Athena Developer Guide

Athena Chapter 14 Tools and ToolSvc Version/Issue: 2.0.0
algorithm whereas the initialize() and finalize() methods are invoked automatically by the
framework while initializing the parent algorithm. The properties of a sub-algorithm are automatically
set by the framework but the parent algorithm can change them during execution. Sharing of data
between nested algorithms is done via the Transient Event Store.
Both Services and Algorithms are created during the initialization stage of a job and live until the jobs
ends.
Sometimes an encapsulated piece of code needs to be executed only for specific events, in which case it
is desirable to create it only when necessary. On other occasions the same piece of code needs to be
executed many times per event. Moreover it can be necessary to execute a sub-algorithm on specific
contained objects that are selected by the parent algorithm or have the sub-algorithm produce new
contained objects that may or may not be put in the Transient Store. Finally different algorithms may
wish to configure the same piece of code slightly differently or share it as-is with other algorithms.
To provide this kind of functionality we have introduced a category of processing objects that
encapsulate these “light” algorithms. We have called this category Tools.
Some examples of possible tools are single track fitters, association to Monte Carlo truth information,
vertexing between particles, smearing of Monte Carlo quantities.
14.2.1 “Private” and “Shared” Tools
Algorithms can share instances of Tools with other Algorithms if the configuration of the tool
is suitable. In some cases however an Algorithm will need to customize a tool in a specific way in
order to use it. This is possible by requesting the ToolSvc to provide a “private” instance of a tool.
If an Algorithm passes a pointer to itself when it asks the ToolSvc to provide it with a tool, it is
declaring itself as the parent and a “private” instance is supplied. Private instances can be configured
according to the needs of each particular Algorithm via jobOptions.
As mentioned above many Algorithms can use a tool as-is, in which case only one instance of a
Tool is created, configured and passed by the ToolSvc to the different algorithms. This is called a
“shared” instance. The parent of “shared” tools is the ToolSvc.
14.2.2 The Tool classes
14.2.2.1 The AlgTool base class
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The main responsibilities of the AlgTool base class (see Listing 14.1) are the identification of the
tools instances, the initialisation of certain internal pointers when the tool is created and the
management of the tools properties. The AlgTool base class also offers some facilities to help in the
implementation of derived tools.