SSR Hydration

SSR Hydration

Transport server-resolved State to the client for hydration (#563)

Why

In SSR, the server resolves a URL into a State (running matchPath, forwardState, buildPath) and renders HTML. The client needs to commit the same State on hydration so that the first React/Vue render matches server-rendered HTML.

The canonical URL — state.path — is the source of truth. Real-router's start(path) deterministically reproduces the server's State on the client, provided your interceptors are deterministic on URL alone. If they read async client-only state (auth that loads asynchronously, A/B group from a non-cookie source, persistent params from localStorage), that's a design issue: server and client must see the same input given the same URL, otherwise hydration mismatches will appear regardless of how you start the router.

This page covers two helpers in @real-router/core/utils for the SSR transport layer:

Helper	Purpose
serializeRouterState(state)	XSS-safe JSON of State, strips per-navigation transition meta
hydrateRouter(router, source)	Convenience: parses JSON (if string), calls router.start(state.path)

Quick Start

Server (entry-server.tsx)

import { UNKNOWN_ROUTE } from "@real-router/core";
import { cloneRouter } from "@real-router/core/api";
import { serializeRouterState } from "@real-router/core/utils";
import { renderToString } from "react-dom/server";

export async function render(url: string, ctx: RenderContext) {
  const router = cloneRouter(baseRouter, ctx);
  router.usePlugin(ssrDataPluginFactory(loaders));

  try {
    const state = await router.start(url);
    const html = renderToString(
      <RouterProvider router={router}>
        <App />
      </RouterProvider>,
    );

    return {
      html,
      // Inject server-resolved State for client hydration
      script: `<script>window.__SSR_STATE__=${serializeRouterState(state)}</script>`,
      statusCode: state.name === UNKNOWN_ROUTE ? 404 : 200,
    };
  } finally {
    router.dispose();
  }
}

Client (entry-client.tsx)

import { browserPluginFactory } from "@real-router/browser-plugin";
import { hydrateRouter } from "@real-router/core/utils";
import { hydrateRoot } from "react-dom/client";

declare global {
  interface Window {
    __SSR_STATE__?: { path: string };
  }
}

const router = createAppRouter({ /* client deps */ });
router.usePlugin(browserPluginFactory());

const ssrState = window.__SSR_STATE__;

if (ssrState) {
  await hydrateRouter(router, ssrState);
} else {
  await router.start();
}

hydrateRoot(document.getElementById("root")!, <App />);

hydrateRouter extracts state.path and calls router.start(state.path) — a one-line wrapper around JSON.parse + start. You can equivalently write the call inline if preferred:

await router.start(JSON.parse(rawJson).path);

What gets serialized

serializeRouterState(state) keeps:

• state.name
• state.params
• state.path — canonical URL (server's source of truth)
• state.context — plugin context namespaces (e.g. state.context.data from ssr-data-plugin)

serializeRouterState(state) strips:

• state.transition — per-navigation TransitionMeta. Regenerated on commit.

• Any namespace listed in options.excludeContext (second optional argument). Use this when a plugin populates state.context.<ns> with non-JSON-serializable values (e.g. @real-router/rsc-server-plugin writes a ReactNode to state.context.rsc):

  const json = serializeRouterState(state, { excludeContext: ["rsc"] });

XSS escapes (<, >, & → <, >, &) are inherited from the underlying serializeState, so the JSON is safe to embed inside <script>.

Reading server-side context payloads

hydrateRouter only consumes state.path. If your app uses state.context.<namespace> for SSR data transport (e.g., ssr-data-plugin writes state.context.data on the server), read that value separately from window.__SSR_STATE__ before discarding the JSON:

const ssrState = window.__SSR_STATE__ as
  | { path: string; context?: { data?: unknown } }
  | undefined;

const initialData = ssrState?.context?.data;
// hand initialData to your store / cache / data layer

if (ssrState) await hydrateRouter(router, ssrState);

The router doesn't carry state.context across hydration automatically — plugins write context on the client during their own lifecycle hooks. If you want server-rendered data to bypass a re-fetch on the client, manage that at the data layer (TanStack Query dehydrate/hydrate, store rehydration, or a plugin's own initial-data option).

Why path-only?

state.path is the canonical URL produced by the server's full pipeline (matchPath → forwardState → buildPath). When the client calls router.start(state.path):

• The same URL is fed to the same route tree → matchPath produces the same name + params.
• Client-side forwardState and buildPath interceptors run; if they're deterministic on URL (the only correct design), they produce identical results to the server's.
• Activation guards run normally — that's where current-world checks belong (live session, permissions).

Bypassing those interceptors on the client (an alternative considered and rejected) would mask non-idempotent interceptor design rather than fix it. A forwardFn that depends on async client state is a bug; the fix is to make it synchronous on a shared input (cookie, header) — not to skip it on hydration.

Related

• #525 — navigateToState primitive (used by start(path) since #525)
• Plugin Architecture
• @real-router/ssr-data-plugin — Per-route data loading on start()
• @real-router/rsc-server-plugin — Per-route ReactNode (RSC payload) loading on start()
• Server-Side Rendering — Full SSR overview