Each question links back to the mental model and a practice where you can use it.
Start from the measured read path, freshness requirement, and failure budget. Then choose the cache location and invalidation strategy that make those constraints explicit.
A good key spreads writes, supports dominant access patterns, avoids hot tenants, and leaves a credible migration and rebalancing path.
Bound retries by an end-to-end deadline, use backoff and jitter, retry only safe or idempotent work, and shed load before retries amplify an outage.
Name the invariant and the user-visible cost of stale data. Pay coordination cost only where violating that invariant creates unacceptable harm.