Quick reply below.
1.) Do both SCI nodes (master and data) have web GUI?
=> No, only the master node has a GUI. Data node is a headless node to parallelise the load (though the data node has a simple GUI for setup).
2.) Do we have to log into both SCI nodes to query data?
=> No. You only have to access the master node.
3.) How do controller select SCI node to send data?
=> Job distribution between nodes is handled automatically by SCI, so it's transparent to the controller.
4.) Do these two nodes have replicated data or unique data?
=> They will have replicated data for redundancy. Replication factor is 3 for SCI 3.1.
5.) Once AP is sending data to either of SCI nodes, is it fixed for all the time?
=> AP does not send data directly to SCI. Data is aggregated at the controller, and SCI will retrieve the data from the controller (for ZD and SZ below 3.4.1), or controller will push the data to SCI (SZ 3.5 and above).
6.) Can we have master and data nodes in two geographical locations?
=> Yes, as long as there are no issues with network connectivity. Please refer to the Installation Guide for full list of ports that have to be opened.
7.) Do we have to configure both SCI nodes in controller?
=> No, don't have to.