It's actually cheaper than data transfer across AZs. Within a region, AWS charges 1 cent per GB in each direction. You pay a cent to send data from one instance and another cent to receive data on another instance. But between Virginia and Ohio, they charge 1 cent to send data (in either direction) but receiving data is free. In the end, it's actually cheaper to balance your servers across one AZ in Virginia and one AZ in Ohio, rather than 2 AZs in the same region.
But it's only a penny, right? How much could it matter? It actually matters a lot. At both my previous and current employers, a surprisingly large portion of our hill comes from data transfer across AZs. All the microservices, all the memcache hits, and all the database replication add up quickly.
But it's only a penny, right? How much could it matter? It actually matters a lot. At both my previous and current employers, a surprisingly large portion of our hill comes from data transfer across AZs. All the microservices, all the memcache hits, and all the database replication add up quickly.