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From Core Mnutes March 20, 2007:
ROOT data indexing: (Eric C.) ROOT-based metadata scheme (used by BABAR) containing pointers to locate all known data for a given event. The basic idea involves storing (File, Tree, Entry) in an external table, pointed to by a single pointer in an NTuple (index file), but with some additional flexibility (works with xrootd, including when data has been migrated to tape). Tools to read & copy the event data also provided. See https://confluence.slac.stanford.edu/download/attachments/20011/Event_Collections.pdf. BABAR code is all online in their CVS repository, module KanEvent. This scheme might find application in GLAST for interleave, skimming, and analysis scenarios.
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Use Cases
Level 0 Interleave
Feed the simulation a library of background events it can randomly access in full from the original data files. So would we envision Interleave querying the DB directly during a run to get the next background event?For the interleave we might try to do it like the ntuple interleave, where you set up to read all events within a given bin, on the assumption that several events will be generated from that bin before going to a new bin. But I think there are few enough interleave events that querying the db each time you want one is not so much overhead. I suppose we will see!
Pruner Skimming
A search is performed and N events are found that satisfy the criteria - with run/event ids in hand, the system finds the data files containing the events, pulls them out and ships them off for delivery to the user doing the search.
User Analysis
User Joe has his favorite N events and he wants to keep a run/event list handy to share with his friends. His friends can then use that list to retrieve the full data themselves.right. the db produces a list which the root reading code uses to return the events as you need them.