Jonathan Eagles, thanks for your feedback, which is useful suggestion. If you have an more detailed design doc, please share with the community.
As JHS has lived for a long time, we should rebase it on the timeline server gradually. IMHO, the first step is to make the MR job not only write the history events to the history files on HDFS, but also publish them to the timeline server, and it is configurable whether the MR job should write history files or upload timeline data.
Once this is done, we can make JHS have the option to source the timeline server for MR jobs' history information, while we preserve the web front. Again it is the configurable to choose the data source.
Once the data layer is stable, we can further think about migrating the UI. We can either build web UI on the third-party tool (such as Ambari) or even create web UI plugin in the timeline server. Any thoughts?