# TanStack Query: Cache Management

> Directly read, write, and remove cache entries with QueryClient methods

## When to Use

- Updating related cache entries after a mutation without an extra network request
- Pre-populating the cache with data received from a WebSocket or server-sent event
- Removing cached data when it is known to be invalid (e.g., after logout)
- Inspecting cached values in tests to assert cache state after operations
- Implementing advanced patterns like cache federation or cross-query updates

## Instructions

1. Use `queryClient.setQueryData(key, updaterOrValue)` to write a value directly to the cache — the query does not refetch.
2. Use `queryClient.getQueryData(key)` to read a cached value synchronously — returns `undefined` if not cached.
3. Use `queryClient.cancelQueries({ queryKey })` before manual cache writes to prevent in-flight fetches from overwriting your change.
4. Use `queryClient.removeQueries({ queryKey })` to evict cache entries entirely — they refetch fresh on next mount.
5. Use `queryClient.clear()` to clear the entire cache — call this on logout to remove all user data.
6. Use `queryClient.getQueriesData(filter)` to bulk-read multiple cache entries matching a filter.
7. Use updater functions in `setQueryData` for immutable updates — receive current value, return new value.

```typescript
// Real-time cache update from WebSocket
function useLivePostUpdates() {
  const queryClient = useQueryClient();

  useEffect(() => {
    const ws = new WebSocket('/ws/posts');

    ws.onmessage = (event) => {
      const updatedPost: Post = JSON.parse(event.data);

      // Update the specific post detail
      queryClient.setQueryData(postKeys.detail(updatedPost.id), updatedPost);

      // Update the post in all list caches
      queryClient.setQueriesData<PostsPage>({ queryKey: postKeys.lists() }, (old) => {
        if (!old) return old;
        return {
          ...old,
          posts: old.posts.map((p) => (p.id === updatedPost.id ? updatedPost : p)),
        };
      });
    };

    return () => ws.close();
  }, [queryClient]);
}

// Logout — clear all user data from cache
function useLogout() {
  const queryClient = useQueryClient();
  const router = useRouter();

  return async () => {
    await fetch('/api/auth/logout', { method: 'POST' });
    queryClient.clear(); // remove all cached data
    router.push('/login');
  };
}

// Seeding cache after create mutation — avoid refetch
const { mutate: createPost } = useMutation({
  mutationFn: (data: CreatePostInput) =>
    fetch('/api/posts', { method: 'POST', body: JSON.stringify(data) }).then((r) => r.json()),
  onSuccess: (newPost: Post) => {
    // Add to detail cache directly
    queryClient.setQueryData(postKeys.detail(newPost.id), newPost);
    // Invalidate lists — server determines list membership
    queryClient.invalidateQueries({ queryKey: postKeys.lists() });
  },
});
```

## Details

TanStack Query's cache is a `Map`-like structure keyed by serialized query keys. `setQueryData`, `getQueryData`, and `removeQueries` give direct programmatic access to this cache outside the normal fetch lifecycle.

**`setQueryData` vs `invalidateQueries`:** `setQueryData` writes a value and marks the query as `fresh` (it will not refetch until `staleTime` expires). `invalidateQueries` marks the query as `stale` and triggers a background refetch. Use `setQueryData` when you have the correct server response in hand (e.g., from a create mutation); use `invalidateQueries` when you know data has changed but do not have the new value.

**`setQueriesData` for list updates:** When a single entity update needs to reflect in multiple list queries (e.g., filtered views), use `setQueriesData(filter, updater)` to update all matching caches in one call. The filter uses the same matching logic as `invalidateQueries`.

**Observer pattern:** Use `queryClient.getQueryCache().subscribe(callback)` to observe all cache changes. This is useful for syncing query state to external systems (analytics, debug logging). Unsubscribe on cleanup.

**Type safety:** `getQueryData<T>(key)` returns `T | undefined`. Always handle the `undefined` case — the cache may not have the value if it was never fetched or was garbage collected.

**Testing assertions:** In tests, call `queryClient.getQueryData(key)` after mutations to assert that the cache was updated correctly — without needing to render a component and inspect the DOM.

## Source

https://tanstack.com/query/latest/docs/reference/QueryClient

## Process

1. Read the instructions and examples in this document.
2. Apply the patterns to your implementation, adapting to your specific context.
3. Verify your implementation against the details and edge cases listed above.

## Harness Integration

- **Type:** knowledge — this skill is a reference document, not a procedural workflow.
- **No tools or state** — consumed as context by other skills and agents.

## Success Criteria

- The patterns described in this document are applied correctly in the implementation.
- Edge cases and anti-patterns listed in this document are avoided.
