- Up to 15 Read Replicas.
- Can be deployed within the same AZ, across AZs, or cross-region.
- Asynchronous replication → reads are eventually consistent.
- Replicas can be promoted to standalone DBs.
- Applications must update connection strings to query replicas.
- Reads only (SELECT) — not suitable for writes (INSERT/UPDATE/DELETE).
Architecture
- Main RDS instance handles all writes.
- Read Replicas handle read queries to offload the primary DB.
- Replication happens asynchronously in the background.
Use Cases
- Scaling read-heavy applications.
- Running analytics/reporting without impacting production DB performance.
- Geographic data distribution.
Network Cost
- Same region (even across AZs) → Free replication.
- Cross-region → Network charges apply.