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AWS Solutions Architect · Lesson

Multi-AZ Patterns for Stateful Services

Apply Multi-AZ to RDS, ElastiCache, EFS, and ELB to eliminate single points of failure within a Region.

Why Stateful Services Need Multi-AZ

Stateful services — databases, caches, file systems — are the hardest components to make highly available because they hold data that must survive failures. If a single-AZ database fails, your entire application loses its data store. AWS's answer is Multi-AZ deployments, where the service maintains a synchronous or near-synchronous replica in a second Availability Zone that can take over rapidly when the primary fails.

RDS Multi-AZ: Synchronous Standby

RDS Multi-AZ maintains a synchronous standby replica in a different AZ. Every write to the primary is synchronously replicated before acknowledging success — this means zero data loss (RPO=0) but a slight write latency increase. When the primary fails, RDS automatically updates the DNS endpoint to point to the standby in 60-120 seconds. Your application only needs to reconnect to the same endpoint — no code changes required.

# Enable Multi-AZ on existing RDS instance
aws rds modify-db-instance \
  --db-instance-identifier mydb \
  --multi-az \
  --apply-immediately

# RDS endpoint stays the same after failover
# Application reconnects to same DNS name

All lessons in this course

  1. HA vs Fault Tolerance: Definitions and Trade-offs
  2. Multi-AZ Patterns for Stateful Services
  3. Multi-Region Active-Active and Active-Passive
  4. Health Checks, Circuit Breakers, and Retry Logic
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