Decide how stale a cache may be

How to do it

Prefer short expiry times for data that changes, and delete the cache entry right after the write commits, in the same code path. Keep the expiry even then, so a missed delete heals itself. Never cache permissions, prices at checkout or anything that must be exact at the moment of use. Include the user or tenant in the cache key for private data.

Example

await db.none('UPDATE products SET price_cents = $1 WHERE id = $2', [price, id]);
await cache.del(`product:${id}`);

cursor.execute("UPDATE products SET price_cents = %s WHERE id = %s", (price, product_id))
cache.delete(f"product:{product_id}")

How to check

Change a cached value through the app. The next read shows the new value, or the stale one only within its stated expiry.