In a distributed system, you can only guarantee two of three:
| Use Case | Recommendation | Reason |
|---|---|---|
| Complex relationships, ACID | PostgreSQL | Relational integrity, transactions |
| Flexible schema, scale-out | MongoDB | Document model, horizontal sharding |
| High-speed caching | Redis | In-memory, sub-millisecond latency |
| Time-series data | InfluxDB / TimescaleDB | Optimized for append, range queries |
| Graph relationships | Neo4j | Native graph traversal |