Athena Developer Guide

Athena
Chapter 15 Converters Version/Issue: 2.0.0
Listing 15.1 An example converter class
// Converter for class UDO.
extern const CLID& CLID_UDO;
extern unsigned char OBJY_StorageType;
static CnvFactory s_factory;
const ICnvFactory& UDOCnvFactory = s_factory;
class UDOCnv : public Converter {
public:
UDOCnv(ISvcLocator* svcLoc) :
Converter(Objectivity_StorageType, CLID_UDO, svcLoc) { }
}
createRep(DataObject* pO, IOpaqueAddress*& a); // transient->persistent
createObj(IOpaqueAddress* pa, DataObject*& pO); // persistent->transient
fillObjRefs( ... ); // transient->persistent
fillRepRefs( ... ); // persistent->transient
15.6 Storing Data using the ROOT I/O Engine
One possibility for storing data is to use the ROOT I/O engine to write ROOT files. Although ROOT by
itself is not an object oriented database, with modest effort a structure can be built on top to allow the
Converters to emulate this behaviour. In particular, the issue of object linking had to be solved in order
to resolve pointers in the transient world.
The concept of ROOT supporting paged tuples called trees and branches is adequate for storing bulk
event data. Trees split into one or several branches containing individual leaves with data. The data
structure within the Athena data store is tree like (as an example, part of the LHCb event data model is
shown in Figure 15.1).
In the transient world Athena objects are sub-class instances of the “DataObject”. The DataObject
offers some basic functionality like the implicit data directory which allows e.g. to browse a data store.
This tree structure will be mapped to a flat structure in the ROOT file resulting in a separate tree
representing each leaf of the data store. Each data tree contains a single branch containing objects of the
same type. The Athena tree is split up into individual ROOT trees in order to give easy access to
individual items represented in the transient model without the need of loading complete events from
the root file i.e. to allow for selective data retrieval. The feature of ROOT supporting selective data
reading using split trees did not seem too attractive since, generally, complete nodes in the transient
store should be made available in one go.
However, ROOT expects “ROOT” objects, they must inherit from TObject. Therefore the objects
from the transient store have to be converted to objects understandable by ROOT.
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