Meeting Room Scheduler

Model room reservations as half-open intervals of explicit instants. Enforce overlap prevention in the durable repository and make creation, rescheduling and cancellation idempotent or version guarded.

1. Validate start < end, attendees and required equipment. Convert unambiguous timezone inputs to instants; this exercise handles one-time reservations, not recurrence expansion.
2. AvailabilityPolicy filters eligible rooms. A shared transactional repository serializes per-room booking or uses an overlap exclusion constraint before inserting an active reservation.
3. A reschedule checks the expected reservation version and validates the replacement interval in one transaction. On conflict or stale version it leaves the prior reservation intact.
4. Cancellation marks a reservation inactive once and increments its version. Idempotency keys return matching previous create results and reject different payloads with the same key.

• Adjacent intervals succeed while genuine overlaps conflict.
• Concurrent overlapping requests produce one active booking.
• A failed reschedule preserves the original interval and version.
• Stale edits, duplicate keys and ambiguous local-time inputs produce explicit errors.

Trade-offs to explain

• Serializing per-room mutations is simple and sufficient until a room becomes a hot resource.
• UTC instants simplify overlap checks but display and future recurrence still require zone-aware policies.
• Availability search is advisory; final reservation must always repeat the atomic conflict check.