apache/spark — pull request #718 by vanzin (2014-05-09 21:42:14+00:00)
Title: [SPARK-1768] History server enhancements.

Two improvements to the history server:

- Separate the HTTP handling from history fetching, so that it's easy to add
new backends later (thinking about SPARK-1537 in the long run)

- Avoid loading all UIs in memory. Do lazy loading instead, keeping a few in
memory for faster access. This allows the app limit to go away, since holding
just the listing in memory shouldn't be too expensive unless the user has millions
of completed apps in the history (at which point I'd expect other issues to arise
aside from history server memory usage, such as FileSystem.listStatus()
starting to become ridiculously expensive).

I also fixed a few minor things along the way which aren't really worth mentioning.
I also removed the app's log path from the UI since that information may not even
exist depending on which backend is used (even though there is only one now).