Putting It All Together

A full LSM-tree KV with transactions:

What we glossed over:

• Replication: leader-follower, Raft consensus, Paxos.
• Sharding: partition keys across nodes.
• Distributed transactions: 2-phase commit, percolator, FoundationDB's transaction protocol.
• Snapshots / backups: consistent points for restore.
• Encryption: at-rest and in-transit.
• Online schema changes: DDL operations without downtime.
• Time-series optimizations: time as part of the key; older data compacted differently.

Real systems you should look at:

• LevelDB: Google, ~50k LoC. The reference LSM-tree implementation.
• RocksDB: Facebook fork; many production tweaks.
• Cassandra: distributed LSM with size-tiered compaction.
• etcd: BoltDB-backed, MVCC, strongly consistent, used by Kubernetes.
• FoundationDB: distributed ACID, stunning architecture (Apple uses it everywhere).
• TiKV: distributed RocksDB + Raft.
• CockroachDB: distributed SQL on top of TiKV-like storage.

You've now built a meaningful subset of all of these.