NTuple service example

[tmadlener]

We have some examples here that use the histogram service. Do we have similar examples (or can we come up with examples) that let us produce ntuples (outside of EDM4hep)? Would make some monitoring tasks a bit easier potentially. At the moment one would have to create a podio::UserDataCollection for every branch that we want to have in the ntuple which again requires changing the algorithm type in some cases.

[Zehvogel]

Hi @tmadlener, I came up with an example here: https://github.com/Zehvogel/k4Performance/blob/main/PFlowValidation/components/PFONTupleWriter.cpp
There is much room for improvement, though. Making it thread-safe would only need the addition of one lock, as here, I think.

In general, this was inspired by the NTupleWriter that was recently added to Gaudi. I just did not manage to use it directly with the extra handling that we do around all the EDM4hep stuff.

[Zehvogel]

#364

[tmadlener]

Checking #364 in a bit more details, I don't think it covers exactly the same use case as an NTuple service would be targeting. In my mind the NTuple service would mainly be used for creating some monitoring outputs that potentially all go into the same root file or even TTree (or RNTuple). This is useful if one is interested in exact correlations between some variables which will invariably be lost if put into multidimensional histograms (plus those tend to end at some rather finite number of dimensions).

On the other hand the algorithm that is introduced in #364 (at least for me) would form the basis of generating some form of analysis level ntuples that simply extract a more minimal version of the datamodel.