DevelopKit

Intro to the Gaudi framework

Overview

Teaching: 20 min
Exercises: 30 min
Questions

  • What does Gaudi do?

Objectives

  • Learn the basic concepts of Gaudi.

What is the Gaudi framework?

The Gaudi framework runs over a list of events, providing ways to process them and store data in a new format. It creates and manages Data Objects, which can hold a variety of data. A Transiant Event Store (TES) stores data objects in a way to make them accessible to the rest of the framework, and parts of it can be made persistent in a ROOT format file. The data in the TES is created and accessed by Algorithms, which produce data objects and process data objects. Gaudi also provides Services, which provide access to other parts of the framework, such as histograms. Tools are lightweight routines that are also available. The Application Manager manages these components.

Algorithms

This is the most important component of the framework for an user to know. Algorithms are called once per physics event, and (traditionally) implement three methods beyond constructor/destructor: initialize, execute, and finalize (see the details on the upgrade). Also, beginRun and endRun are available, though be careful not to misuse state.

Properties

Algorithms are a Configurable, which means they can be accessed in Python and Properties can be manipulated there. In the classic API, a property is declared in the constuctor, using:

declareProperty("PropertyName", f_value, "Description of property");

Here, f_value is a reference to a variable for an int, string, etc. It is almost always a member variable for the class so that you can access it in the other methods.

To use a property, you can simply access it on the configurable in Python:

my_algorithm.PropertyName = 42

Data Objects

To place an item on the TES, it must inherit from DataObject (in the classic system) and must have a unique CLID identifier. This can be done manually, or can be automated with the GaudiObjDesc (GOD) system in the LHCb project.

GaudiObjDesc example

To use GOD, add a directory xml to your package, and place a GOD xml file in it, with a structure similar to:

xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE gdd SYSTEM "gdd.dtd">
<gdd>
    <package name="ThreeVecPackage">
        <class
          name="ThreeVecEx"
          author="tbd"
          desc="Some description"
        >
            <base name="DataObject"/>
            <attribute name="x" type="double" desc="Value x"/>
            <attribute name="y" type="double" desc="Value y"/>
            <attribute name="z" type="double" desc="Value z"/>
        </class>
    </package>
</gdd>

GOD will automatically create accessors for you, such as GetX and SetX, and will specialize as needed if the base class is DataObject. The output header file will be accessable as "Event/ThreeVecEx", and the class will be in the LHCb namespace.

The project you are in must be one that includes at least LHCb (such as Lbcom, DaVinci, etc) and you need the following lines in your CMakeLists:

include(GaudiObjDesc)
god_build_headers(xml/*.xml)
god_build_dictionary(xml/*.xml)

Transient Event Store (TES)

The TES is a place where you can store items on a per-event basis. It should be viewed as non-mutable; meaning that once you place an item in it, it should never change. Persistency is optional and will be covered later. The path to an event should always start with "/Event", though Gaudi is smart enough to assume that a path that does not start with a slash is a relative path, and will get "/Event/" prepended to it. In the classic framework, you used get and put functions to access the TES. For this you must use GaudiAlgorithm instead of Algorithm, which is a specialization to add access to the TES. You will need to add GaudiAlgLib to the linked library list for the GaudiAlgorithm.

To place an item in the event store, create a pointer to a new object, and then put it in the event store in an execute method:

auto data = new DataObject();
put(data, "/Event/SomeData");

The event store will take ownership of the object, so do not delete it.

To retrieve it, also in an execute method:

auto data = get<DataObject>("/Event/SomeData");

Project: Data communication

Set up two algorithms, one that produces data and one that consumes it. You can copy the above GOD file to give yourself an object to pass.

You can get a blank template to start this project with:

$ svn export https://github.com/lhcb/DevelopKit/trunk/code/GaudiTemplate MyGaudiProject

This will give you a directory called MyGaudiProject with the basics ready. Edit the CMakeLists and directory name inside the project, then you will just need to work on the files in src and options.

To facilitate building, a build_and_run.sh script is included in the project. This will automatically run the commands needed to build the project and run the options file.

Solution

Code solving this is in /code/GaudiClassicAPI/GaudiDataTrans

Futher reading

There are further examples in the Gaudi repository: Gaudi/GaudiExamples/src.

Key Points