Design a library management system shared by multiple library branches. It must support catalog search, branch-level availability, reservations, checkout, and return. A bibliographic title may have many physical copies across branches, but the same copy must never have overlapping loans. ### Constraints & Assumptions - Separate title-level metadata such as ISBN from the identity and state of each physical copy. - A return should make the copy available and complete the loan even if a downstream notification is temporarily unavailable. - Search must support title lookup and finding branches that currently have a requested title. - Reservation policy, loan duration, and transfer between branches should be clarified rather than invented. ### Clarifying Questions to Ask - Is a reservation for any copy of a title at one branch, or for one specific physical copy? - Can a copy be transferred between branches while reserved or checked out? - Must checkout and return work while a branch is temporarily disconnected? - What consistency is required between the search index and transactional copy availability? ```hint Put contention on the physical copy Two patrons can request the same title concurrently, but only a conditional state transition on one copy can create a valid loan. ``` ### What a Strong Answer Covers - Catalog, copy, branch, patron, reservation, and loan models with explicit identifiers and state transitions. - Search and availability indexes, including their consistency relationship to the transactional source of truth. - Concurrency-safe reservation, checkout, and return operations that prevent overlapping loans. - Transactional outbox notifications, idempotency, failure recovery, indexing, partitioning, and auditability. ### Follow-up Questions - How would you prevent two branches from checking out the same transferred copy during a race? - What happens when a return commits but the hold-notification service is down? - How would you answer “which nearby branches have this ISBN now?” without trusting a stale search document?