1. Event Data Definition in LHCb
Marco Cattaneo
Markus Frank
Silvia Miksch
Gloria Corti
Pere Mato Vila
Stefan Roiser

2. Content The problem Description of the data-model Example
Benefits / Drawbacks
Current Status / Prospects
Migration to LCG software
Conclusion
Mar. 24th, 2003
Stefan Roiser

3. The Problem Long lifetime of experiment (> 10 years)
Change of programming-paradigms
Change of implementation-language
Storing large amounts of data
Uniform way of handling objects
Many sub-detectors define bits of event-model
Coherency is an important issue
Avoid different look and feel
Mar. 24th, 2003
Stefan Roiser

4. Gaudi Object Description (GOD)
The Goals
Definition of objects on a higher level
Easy language for defining objects
Ability to derive several implementations from this source
Uniform layout of objects
Easily extensible
Gaudi Object Description (GOD)
Mar. 24th, 2003
Stefan Roiser

5. Overall View of Gaudi Object Description
LHCb rules
internal
model
specific C++
mapping rules
C++
Dictionary
XSLT
java
C++
headers
dictionary
headers
html
docu
java
classes
doxygen
make .so
docu
dictionary
library
Mar. 24th, 2003
Stefan Roiser

6. Object Definition Languages
ASCII-text
simple to write but parsing difficult
IDL
interface-language, not flexible enough
UML
description-language also not flexible
XML
strong syntax (DTD, XML Schema)
still flexible and easily extendable
Mar. 24th, 2003
Stefan Roiser

7. Example
<class name=“MCParticle” id=“210” location=“MC/Particles” author=“Gloria Corti” desc=“The MC particle kinematics information”>
<base name=“KeyedObject<int>”/>
<attribute name=“momentum” type=“HepLorentzVector” init=“0.0,0.0,0.0,0.0” desc=“4-momentum-vector”/>
<relation name=“endVertices” type=“MCVertex” mulitplicity=“M” desc=“Vector of Pointers to decay vertices”/>
<method name=“virtualMass” type=“double”
const=“TRUE” desc=“Retrieve virtual mass”/>
<code> return m_momentum.m(); </code>
</method>
</class>
Mar. 24th, 2003
Stefan Roiser

8. C++ Code Generation Unknown types looked up in database and included
Generation of setters/getters for attributes
Depending on multiplicity handling of relations
only setter/getter for 1-1 relation
additional addTo/removeFrom/clear vector for 1-M relations
Use of smart-pointers for relations
Generation of class-id
Documentation in doxygen-style
Generation of serializers/deserializers
Handling plurals …
Mar. 24th, 2003
Stefan Roiser

9. Example (cont’d)
Mar. 24th, 2003
Stefan Roiser

10. .h-files + dictionary-files
Ratio*
.h-files + dictionary-files
( loc)
1:12,5
dictionary-files
( loc)
1:8,2
.h-files
( loc)
1:4,3
.h-files
( loc)
1:4,3
xml-source
(5.427 loc)
*(LHCb Event-model taken from LHCb-project v11r9)
Mar. 24th, 2003
Stefan Roiser

11. Benefits & Drawbacks XML XML Automatic Documentation
Strong syntax (DTD)
Still flexible for expansion (new elements/attributes)
Use of default-values
Simple (13 elements, ~4 attributes/element)
Automatic Documentation
Obeys coding-conventions
Object-introspection
Other backends possible
No change of source-code
XML
verbose language
escape sequences (eg. “<”)
Mar. 24th, 2003
Stefan Roiser

12. Reflection
Reflection is the ability to obtain information about an object at runtime and interact with it
query it’s layout
get/set values
invoke functions
Can be used in:
Persistency
Interactive environments
Reflection-information of event-objects is generated from the same xml-source
Development started in LHCb, now LCG/SEAL
Mar. 24th, 2003
Stefan Roiser

13. Reflection Model Item Class Field Method PropertyList Array Modifiers
declaringClass
propertyList
superclasses
Class
Field
Method
PropertyList
Array
type
fields
returnType
Modifiers
argumentTypes
methods
Mar. 24th, 2003
Stefan Roiser

14. Current Status and Prospects
Gaudi Object Description proved to be usable and stable
Used for 24 iterations of the LHCb-event-model (since Dec. 2001)
Features seem to be sufficient for doing LHCb physics
Adaptations maybe needed for future iterations of the event-model
Review of LHCb-event-model scheduled for this autumn
Mar. 24th, 2003
Stefan Roiser

15. Migration to LCG Software
LCG/SEAL
Dictionary-information-generation will be adapted to work with SEAL-Reflection
Python binding to LCG-Reflection
LCG/POOL
Dictionary and meta-information about objects will be used for persistency
No more need for generated serializers/deserializers
Mar. 24th, 2003
Stefan Roiser

16. Conclusions
Gaudi Object Description has been in production for more than a year
Accepted by physicists
Model proved to be useable
XML as underlying language was a good choice
Model is flexible enough for short-term adaptations
Good input/output ratio
Even more advantages on long-term
Integration with LCG software
We don’t expect major difficulties
Mar. 24th, 2003
Stefan Roiser