Scheduling/booking is tricky. Postgres exclusion constraints prevent overlapping time ranges at the database layer—far more reliable than application checks. Rails can still validate, but the DB is the source of truth.

With replicas, a user can create a record then immediately not see it due to replication lag. A simple mitigation is to pin subsequent reads to primary for a short window after writes.

Retries can become infinite noise. For certain failure classes, record the payload + error in a DLQ table and alert/triage. This keeps queues healthy and makes failures actionable.

Presence is often more about “who is here now” than perfect accuracy. Use Redis sets with expiries updated by pings. This keeps the system simple and operationally understandable.

Avoid loading whole records when you only need to know if something exists. exists? is good; for complex joins, a scoped select(1).limit(1) can be clearer and keeps the DB workload low.

Sometimes you need “only one runner globally” (backfills, refresh jobs). A Redis mutex with TTL avoids deadlocks if the process dies. It’s not perfect, but it’s a solid pragmatic tool.

Production debugging is about logs that are searchable. Emit JSON with consistent keys (requestid, memberid, duration). Even if you later add Lograge, this structure maps cleanly.

When controllers become parameter jungles, use a form object. It centralizes coercion, validations, and save logic. This pattern is extremely effective for admin panels and multi-step flows.

Inventory systems are concurrency systems. Lock the row, verify available quantity, then write the reservation in the same transaction. It’s the simplest correct starting point.

Webhooks are a security boundary. Verify signatures with constant-time compare, include a timestamp window to prevent replay, and store processed event IDs to make handlers idempotent.

When cache structures change, you want to invalidate safely without flushing the world. Use a namespace version key (per feature) and incorporate it into cache keys.

Callback chains become spooky action at a distance. A simple in-app event bus keeps side effects explicit and testable. This isn’t about Kafka—it’s about clarity and seams.