Athena Developer Guide

Athena
Chapter 15 Converters Version/Issue: 2.0.0
Chapter 15
Converters
15.1 Overview
Consider a small piece of detector; a silicon wafer for example. This “object” will appear in many
contexts: it may be drawn in an event display, it may be traversed by particles in a Geant4 simulation,
its position and orientation may be stored in a database, the layout of its strips may be queried in an
analysis program, etc. All of these uses or views of the silicon wafer will require code.
One of the key issues in the design of the framework was how to encompass the need for these different
views within Athena. In this chapter we outline the design adopted for the framework and look at how
the conversion process works. This is followed by sections which deal with the technicalities of writing
converters for reading from and writing to ROOT files.
15.2 Persistency converters
Athena gives the possibility to read event data from, and to write data back to, ROOT files. The use of
ODBC compliant databases is also possible, though this is not yet part of the Athena release. Other
persistency technologies have been implemented for LHCb, in particular the reading of data from
LHCb DSTs based on ZEBRA.
Figure 15.1 is a schematic illustrating how converters fit into the transient-persistent translation of
event data. We will not discuss in detail how the transient data store (e.g. the event data service) or the
persistency service work, but simply look at the flow of data in order to understand how converters are
used.
One of the issues considered when designing the Athena framework was the capability for users to
“create their own data types and save objects of those types along with references to already existing
page 137