MSA is a decision about which criterion to use to decompose the system. Domain boundary, data ownership, scale pattern, failure isolation — the chosen criterion creates the service boundaries, and those boundaries decide communication and data in turn.
ES and CQRS address how a system’s source of truth is shaped and how its views are separated from it. Adoption cost spreads across the system, so I lean toward adopting only when the value can be stated explicitly.
Distributed transactions are about how a single ACID transaction decomposes across services and how its pieces are reassembled. The roles and trade-offs of 2PC, Saga (Choreography vs Orchestration), and Outbox.
Core Kafka concepts (topics, partitions, consumer groups, replication) and the background behind KRaft mode, which removes the ZooKeeper dependency.