Design a cloud database write path and recovery

## System Design (Engine-level): Write Path + Crash Recovery Design a core subsystem for a cloud-native relational database (Aurora-like) where **compute is separated from durable distributed storage**. ### Goal Support transactional writes with: - high throughput - low commit latency - crash recovery - strong durability guarantees (clearly specify what guarantees) ### Requirements / prompts 1. **Write path**: Describe how an UPDATE/INSERT flows from compute to durable storage. Where do you place the log (WAL)? 2. **Commit protocol**: When does a transaction commit succeed? What acknowledgements are required? 3. **Replication & consistency**: How many replicas, what quorum rules, and how do you handle network partitions? 4. **Crash recovery**: If the compute node crashes, how does a new node recover state and resume service? What data structures/checkpoints exist? 5. **Write amplification**: Identify sources (WAL, page rewrites, compaction) and propose reductions. 6. **Scalability**: How do you scale storage and compute independently? Discuss sharding, rebalancing, and hotspot handling. 7. **Observability**: What metrics and logs would you add to detect replication lag, redo backlog, and tail latency?