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Server-side Rendering

By default, Yew components render on 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 on 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 the initial render has been completed. This can result in a poor experience for users on 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 on 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) 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 as effects are not executed during server-side rendering.

Struct Components

While it is possible to use Struct Components with server-side rendering, there are 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 the 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 points 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 was 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 is being requested during the 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.

This not only 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 the hydration process to make sure that the virtual DOM returned by the 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 starting 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.

warning

The hydration can only succeed if the real DOM matches the expected DOM after initial render of the SSR output (static HTML) by browser. If your HTML is not spec-compliant, the hydration may fail. Browsers may change the DOM structure of the incorrect HTML, causing the actual DOM to be different from the expected DOM. For example, if you have a <table> without a <tbody>, the browser may add a <tbody> to the DOM

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 your component is free of side effects. It should not mutate any states or trigger additional renders. If your component currently mutates states or triggers additional renders, move them into a use_effect hook.

It is 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 the 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

Single thread mode​

Yew supports single thread mode for server-side rendering by yew::LocalServerRenderer. This mode would work in a single thread environment like WASI.

// Build it by `wasm32-wasip1` target or `wasm32-wasip2` target (after rustc 1.78).
// You can still use `wasm32-wasi` target to build if you are using older version of rustc (before 1.84).
// See https://blog.rust-lang.org/2024/04/09/updates-to-rusts-wasi-targets.html for more information.

use yew::prelude::*;
use yew::LocalServerRenderer;

#[function_component]
fn App() -> Html {
    use yew_router::prelude::*;

    html! {
        <>
            <h1>{"Yew WASI SSR demo"}</h1>
        </>
    }
}

pub async fn render() -> String {
    let renderer = LocalServerRenderer::<App>::new();
    let html_raw = renderer.render().await;

    let mut body = String::new();
    body.push_str("<body>");
    body.push_str("<div id='app'>");
    body.push_str(&html_raw);
    body.push_str("</div>");
    body.push_str("</body>");

    body
}

#[tokio::main(flavor = "current_thread")]
async fn main() {
    println!("{}", render().await);
}

Example: wasi_ssr_module

note

If you are using the wasm32-unknown-unknown target to build a SSR application, you can use the not_browser_env feature flag to disable access of browser-specific APIs inside of Yew. This would be useful on serverless platforms like Cloudflare Worker.

caution

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