AWS charges for internal traffic too, both cross-az and cross-region.

The inter-AZ charges kill me. They’re charging you for traffic over what is almost certainly private fiber, of a distance no more than 50 miles or so. Just absurd.

Then, they have the audacity to recommend you make everything Multi-AZ. “Don’t worry, RDS-RDS traffic isn’t billed!” Yes, but the apps running in us-east-1a that are hitting -1b and -1c due to load balancers most assuredly do bill for traffic.

Not billing for S3 and RDS replication traffic is also an unfair subsidy for those services.

Isn't that baked into the storage costs and put/get operations?

What’s extra special is that the cloud providers are now disincentivised from making those load balancers zone topology aware. If they made them efficiently pick the same zone whenever possible, they would be missing out on cross-AZ traffic billing, which is basically a money printer.

AWS ELB certainly is zone aware, iirc cross-zone load-balancing is opt-in

Azure load balancers certainly aren’t.

I guess that’s just one more reason to prefer AWS over Azure.

BTW, if nothing changed the last year or two, then even if you want to run in a single AZ, but also to have (automatically) replicated RDS instance, then I believe the replica MUST be in another AZ. Meaning that any potential failover puts you in exactly that position you have mentioned - apps running in one AZ, database in the other. Suddenly you pay 0.01USD/GB for transfer between those, billed in each direction separately.

Correct. The same thing also happens if you use the Blue/Green functionality to do an upgrade – unless you specify (and even then, I'm not sure if you can do so), the Blue instance will be in a different AZ than Green.