AWS Relational Database Service (RDS): Read Replicas & Multi-AZ Instances

Friday morning. Second cup of coffee. I remember the discussion I had yesterday about the differences of RDS read replicas vs. multi-AZ instances in the Amazon compute cloud.

Actually these concepts are completely different. So it is important to pick the right one that supports your design. Here is a short comparison:

Multi-availability zone instances Read replicas
Purpose Availability Scalability
Number of copies 1 primary + 1 secondary 1 primary + n secondaries
Secondaries used during normal operation? No Yes
Synchronization Synchronous Asynchronous
Use with ELB No (only one active) Not supported
Use with elastic IPs Not supported Not supported
Cost Doubles Per Instance

And don’t forget you can use a combination of both.

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