How to store run information

[tmadlener]

This came up in the context of converting LEP data and was discussed during the EDM4hep meeting on Nov 19. This should serve as a discussion thread / issue to collect relevant data and information.

From the meeting notes:

• E.g. Conversion from LEP data (run energy)
  • Run frame parameters? Dedicated datatype (RunHeader)?
• Want to access run data for each event
  • Write them separately from the events
• Lookup runs via the runNumber from the EventHeader
  • Cache Run
• Gaudi has EventContext for storing run number, lumi block, etc.
  • Has to be filled by some service at beginning of event loop, algorithms only get const EventContext&
  • Created by EventSelector in Gaudi
• Use LEP data as starting point and define contents from there

A dedicated edm4hep::RunHeader datatype seems to make sense. Since the edm4hep::EventHeader already stores a runNumber it should be possible to retrieve the appropriate RunHeader via that (plus some caching).

EDM4hep/edm4hep.yaml

Lines 242 to 247 in fe5a540

	edm4hep::EventHeader:
	Description: "Event Header. Additional parameters are assumed to go into the metadata tree."
	Author: "EDM4hep authors"
	Members:
	- int32_t eventNumber // event number
	- int32_t runNumber // run number

The details need to be sorted out and most likely will also require some work in k4FWCore.