RDS: Multi-AZ, Read Replicas & Scaling

Multi-AZ Deployment

Multi-AZ maintains a synchronous standby replica in a different Availability Zone. Failover happens automatically in 60–120 seconds when AWS detects an issue. The standby is not readable — it exists only for HA.

• Synchronous replication: every write is confirmed on standby before ACK to application

• Automatic failover: DNS endpoint switches to standby — application just reconnects

• Failover triggers: primary AZ outage, primary instance failure, manual failover, OS patching

• RTO: typically 60–120 seconds; RPO: 0 (synchronous replication)

• Multi-AZ does NOT improve read performance — use Read Replicas for that

# Enable Multi-AZ on existing instance
aws rds modify-db-instance \
  --db-instance-identifier my-postgres \
  --multi-az \
  --apply-immediately   # or --no-apply-immediately for next maintenance window

# Manually trigger failover (for testing)
aws rds reboot-db-instance \
  --db-instance-identifier my-postgres \
  --force-failover

Multi-AZ DB Cluster (newer option)

Multi-AZ DB Cluster uses 3 DB instances across 3 AZs with semi-synchronous replication. One writer, two readers. Readers ARE accessible for reads. Faster failover than classic Multi-AZ (~35 seconds). Available for PostgreSQL and MySQL.

Read Replicas

Read replicas use asynchronous replication. They are readable, reducing load on the primary. You can have up to 15 replicas (PostgreSQL). Promote a replica to a standalone instance in disaster scenarios.

# Create read replica (same region)
aws rds create-db-instance-read-replica \
  --db-instance-identifier my-postgres-replica-1 \
  --source-db-instance-identifier my-postgres \
  --db-instance-class db.t3.medium

# Cross-region read replica
aws rds create-db-instance-read-replica \
  --db-instance-identifier my-postgres-us \
  --source-db-instance-identifier arn:aws:rds:eu-west-1:123456789012:db:my-postgres \
  --db-instance-class db.t3.medium \
  --region us-east-1

# Promote replica to standalone (for disaster recovery or migration)
aws rds promote-read-replica \
  --db-instance-identifier my-postgres-replica-1

# Monitor replication lag
aws cloudwatch get-metric-statistics \
  --namespace AWS/RDS \
  --metric-name ReplicaLag \
  --dimensions Name=DBInstanceIdentifier,Value=my-postgres-replica-1 \
  --start-time 2024-01-01T00:00:00Z \
  --end-time 2024-01-01T01:00:00Z \
  --period 60 \
  --statistics Average

RDS Proxy

RDS Proxy sits between your application and RDS. It pools and reuses database connections, dramatically reducing connection overhead for Lambda functions and high-concurrency apps. Supports IAM auth and Secrets Manager.

• Reduces connection overhead: Lambda can open thousands of concurrent "connections" — Proxy multiplexes them

• Faster failover: Proxy maintains warm connections to standby, reduces failover impact on app

• IAM database authentication: authenticate with IAM token instead of password

• Automatic secrets rotation: works with Secrets Manager, no app restarts needed

# Create RDS Proxy
aws rds create-db-proxy \
  --db-proxy-name my-postgres-proxy \
  --engine-family POSTGRESQL \
  --auth '[{"AuthScheme":"SECRETS","SecretArn":"arn:aws:secretsmanager:eu-west-1:123:secret:rds-creds","IAMAuth":"REQUIRED"}]' \
  --role-arn arn:aws:iam::123456789012:role/rds-proxy-role \
  --vpc-subnet-ids subnet-abc subnet-def \
  --vpc-security-group-ids sg-proxy

# Application uses proxy endpoint instead of RDS endpoint
# proxy-endpoint: my-postgres-proxy.proxy-abc.eu-west-1.rds.amazonaws.com

Vertical Scaling

# Change instance class (requires brief downtime, use maintenance window)
aws rds modify-db-instance \
  --db-instance-identifier my-postgres \
  --db-instance-class db.m6g.large \
  --apply-immediately   # or schedule for maintenance window

Blue/Green Deployments

Blue/Green creates a staging environment (Green) with your changes (new engine version, parameter changes). Traffic switches instantly. Rollback is just switching back. Zero-downtime upgrades for minor versions; brief cutover for major.

# Create Blue/Green deployment
aws rds create-blue-green-deployment \
  --blue-green-deployment-name my-postgres-upgrade \
  --source arn:aws:rds:eu-west-1:123456789012:db:my-postgres \
  --target-engine-version 16.1

# After testing, switch over
aws rds switchover-blue-green-deployment \
  --blue-green-deployment-identifier bgd-abc123