Athena Developer Guide

Athena
Chapter 13 Framework services Version/Issue: 2.0.0
13.8 The Random Numbers Service
When generating random numbers two issues must be considered:
• reproducibility and
• randomness of the generated numbers.
In order to ensure both, Athena implements a single service ensuring that these criteria are met. The
encapsulation of the actual random generator into a service has several advantages:
• Random seeds are set by the framework. When debugging the detector simulation, the
program could start at any event independent of the events simulated before. Unlike the
random number generators that were known from CERNLIB, the state of modern generators
is no longer defined by one or two numbers, but rather by a fairly large set of numbers. To
ensure reproducibility the random number generator must be initialized for every event.
• The distribution of the random numbers generated is independent of the random number
engine behind. Any distribution can be generated starting from a flat distribution.
• The actual number generator can easily be replaced if at some time in the future better
generators become available, without affecting any user code.
The implementation of both generators and random number engines are taken from CLHEP. The
default random number engine used by Athena is the RanLux engine of CLHEP with a luxury level of
3, which is also the default for Geant4, so as to use the same mechanism to generate random numbers as
the detector simulation.
Figure 13.1 shows the general architecture of the Athena random number service. The client interacts
with the service in the following way:
• The client requests a generator from the service, which is able to produce a generator
according to a requested distribution. The client then retrieves the requested generator.
• Behind the scenes, the generator service creates the requested generator and initializes the
object according to the parameters. The service also supplies the shared random number
engine to the generator.
• After the client has finished using the generator, the object must be released in order to inhibit
resource leaks
Distribution:
Gauss
RndmGenSvc
owns & initializes
owns
RndmGen
uses
RndmEngine
Figure 13.1 The architecture of the random number service. The client requests from the service a random
number generator satisfying certain criteria
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