Recently there was a question asking architecture differences between Tableau 8 to 9 (not features but Tableau Server architecture difference). I thought that this was a good question although Tableau 9 was released one year ago (see my post for all Tableau historic releases @ | Tableau Public). I put my answers to that new user and he is very happy about the summary. Hope this will help others as well. Here is my summary of architecture differences between Tableau 8 to 9.
HA and fail-over new components introduced in Tableau 9 : Coordinator Service (manages leader election and ensures that there is a quorum for making decisions during failover.) & Cluster Controller (report process status and coordinate failover for HA and failover).
New File Store in Tableau 9 to ensure extracts are available on all nodes of a cluster
New Cache Server manages a shared query cache across the server cluster and is used by the VizQL Server, Backgrounder, and Data Server processes
New minimum hardware requirements – Tableau 9 will not install if the hardware does not meet the minimum requirements
New API server – This process is used when you interact with the server via REST API.
Data Engine is no longer limited to running only two data engine nodes per cluster. This new flexibility can improve server clusters that are used for extract-heavy scenarios
Gateway can be configured in multiple nodes for better HA.
You must have minimum 3-node in the cluster to achieve full HA mode starting Tableau 9