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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