Columnar table mutations
Design how a columnar lakehouse table format (think Delta/Iceberg-class, immutable column files on object storage) supports row-level UPDATEs and DELETEs and GDPR 'delete this user' without rewriting whole multi-GB files on every change. Constraint: column files are immutable on the object store, queries scan them directly, and you need ACID semantics (a reader sees a consistent table version) plus reasonable read performance after many small deletes. Design the mutation mechanism, how reads reconcile it, and the compaction that keeps reads fast.
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.