading everything
    • Load only (what|when) you need
    • O(1), not O(n)

You should definitely try! 😻

References 📚