Server-side Rendering

By default, Yew components render at the client side. When a viewer visits a website, the server sends a skeleton html file without any actual content and a WebAssembly bundle to the browser. Everything is rendered at the client side by the WebAssembly bundle. This is known as client-side rendering.

This approach works fine for most websites, with some caveats:

1. Users will not be able to see anything until the entire WebAssembly bundle is downloaded and initial render has completed. This can result in poor user experience if the user is using a slow network.
2. Some search engines do not support dynamically rendered web content and those who do usually rank dynamic websites lower in the search results.

To solve these problems, we can render our website on the server side.

How it Works

Yew provides a ServerRenderer to render pages on the server-side.

To render Yew components at the server-side, you can create a renderer with ServerRenderer::<App>::new() and call renderer.render().await to render <App /> into a String.

use yew::prelude::*;
use yew::ServerRenderer;
#[function_component]
fn App() -> Html {
    html! {<div>{"Hello, World!"}</div>}
}
// we use `flavor = "current_thread"` so this snippet can be tested in CI,
// where tests are run in a WASM environment. You likely want to use
// the (default) `multi_thread` favor as:
// #[tokio::main]
#[tokio::main(flavor = "current_thread")]
async fn no_main() {
    let renderer = ServerRenderer::<App>::new();
    let rendered = renderer.render().await;
    // Prints: <div>Hello, World!</div>
    println!("{}", rendered);
}

Component Lifecycle

The recommended way of working with server-side rendering is function components.

All hooks other than use_effect (and use_effect_with_deps) will function normally until a component successfully renders into Html for the first time.

Web APIs are not available!

Web APIs such as web_sys are not available when your component is rendering on the server-side. Your application will panic if you try to use them. You should isolate logics that need Web APIs in use_effect or use_effect_with_deps as effects are not executed during server side rendering.

Struct Components

Whilst it’s possible to use Struct Components with server-side rendering, there’s no clear boundaries between client-side safe logic like the use_effect hook for function components and lifecycle events are invoked in a different order than client side.

In addition, Struct Components will continue to accept messages until all of its children are rendered and destroy method is called. Developers need to make sure no messages possibly passed to components would link to logic that makes use of Web APIs.

When designing an application with server-side rendering support, prefer function components unless you have a good reason not to.

Data Fetching during Server-side Rendering

Data fetching is one of the difficult point with server side rendering and hydration.

Traditionally, when a component renders, it is instantly available (outputs a virtual dom to be rendered). This works fine when the component does not want to fetch any data. But what happens if the component wants to fetch some data during rendering?

In the past, there’s no mechanism for Yew to detect whether a component is still fetching data. The data fetching client is responsible to implement a solution to detect what’s being requested during initial render and triggers a second render after requests are fulfilled. The server repeats this process until no more pending requests are added during a render before returning a response.

Not only this wastes CPU resources by repeatedly rendering components, but the data client also needs to provide a way to make the data fetched on the server-side available during hydration process to make sure that the virtual dom returned by initial render is consistent with the server-side rendered DOM tree which can be hard to implement.

Yew takes a different approach by trying to solve this issue with <Suspense />.

Suspense is a special component that when used on the client-side, provides a way to show a fallback UI while the component is fetching data (suspended) and resumes to normal UI when the data fetching completes.

When the application is rendered on the server-side, Yew waits until a component is no longer suspended before serializing it into the string buffer.

During the hydration process, elements within a <Suspense /> component remains dehydrated until all of its child components are no longer suspended.

With this approach, developers can build a client-agnostic, SSR ready application with data fetching with very little effort.

SSR Hydration

Hydration is the process that connects a Yew application to the server-side generated HTML file. By default, ServerRender prints hydratable html string which includes additional information to facilitate hydration. When the Renderer::hydrate method is called, instead of start rendering from scratch, Yew will reconcile the Virtual DOM generated by the application with the html string generated by the server renderer.

caution

To successfully hydrate an html representation created by the ServerRenderer, the client must produce a Virtual DOM layout that exactly matches the one used for SSR including components that do not contain any elements. If you have any component that is only useful in one implementation, you may want to use a PhantomComponent to fill the position of the extra component.

Component Lifecycle during hydration

During Hydration, components schedule 2 consecutive renders after it is created. Any effects are called after the second render completes. It is important to make sure that the render function of the your component is side-effect free. It should not mutate any states or trigger additional renders. If your component currently mutates states or triggers additional renders, move them into an use_effect hook.

It’s possible to use Struct Components with server-side rendering in hydration, the view function will be called multiple times before the rendered function will be called. The DOM is considered as not connected until rendered function is called, you should prevent any access to rendered nodes until rendered() method is called.

Example

use yew::prelude::*;
use yew::Renderer;
#[function_component]
fn App() -> Html {
    html! {<div>{"Hello, World!"}</div>}
}
fn main() {
    let renderer = Renderer::<App>::new();
    // hydrates everything under body element, removes trailing
    // elements (if any).
    renderer.hydrate();
}

Example: simple_ssr Example: ssr_router

caution

Server-side rendering is currently experiemental. If you find a bug, please file an issue on GitHub.