API-Frontend Integration

When to Use

Activate when connecting a backend API to a frontend application, setting up data-fetching patterns, implementing optimistic updates, or establishing type-safe API contracts.

Integration Strategy Selection

Approach	Type Safety	Setup Cost	Best For
tRPC	Full E2E	Low	Monorepos, same-team ownership
OpenAPI codegen	Generated	Medium	REST APIs, multi-team
GraphQL + codegen	Generated	Medium	Complex relational data queries
Manual fetch + Zod	Runtime	Low	Simple APIs, third-party services

tRPC Integration (Recommended for Monorepos)

1. Define routers in packages/api/src/routers/ with input validation via Zod.
2. Export the AppRouter type -- never the runtime object -- to the frontend.
3. Create a tRPC client with httpBatchLink or httpSubscriptionLink.
4. Wrap the app with trpc.Provider and QueryClientProvider.
5. Use trpc.useQuery() and trpc.useMutation() -- types flow automatically.

REST Client Generation

• openapi-typescript -- generates TypeScript types from OpenAPI 3.x specs. Pair with openapi-fetch for typed fetch calls.
• orval -- generates React Query hooks, Zod schemas, and mock service workers from OpenAPI specs. Preferred for complex REST APIs.
• Automate generation in CI: "codegen": "orval --config orval.config.ts".

Data Fetching Patterns (TanStack Query / SWR)

// Query with proper typing, stale time, and error handling
const { data, isLoading, error } = useQuery({
  queryKey: ['users', filters],
  queryFn: () => api.users.list(filters),
  staleTime: 5 * 60 * 1000,
  retry: 3,
  retryDelay: (attempt) => Math.min(1000 * 2 ** attempt, 10000),
});

Optimistic Updates

1. Use onMutate to snapshot previous data and apply the optimistic change.
2. Use onError to rollback to the snapshot on failure.
3. Use onSettled to invalidate queries and refetch the authoritative state.
4. Show a subtle indicator (toast, inline badge) so the user knows the change is pending.

Error Handling Strategy

• API errors -- define a shared error shape ({ code, message, details? }). Parse with Zod on the client.
• Network errors -- distinguish between offline, timeout, and server errors. Show different UI for each.
• Error boundaries -- wrap route segments with React Error Boundaries. Provide a "Retry" action.
• Form validation -- validate on the client first (Zod + react-hook-form), then re-validate on the server. Surface server-side errors next to the relevant field.

Loading and Empty States

• Use skeleton loaders (not spinners) for content areas to reduce perceived latency.
• Always handle the empty state: "No results found" with a clear call-to-action.
• Show stale data with a background refresh indicator rather than a full loading state on refetch.

Common Pitfalls

• Fetching inside useEffect without cancellation -- always use TanStack Query or SWR.
• Forgetting to invalidate related queries after a mutation.
• Not deduplicating concurrent identical requests (TanStack Query handles this automatically).
• Ignoring error responses and only checking data -- always handle the error path.