Not exhaustive, just list a few.
- Scala, Java and Python programming languages.
- Distributed relational databases and its performance caveats.
- Relational query plan generation and materialization.
- RPC based distributed services (stateful & non-stateful) and troubleshooting.
- Distributed messaging queues and data loss models.
- Hadoop frameworks and application development based on it.
- Zookeeper used for service discovery and configuration management.