Ref: https://learn.cantrill.io/courses/1820301/lectures/41301424

• Summary:
• Contents:

RDS Read-Replicas (RRs) - Architecture

• 🔧 RDS Read-Replica (RR) = a read-only replica of an RDS instance
  • Can be used for reads (unlike standby replica in Multi-AZ instance)
    • 👍 allow read performance scaling
  • An instance can have up to 5 direct RRs
    • Can be created in the same region or in a different region (cross-region RRs)

• ‼️ RRs are separate from the main RDS architecture!
  • Each RR has its own endpoint address, independent from RDS instance endpoints
    • Requires app support: app needs to be adjusted to use a RR
    • ❗ Apps by default know nothing about RRs
  • No automatic failover
• ❗ Asynchronous replication
  • Data committed when written to main instance. After that, replicated to its RRs.
  • Lag (can be noticeable depending on NW conditions & amount of writes)
    • 💡 RRs can have their own RRs, but lag starts to be even more noticeable!
• Cross-region RRs allow global performance improvement of read workloads
  • Users can read DBs from different regions more efficiently
  • Cross-region NWing handled transparently by AWS (data fully encrypted in transit)
• 💡 Multi-AZ cluster deployment is like a combination of Multi-AZ instance deployment + RRs
  • ‼️ BUT! The 2 Reader Replicas in Multi-AZ cluster deployment are part of the main architecture! External RRs should be considered something separate!
  • For exam: synchronous replication → multi-AZ; asynchronous → RRs (excluding Aurora)

Promotion of RDS Read-Replicas & Disaster Recovery

• 🔧 RRs are read-only until promoted. Upon promotion, they become a normal RDS instance
  • Promotion can be done very quickly → 👍 low RTO
  • 👍 Improves global availability/resilience
    • RR in a different (failover) region can be quickly promoted if main region outage
  • 👍 RRs also offer near 0 RPO
    • data synced constantly from the main DB instance (very little potential for data loss)
  • 💡 RRs are great for quickly recovering from failure, as long as there's no data corruption
• ‼️ Use RRs ONLY when recovering from failure/outage, NOT from data corruption!!
  • 👎 Because data is constantly replicated to RR, data corruption is also replicated to RR!
  • 💡 If data corruption → must rely on snapshots & backups
    • higher frequency and/or higher quality of snapshots/backups improves RPO