Here is my summary of architecture differences between Tableau 8 to 9. Pls ref Tableau Server 9.0 High Availability: Delivering Mission-Critical Analytics in the Flow | Tableau Software  for details:

1. HA and fail-over new components introduced in 9.0 : Coordinator Service & Cluster Controller (report process status and coordinate failover for HA and failover).
2. New File Store in Tableau 9 to ensure extracts are available on all nodes of a cluster
3. New Cache Server manages a shared query cache across the server cluster and is used by the VizQL Server, Backgrounder, and Data Server processes
4. New minimum hardware requirements - Tableau 9 will not install if the hardware does not meet the minimum requirements
5. New API server - This process is used when you interact with the server via REST API.
6. 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
7. Gateway can be configured in multiple nodes for better HA.
8. You must have minimum 3-node in the cluster to achieve full HA mode starting Tableau 9

HI Balaji

I wrote some articles last year on this topic. Hope that they are helpful

What's new in Tableau Server 9.0

http://interworks.co.uk/blog/whats-new-in-tableau-server-9-0-for-the-administrator/

http://interworks.co.uk/blog/comparing-performance-tableau-8-3-server-vs-tableau-9-0-server/

These were for 9.0. Since then there have been further changes so they are a little out of date

All the best

Glen