A couple of months back a customer asked why we are proposing a three Availability Zone (AZ in short) architecture instead of two. Their main point was which failure modes 3 AZs guard against that 2 AZs can’t do. We gave the following two reasons:
- We proposed 3 AZs for improved availability. Also, since services and instances will be deployed across 3 AZs then if one AZ goes down then with 3 AZs you lose 1/3 capacity. With two AZs you can lose half the capacity.
- If there are services(like you want run your own Cassandra or something) where we need to manage quorum it is better to have three
They were not very convinced so we agreed to start with the two AZs solution.
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