RDS Read Replicas vs Multi AZ¶

RDS Read Replicas for read scalability¶

• Up to 5 read replicas
• Within AZ, Cross AZ or Cross Region
• Replication is ASYNC so reads are eventually consistent
• Replicas can be promoted to their own DB
• Application must update the connection string to leverage read replicas

RDS Read Replicas - Use Cases¶

• You have a production database that is taking on normal load
• You want to run a reporting application to run some analytics
• You create a read replica to run the new workload there
• The production application is unaffected
• Read replicas are used for SELECT (read) only kind of statements (not INSERT, UPDATE, DELETE)

RDS Read Replicas - network cost¶

• In AWS there's a network cost when data goes from one AZ to another
• For RDS Read Replicas within the same region, you don't pay that fee.

So, if you have an RDS instance in us-east-1a and a read replica in us-east-1b - the replication will be free.

If you have an RDS instance in us-east-1a and you make cross-region replication to eu-west-1b, it will cost you the network fee.

RDS Multi AZ (Disaster Recovery)¶

• SYNC replication
• One DNS name - automatic app failover to standby
• Increase availability
• Failover in case of loss of AZ, loss of network, instance or storage failure
• No manual intervention in apps

• Not used for scaling

• The read replicas can be setup as Multi AZ for Disaster Recovery (DR).

RDS - From Single-AZ to Multi-AZ¶

• Zero downtime operation (no need to stop the DB)
• Just click on "modify" for the database
• The following happens internally:
  • A snapshot is taken
  • A new DB is restored from the snapshot in a new AZ
  • Synchronization is established between the two databases