  High energy physics detectors can be described hierarchically from the
different subsystems to their divisions in r, phi, theta and to the individual
readout channels. An identification schema that follows the logical
decomposition of the ATLAS detector has been introduced allowing identification
of individual readout channels as well as other parts of the detector, in
particular detector elements. These identifiers provide a sort of ?glue?
allowing, for example, the connection of raw event data to their detector
description for position calculation or alignment corrections, as well as fast
access to subsets of the event data for event trigger selection. There are two
important requirements on the software to support such an identification
scheme. First is the possibility to formally specify these identifiers in terms
of their structure and allowed values. And second is to generate different
forms of the identifiers optimised in terms of access efficiency to information
content, compactness or search key efficiency. We present here the generic
toolkit developed in the context of the ATLAS experiment to primarily provide
the identification of the readout channels and detector elements. The
architecture of the toolkit is decomposed into three parts: an XML-based
dictionary containing the formal specification of a particular range of
identifiers, a set of various identifier classes (offering various level of
compaction), and finally a set of ?helper? classes, specific for each detector
system, which serve as intermediaries between the dictionary and the identifier
classes to create, manipulate and interpret the identifiers. This architecture
will be described as well as the various applications of this identification
scheme.
