Database questions are data-contract questions: which queries must be fast, which transactions must be atomic, how large can data grow, and what consistency can clients observe?
Relational availability is not read scaling
RDS Multi-AZ maintains a synchronous standby for failover. It is not normally a read target. Read replicas scale reads asynchronously and can promote, but are not the same automatic-HA contract. Aurora separates distributed storage from compute, exposes writer and reader endpoints, and supports replicas, Serverless, and Global Database patterns.
RDS Proxy pools connections and protects a relational database from bursts such as Lambda concurrency. It does not cache query results.
DynamoDB starts with access patterns
DynamoDB provides key-value/document access with partition and sort keys. On-demand capacity fits uncertain traffic; provisioned capacity fits predictable traffic with active management. GSI adds a new partition-key access pattern and has separate capacity implications; LSI shares the base partition key and must be defined at table creation.
Global Tables provide multi-Region active-active replication. DAX caches DynamoDB reads. DynamoDB Streams records item changes for event-driven processing.
Cache and purpose-built stores
ElastiCache Redis supports richer data structures, replication, persistence options, and sorted sets; Memcached is simpler distributed caching. A cache improves latency and load but creates invalidation and staleness decisions.
Use DocumentDB for MongoDB-compatible document workloads, Neptune for graph relationships, Keyspaces for Cassandra-compatible wide columns, and Timestream for time-series data. “Compatible” does not mean identical to the upstream database.
Exam lens: pick the data model first. Then add availability, read scaling, caching, and global replication as separate requirements.
Review card
- Multi-AZ is HA; read replica is read scaling.
- RDS Proxy pools connections; ElastiCache caches data.
- DynamoDB design begins with partition keys and access patterns.
- DAX is specifically a DynamoDB cache.
- Purpose-built databases win when the query model is genuinely specialized.