Platform & DevOps·What the platform costs
spreading replicas across zones for resilience quietly added a per-gigabyte charge to every internal call between them.
Cross-zone traffic cost
Also calledinter-AZ transfer, cross-AZ charges
The charge for data moving between availability zones inside a region, applied in both directions on many providers. It is invisible in architecture diagrams and material for chatty services, replicated databases and meshes that route without regard to topology. Zone-aware routing — preferring a local endpoint when one is healthy — is the standard mitigation, and it trades some balance for a real saving.