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Data architecture: where we stand, what we think

Definition

Data product: a class representing data, that is stored in a ROOT tree

Charge1

  • periodically review the architecture of selected components and subsystems
  • also consider issues raised ad hoc
  • draft findings and recommendations for changes and improvements to the architecture
  • ensure that solutions will work for all the experiments

Design goals

  • LArSoft should offer data products that can be read by other frameworks with no change in their source code

Design principles

  • each data product must represent to specific concept2
    (different concepts want different data products)
  • data must be readable with bare C++ plus ROOT to allow different code to use it2
  • data products should be fully expressed by pure C++ means

Questions

Q. Are dictionary libraries independent of art etc.?
A. Currently they are not.
It might be possible to fix that for some of the products which do not explicitly require art.


Dependencies

  • C++11 is required to fully compile the data product classes
  • header files must be parsable with ROOT GenReflex2
    (C++11-specific headers and constructs need to be enclosed in #ifndef __GCCXML__ blocks)
  • data products can depend on ROOT2
  • implementation of classes is allowed to use MessageFacility2

Question

Is CET library (cetlib) allowed in the implementation too2?
  • is contains the popular cet::exception
  • some other part of the code might have to use cet search paths anyway
  • it requires Fermilab's cpp0x library (from LArSoft point of view, for no reason since we do use C++11)
  • it requires Boost libraries

The only part positively needed is an exception that can be recognized by the frameworks (it could be derived from std::exception).

Technical guidelines

Data products can contain data members:
  • plain data
  • STL containers from standard C++03
  • ROOT objects
  • custom structures based on the above
They must not contain:
  • C++11 structures2 (e.g. no std::array)
    (ROOT GenReflex support would be very problematic)
  • CLHEP objects2
    (typically its data structures have an equivalent in ROOT)
  • art objects2
    (for framework independence)

The data members will be /private/, accessed through the class interface2.
The interface should offer a read-only access2.
The object will not operate on its data, leaving the task to additional layers and algorithms2.


Recommended data structures

Suggestions:

3-vectors (position...) TVector3 (unless there are data size issues) or double[3]? (both are currently used)
3-vectors (momentum...) TVector3 or float[3] (as above, except that we don't have, nor need, more than 6 digits precision)
4-vectors TLorentzVector (unless there are data size issues)
error/correlation matrices TMatrixTSym<> or float[6]? (both TMatrixD and double[6] are currently used)
wire/time pair short int[2], float[2]
...

Semantics

Each data product represents a physics quantity or some concept.
First step: identify and define those concepts for the existing and needed objects.

In the standard MicroBooNE simulation + reconstruction chain, 34 classes from LArSoft (lardata) are used as data products, plus 4 more for simulation from nutools.

Let's start from the largest chunk of correlate objects: reconstruction (base)
  • live in recob namespace
  • connected to simulation (sim namespace) and raw data (raw namespace)
This encompasses 13 LArSoft classes:
  • raw::RawDigit
  • recob::Cluster
  • recob::EndPoint2D
  • recob::Hit
  • recob::PFParticle
  • recob::Shower
  • recob::SpacePoint
  • recob::Track
  • recob::Vertex
  • recob::Wire
  • anab::Calorimetry
  • anab::CosmicTag
  • anab::ParticleID


"(version with modules creating each associations)":recoblock_assns.svg

raw::RawDigit

The sequence of (uncalibrated?) ADC counts measured on one channel in the full time window.

  • can be compressed with one of a few standard algorithms

recob::Wire

The signal deposited on a single channel as function of data acquisition time (TDC).

The signal may have been be zero-suppressed, filtered from noise and deconvoluted from electronics effects.
  • includes geometrical identification: type of signal (collection or induction) and wire direction (U, V or X)
  • it is possible to store exclusively the time slices where there was activity (regions of interest)
  • it can point to the raw::RawDigit object on its channel

Recommended actions

  • remove art::Ptr from the data products3
  • have modules create associations between recob::Wire and raw::RawDigit [3]
  • add channel ID data member to recob::Wire [3][4]

recob::Hit

The observed charge from a travelling particle that drifted to a single wire .

A hit describes the charge profile in time using a shape hypothesis .
Its position information is two-dimensional: wire location and time.
It includes:
  • parameters of the signal shape best fitting the calibrated charge (peak, width...)
  • the portion of wire readings it fits directly, and timing information
  • full geometrical identification (geo::WireID)
  • references to the channel (wire) where the charge was collected and its raw data

Recommended actions

  • remove art::Ptr from the data products3
  • have modules create associations between recob::Hit and recob::Wire [3]
  • add channel ID data member to recob::Hit [3][4]
  • make fHitSignal member private, add constructors to initialize it5

Pending questions

  • fHitSignal is never filled in LArSoft code... should it be? or should it be removed?
  • fView and fSignalType are felt by MicroBooNE useless5

recob::Cluster

A sequence of hits showing geometrical correlation.

Its position information is two-dimensional.
It includes:
  • initial and final position of the sequence (in wire×time), with uncertainties
  • full geometrical information (geo::PlaneID, wire direction)
  • total observed charge, dQ/dW and dT/dW (and errors)

Recommended actions

  • replace each of the positions and errors with two float or a float[2]
  • replace double items with float

recob::EndPoint2D

Two-dimensional coordinate on a plane

Questions
  • what is this supposed to represent?

recob::Track

The observed information from a single physical particle with well-defined trajectory.

Questions

Q. All the information? or just trajectory? or...?

Recommended action

  • split into detected information (hit collection) and trajectory (geometrical)

recob::Shower

The observed information from a single physical particle evolving in a shower.

recob::SpacePoint

A geometrical point in the detector.

recob::Vertex

A point in the detector origin of particles

Questions

  • need uncertainty on vertex position... error matrix? independent variances?

anab::Calorimetry

Calorimetric information of a particle.

anab::CosmicTag

Cosmic-ray-like attributes of a particle.

anab::ParticleID

Hypothesis on the nature of a particle.

recob::PFParticle

The evolution of a particle in the event (particle flow).

  • it is possible to attribute a very rough particle hypothesis
  • interface with pandora


1 From discussion on September 15, 2014 .

2 From discussion on October 6, 2014 .

3 From discussion on October 13, 2014 .

4 After discussion at [3], two proposals have been evaluated: adding the channel ID as a new data member, or adding it in the geo::WireID structure. No compelling reason was identified to prefer either. The decision was in the end taken from the idea of having geo::WireID reflect a strictly geometric concept, as opposed to integrate a data acquisition concept.

5 From discussion on MicroBooNE Analysis Tools meeting, October 16, 2014, and following private e-mail exchange with Brian Rebel.