Feature Backend

Feature Backend

Multi-User Architecture

Features are always multi-user. The backend serves requests from many users concurrently. Every design decision must account for this.

Per-user vs. shared state:

Common mistakes:

• ❌ Storing a user's OAuth token in an env var or in-memory config → all users share one token
• ❌ Storing a user's preference in a module-level variable → last user's preference wins for everyone
• ❌ Using env vars for per-user settings or selections → same value for everyone
• ✅ Store per-user data in cookies or dashboard rows keyed by user
• ✅ Use env vars only for credentials/config shared across all users (e.g. OAuth client ID/secret)

When to Add a Backend

A backend is optional. Most features work fine with the Dashboard SDK alone (client-side calls to the dashboard service). Only add backend/ when the feature genuinely needs:

• Custom business logic (aggregations, validations, workflows)
• Real-time push via WebSockets
• Server-side API composition or proxying
• Background processing or scheduled tasks
• Operations that cannot run in the browser (secrets, heavy computation)

Do NOT add a backend just for CRUD on dashboard data — use the Dashboard SDK directly from the SPA.

Structure

features/my-feature/
  package.json              ← SPA deps (unchanged)
  vite.config.ts
  src/                      ← SPA code
  backend/                  ← backend (only if needed)
    package.json            ← backend-only deps
    tsconfig.json
    src/
      index.ts              ← entrypoint
      routes/               ← route handlers
      ws/                   ← WebSocket handlers (if needed)

Key points:

• backend/ has its own package.json — keeps backend deps (Hono, ws libs) out of the SPA bundle
• No code is shared between SPA and backend — each side defines its own types independently. Do not create a shared/ directory
• Backends are not shared among features — only the feature that owns the backend/ folder can access it. Each feature must have its own backend if it needs one; one feature cannot call another feature's backend.
• The SPA package.json remains unchanged — no backend deps leak in

Framework: Hono

Use Hono for the backend. It is TypeScript-first, lightweight, and has built‑in WebSocket support. It runs on Node.js and Bun.

backend/package.json

{
  "name": "my-feature-server",
  "private": true,
  "type": "module",
  "scripts": {
    "dev": "tsx watch src/index.ts",
    "build": "tsup src/index.ts --format esm --out-dir dist",
    "start": "node dist/index.js",
    "lint": "eslint . --max-warnings 0"
  },
  "dependencies": {
    "hono": "^4.x",
    "@hono/node-server": "^1.x",
    "@hono/node-ws": "^1.x"
  },
  "devDependencies": {
    "tsx": "^4.x",
    "tsup": "^8.x",
    "typescript": "^5.x"
  }
}

backend/tsconfig.json

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "ESNext",
    "moduleResolution": "bundler",
    "strict": true,
    "esModuleInterop": true,
    "outDir": "dist",
    "rootDir": "."
  },
  "include": ["src"]
}

backend/src/index.ts (minimal)

import { Hono } from 'hono'
import { serve } from '@hono/node-server'

const app = new Hono().basePath('/api')

app.get('/health', (c) => c.json({ ok: true }))

// Add routes:
// import { itemsRoutes } from './routes/items'
// app.route('/items', itemsRoutes)

const port = Number(process.env.BACKEND_PORT) || 3001

serve({ fetch: app.fetch, port }, () => {
  console.log(`Server running on port ${port}`)
})

export default app

Adding WebSockets

import { Hono } from 'hono'
import { serve } from '@hono/node-server'
import { createNodeWebSocket } from '@hono/node-ws'

const app = new Hono().basePath('/api')

const { injectWebSocket, upgradeWebSocket } = createNodeWebSocket({ app })

app.get('/ws', upgradeWebSocket((c) => ({
  onMessage(event, ws) {
    // handle incoming message
    ws.send(JSON.stringify({ echo: event.data }))
  },
  onClose() {
    console.log('Client disconnected')
  },
})))

const port = Number(process.env.BACKEND_PORT) || 3001
const server = serve({ fetch: app.fetch, port })
injectWebSocket(server)

Routing: /api is Reserved for the Backend

When a feature has a backend, the /api path prefix is reserved for the backend. The SPA must not define client-side routes under /api.

• Backend routes: /api/* (REST endpoints, WebSocket upgrades)
• SPA routes: everything else (/, /items/:id, /settings, etc.)

Dev Proxy

fusebase dev start automatically proxies /api HTTP requests and WebSocket upgrades to the backend dev server.

The BACKEND_PORT env var is assigned by fusebase dev start and injected into both the SPA and backend processes, allowing multiple features to run backends concurrently without port conflicts.

fusebase.json Backend Config

When a feature has a backend, add the backend block to its entry in fusebase.json:

{
  "features": [
    {
      "id": "feature-id",
      "path": "features/my-feature",
      "dev": { "command": "npm run dev" },
      "build": { "command": "npm run build", "outputDir": "dist" },
      "backend": {
        "dev": { "command": "npm run dev" },
        "build": { "command": "npm run build" },
        "start": { "command": "npm run start" }
      }
    }
  ]
}

Backend commands (dev, build, start) run from the backend/ subdirectory of the feature path.

Deriving the Public Base URL from the Request

NEVER hardcode localhost in callback/redirect URLs (e.g. OAuth redirect URIs, webhook URLs, links sent to external services). A feature's backend runs behind a proxy — localhost only works during local dev and breaks in production.

Instead, derive the public base URL from the incoming request headers:

/** Derive the public base URL from the incoming request. */
function getBaseUrl(req: Request): string {
  const url = new URL(req.url)
  const forwardedProto = req.headers.get('x-forwarded-proto')
  const forwardedHost =
    req.headers.get('x-forwarded-host') ?? req.headers.get('host')
  if (forwardedHost) {
    const proto = forwardedProto ?? url.protocol.replace(':', '')
    return `${proto}://${forwardedHost}`
  }
  return url.origin
}

Usage example (OAuth redirect URI):

app.get('/auth/url', (c) => {
  const baseUrl = getBaseUrl(c.req.raw)
  const redirectUri = `${baseUrl}/api/auth/callback`
  // Use redirectUri when building the OAuth authorization URL
})

This works in both environments:

• Local dev: resolves to http://localhost:<port> (via Fusebase dev server proxy forwarding host)
• Deployed: resolves to https://<subdomain>.{FUSEBASE_APP_HOST} (platform sets x-forwarded-host / x-forwarded-proto)

Calling the Backend from the SPA

Use standard fetch with relative URLs. Same-origin requests automatically include the fbsfeaturetoken cookie, so the backend can authenticate on behalf of the user without depending on a custom header surviving the deployed platform proxy:

// In SPA code
const res = await fetch('/api/items')
const data = await res.json()

If you still send x-app-feature-token from the SPA, treat it as a best-effort dev/proxy optimization only. Backend handlers must always support both sources:

import { getCookie } from 'hono/cookie'

const featureToken =
  c.req.header('x-app-feature-token') || getCookie(c, 'fbsfeaturetoken')

if (!featureToken) {
  return c.json({ error: 'Missing feature token' }, 401)
}

Gate security: fail closed for user-facing routes

When backend routes call Gate on behalf of the current user, keep auth in feature-token context only.

• Do not silently fall back to service-account/service-token auth in user-facing routes.
• On missing/invalid feature token or Gate auth rejection, return 401/403 and require re-auth/permission sync.
• Service-token usage is allowed only for explicitly system/admin routes, not as an automatic fallback path.

For WebSockets:

const ws = new WebSocket(`wss://${window.location.host}/api/ws`)
ws.onmessage = (event) => {
  const msg: WsMessage = JSON.parse(event.data)
  // handle message
}

Stateless Backend — No Filesystem Writes, No In-Memory Persistence

The deployed backend is stateless. The filesystem is ephemeral and in-memory state is lost on restart/redeployment. Do not rely on either for persistent data.

NEVER:

• Write to .env, JSON, or any local file to persist runtime state
• Use fs.writeFileSync / fs.writeFile for data that must survive restarts
• Store tokens, credentials, or user data on the local filesystem
• Use SQLite or file-based databases
• Store persistent state only in backend memory (lost on restart)

Instead, use:

• httpOnly cookies — for per-user credentials obtained at runtime (e.g. OAuth refresh tokens). The browser sends them automatically; the backend stays stateless. This is the preferred approach for user-specific tokens.
• Fusebase dashboards — for persistent runtime data shared across users (via Dashboard SDK in backend code)
• Fusebase secrets (env vars) — for shared credentials set at deploy time (API keys, service-account tokens). Not suitable for per-user or dynamically obtained tokens.
• In-memory caches — acceptable only for short-lived caches (e.g. access tokens derived from a refresh token in a cookie). Must be re-derivable from persistent source.

Example — OAuth token flow (httpOnly cookie): When an OAuth callback returns a refresh token, store it in an httpOnly cookie:

import { setCookie, getCookie } from 'hono/cookie'

// In the OAuth callback handler:
setCookie(c, 'oauth_refresh_token', tokens.refresh_token, {
  httpOnly: true,
  secure: true,
  sameSite: 'Lax',
  path: '/',
  maxAge: 60 * 60 * 24 * 365, // 1 year
})

// In API handlers — read token from cookie, fall back to env var:
const refreshToken = getCookie(c, 'oauth_refresh_token') ?? process.env.REFRESH_TOKEN ?? ''

// ❌ Wrong: writing to filesystem
writeFileSync('.env', `REFRESH_TOKEN=${tokens.refresh_token}`)

// ❌ Wrong: relying solely on in-memory state
config.refreshToken = tokens.refresh_token  // lost on restart

Dev Workflow

1. cd features/my-feature/backend && npm install — install backend deps
2. fusebase secret create --feature <id> --secret "KEY:description" — register secrets (if needed), set values via the printed URL
3. fusebase dev start — starts both SPA and backend; secrets are injected automatically as env vars

No .env files or dotenv needed — fusebase dev start injects secrets into the backend process.

Checklist

Before adding a backend:

• Confirmed the feature genuinely needs backend logic (not just dashboard CRUD)
• Created backend/ with its own package.json and tsconfig.json
• Set up Hono with .basePath('/api')
• Verified fusebase dev start proxies /api to backend (automatic when backend block exists in fusebase.json)
• Updated fusebase.json with backend block
• SPA does not define routes under /api
• No .env files or dotenv — secrets injected by fusebase dev start