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