ACID in practice, read committed, snapshot isolation, serializability, write skew, 2PC, saga pattern, and exactly-once processing.
Advanced
Raft, Paxos, ZooKeeper, etcd. Leader election, FLP impossibility, linearizability vs. serializability, fencing tokens, and distributed locks.
Expert
MapReduce, Spark, Kafka internals, event sourcing, CQRS, change data capture, stream joins, windowing, and exactly-once semantics.
Advanced