Within the current deployment configuration of A2HA, the minimum number of nodes needed for an A2HA cluster is 9 systems:
bastion/management nodes: 1
Automate Front-End Nodes: 1
Chef Infra Server Front-End Nodes: 1
ElasticSearch Nodes: 3
PostgreSQL Nodes: 3
The minimum node count requirements can be quite a leap from existing Chef Infra Server ha/backend/tiered deployments and A2HA is intended to be the ultimate replacement for these solutions. It would be greatly helpful to reduce the minimum number of nodes for smaller deployments or distributed environments where multiple Chef Infra Server clusters might be required (such as regional deployments or DMZ deployments).
As an example solution to this, A2HA could allow for colocation of services on the same node, that could reduce the minimum number of nodes from 9 to 4 ( 1 management node and 3 clustered nodes which run services)