Avoid Shared Module State for Request Data
For React Server Components and client components rendered during SSR, avoid using mutable module-level variables to share request-scoped data. Server renders can run concurrently in the same process. If one render writes to shared module state and another render reads it, you can get race conditions, cross-request contamination, and security bugs where one user's data appears in another user's response.
Treat module scope on the server as process-wide shared memory, not request-local state.
Incorrect (request data leaks across concurrent renders):
let currentUser: User | null = null
export default async function Page() {
currentUser = await auth()
return <Dashboard />
}
async function Dashboard() {
return <div>{currentUser?.name}</div>
}If two requests overlap, request A can set currentUser, then request B overwrites it before request A finishes rendering Dashboard.
Correct (keep request data local to the render tree):
export default async function Page() {
const user = await auth()
return <Dashboard user={user} />
}
function Dashboard({ user }: { user: User | null }) {
return <div>{user?.name}</div>
}Safe exceptions:
- Immutable static assets or config loaded once at module scope
- Shared caches intentionally designed for cross-request reuse and keyed correctly
- Process-wide singletons that do not store request- or user-specific mutable data
For static assets and config, see Hoist Static I/O to Module Level.