Understanding Hydration in Leptos: Boosting SSR Performance

What Is Hydration?

Hydration in Leptos refers to rendering the initial HTML on the server and then, on the client via WebAssembly, injecting event listeners and state management to make the page interactive.

Advantages of Hydration

Fast initial page load : The server‑rendered HTML can be displayed immediately, improving load speed.

SEO optimization : Search engines can index the rendered content, boosting SEO.

Client‑side interactivity : After hydration, full client‑side interactions become available.

Leptos SSR Example Code

The following snippet shows the SSR example provided by Leptos.

// lib.rs
#[cfg(feature = "hydrate")]
#[wasm_bindgen::prelude::wasm_bindgen]
pub fn hydrate() {
    use app::*;
    use leptos::*;
    console_error_panic_hook::set_once();
    mount_to_body(App);
}

Brief Explanation of the Code

The code is compiled only when the hydrate feature is enabled.

The #[wasm_bindgen] attribute makes the function callable from JavaScript.

The function connects Leptos’s client functionality to the server‑rendered HTML.

Workflow

Server renders the initial HTML (SSR).

Client receives the HTML.

The WebAssembly bundle loads and executes the hydrate() function.

The received HTML is activated with dynamic functionality on the client.

This process implements Leptos’s “island architecture” or partial hydration core mechanism.