Deduplicate per-request server work with cache

Wrap server data functions that several components call during one request in React's cache, so each distinct call runs once per request.

Implementation

• Define the cached function at module level, and have every component call that same function.
• Pass primitive arguments, or the same object reference; cache compares arguments with Object.is, so a new object literal is always a miss.
• cache works only in Server Components and lasts for one server request; it does not share results across requests or users.
• In Next.js, fetch requests with the same URL and options are already memoized during rendering, so cache is for non-fetch work such as database queries and authentication checks.

Rationale

In a component tree, several components often need the same data, such as the current user. Passing it down through props couples them, but calling the query in each one repeats it. cache lets each component call the function independently while the query runs once per request.

Examples

Incorrect (counterexample):

export const getUser = cache(async (params: { id: string }) => {
  return db.user.findUnique({ where: { id: params.id } });
});

await getUser({ id: userId });
await getUser({ id: userId }); // new object: runs the query again

Correct:

export const getUser = cache(async (id: string) => {
  return db.user.findUnique({ where: { id } });
});

await getUser(userId);
await getUser(userId); // same argument: returns the cached result

Validation

Log or count the underlying queries while rendering one page, and check that each distinct query runs once. Check that cached functions take primitive arguments.

A query called once per request does not need cache.