React query data fetching

import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
import { ReactQueryDevtools } from '@tanstack/react-query-devtools';

const queryClient = new QueryClient({
  defaultOptions: {
    queries: {
      staleTime: 1000 * 60, // 1 minute: data remains fresh without refetch
      gcTime: 1000 * 60 * 10, // 10 minutes: cache eviction window
      retry: 2,
      refetchOnWindowFocus: false,
    },
    mutations: {
      retry: 0,
    },
  },
});

export function AppProviders({ children }: { children: React.ReactNode }) {
  return (
    <QueryClientProvider client={queryClient}>
      {children}
      <ReactQueryDevtools initialIsOpen={false} />
    </QueryClientProvider>
  );
}

import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
import { api } from '@/lib/api';

export interface User {
  id: string;
  name: string;
  email: string;
  role: 'admin' | 'user';
}

export const userKeys = {
  all: ['users'] as const,
  detail: (id: string) => [...userKeys.all, 'detail', id] as const,
  list: (filters: { page: number; limit: number }) => [...userKeys.all, 'list', filters] as const,
};

export function useUser(id: string) {
  return useQuery({
    queryKey: userKeys.detail(id),
    queryFn: () => api.users.get(id),
    enabled: !!id,
  });
}

export function useUserList(filte

rs: { page: number; limit: number }) { return useQuery({ queryKey: userKeys.list(filters), queryFn: () => api.users.list(filters), keepPreviousData: true, // prevents UI flicker during pagination }); }

### Step 3: Handle Mutations with Cache Invalidation
Mutations modify server state. They must invalidate or optimistically update the cache to maintain consistency.

```typescript
export function useUpdateUser() {
  const queryClient = useQueryClient();

  return useMutation({
    mutationFn: ({ id, data }: { id: string; data: Partial<User> }) =>
      api.users.update(id, data),
    onMutate: async ({ id, data }) => {
      // Cancel outgoing refetches to avoid race conditions
      await queryClient.cancelQueries({ queryKey: userKeys.detail(id) });
      
      // Snapshot current cache
      const previous = queryClient.getQueryData<User>(userKeys.detail(id));
      
      // Optimistically update
      if (previous) {
        queryClient.setQueryData<User>(userKeys.detail(id), { ...previous, ...data });
      }
      
      return { previous };
    },
    onError: (_err, _vars, context) => {
      // Rollback on failure
      if (context?.previous) {
        queryClient.setQueryData(userKeys.detail(context.previous.id), context.previous);
      }
    },
    onSettled: (_data, _error, { id }) => {
      // Invalidate to fetch authoritative server state
      queryClient.invalidateQueries({ queryKey: userKeys.detail(id) });
    },
  });
}

Architecture Decisions & Rationale

1. Query Keys as Arrays: Arrays enable hierarchical cache matching. ['users', 'detail', '123'] allows bulk invalidation via ['users'] while preserving granular entries.
2. Stale Time vs GC Time: staleTime controls when data is considered fresh. gcTime controls when unused cache entries are purged. Separating them prevents premature eviction while maintaining memory hygiene.
3. Optimistic Updates with Rollback: onMutate snapshots state before applying changes. onError restores it. This pattern eliminates loading spinners for predictable operations while preserving data integrity.
4. keepPreviousData for Pagination: Prevents UI flash when navigating pages. The previous result remains visible until the new query resolves.
5. Centralized QueryClient Configuration: Global defaults reduce per-query boilerplate and enforce consistent retry, stale, and garbage collection policies across the application.

Pitfall Guide

1. Over-fetching Due to Missing staleTime

Mistake: Leaving staleTime at 0 causes refetches on every mount or window focus. Impact: Network saturation, rate limit violations, and degraded perceived performance. Fix: Set staleTime to match data volatility. Static metadata: 5-10 minutes. Real-time dashboards: 10-30 seconds. Use refetchOnWindowFocus: false unless explicitly required.

2. Query Key Instability

Mistake: Using dynamically generated objects or non-serializable values in query keys. Impact: Cache misses, duplicate requests, and memory leaks. Fix: Keys must be serializable and stable. Use primitive arrays. Avoid { id: user.id, timestamp: Date.now() }. Prefer ['users', id].

3. Mixing Server and Client State

Mistake: Storing UI toggles, form drafts, or animation states inside query cache. Impact: Cache pollution, incorrect invalidation, and SSR hydration mismatches. Fix: Server state belongs in React Query. Client state belongs in useState, useReducer, or Zustand/Jotai. Never mutate query data with UI-only flags.

4. Ignoring Retry and Error Boundaries

Mistake: Assuming retry: false or relying solely on component-level error handling. Impact: Transient network failures cause hard crashes or silent data loss. Fix: Configure retry based on idempotency. GET requests: 2-3 retries. POST/PUT: 0 retries (unless backend supports idempotency keys). Wrap query consumers in ErrorBoundary components for graceful degradation.

5. Manual Cache Updates Without Invalidation

Mistake: Using setQueryData to patch cache but forgetting to trigger invalidateQueries for dependent lists. Impact: Detail views update, but list views show stale data. UI inconsistency. Fix: Always pair setQueryData with targeted invalidation. Prefer invalidateQueries over direct mutation unless implementing strict optimistic updates with rollback.

6. Using Queries for Static or One-Time Data

Mistake: Fetching configuration files, static assets, or server-rendered initial data through useQuery. Impact: Unnecessary cache entries, hydration mismatches in SSR, and wasted memory. Fix: Inject static data via props or context. Use useQuery only for data that requires caching, background refetching, or invalidation.

7. Forgetting gcTime Configuration

Mistake: Leaving default gcTime (5 minutes) in memory-constrained environments or long-running SPAs. Impact: Memory bloat, especially with high-cardinality query keys (e.g., search results, infinite lists). Fix: Adjust gcTime per route or feature. Use queryClient.removeQueries() for explicit cleanup. Monitor memory via React Query Devtools.

Production Bundle

Action Checklist

• Initialize QueryClient at the application root with environment-aware defaults
• Define query keys as stable, serializable arrays with hierarchical structure
• Configure staleTime and gcTime based on data volatility and memory constraints
• Implement optimistic updates with onMutate snapshots and onError rollbacks
• Replace manual useEffect fetching patterns with useQuery and useMutation
• Add ErrorBoundary wrappers around query-consuming components
• Audit query key stability and remove dynamic/non-serializable values
• Enable React Query Devtools in development for cache visualization

Decision Matrix

Scenario	Recommended Approach	Why	Cost Impact
Static configuration/metadata	Inject via props/context	No caching or invalidation needed; avoids cache pollution	Zero runtime overhead
Real-time dashboard/data	useQuery with staleTime: 10s, refetchInterval: 10s	Balances freshness with network efficiency	Moderate bandwidth, high perceived responsiveness
Form submissions/state changes	useMutation with optimistic updates + invalidateQueries	Eliminates loading spinners while preserving data integrity	Slight complexity increase, major UX improvement
Pagination/infinite scroll	useQuery with keepPreviousData: true, cursor-based keys	Prevents UI flicker, supports back/forward navigation	Minimal bundle impact, improved scroll performance
SSR hydration	hydrate/dehydrate with QueryClient	Prevents double-fetching, matches server-rendered state	Requires build-time coordination, eliminates hydration mismatches

Configuration Template

// src/config/queryClient.ts
import { QueryClient } from '@tanstack/react-query';

export const createQueryClient = () =>
  new QueryClient({
    defaultOptions: {
      queries: {
        staleTime: 1000 * 60,
        gcTime: 1000 * 60 * 10,
        retry: (failureCount, error) => {
          if (error instanceof TypeError) return failureCount < 2; // network errors
          return false; // server errors fail fast
        },
        refetchOnWindowFocus: false,
        refetchOnMount: 'always',
      },
      mutations: {
        retry: 0,
        onError: (error) => {
          console.error('[Query] Mutation failed:', error);
          // Integrate with error tracking (Sentry, LogRocket, etc.)
        },
      },
    },
  });

Quick Start Guide

1. Install dependencies: npm install @tanstack/react-query @tanstack/react-query-devtools
2. Wrap your application root with QueryClientProvider using the template above
3. Replace useEffect + useState fetching blocks with useQuery({ queryKey: [...], queryFn: ... })
4. Add useMutation for data modifications, implementing onMutate/onError/onSettled for cache sync
5. Open React Query Devtools (default: Ctrl/Cmd + Q) to verify cache topology, query status, and invalidation flow