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Chapter 8. openETCS Domain Framework
also needed should be identified during this process, too. Figure 8.2 is a UML class diagram
with an overview of the structural design of the openETCS domain framework. Although
the diagram already holds several classes, the openETCS domain framework consists of more.
Therefore, Figure 8.2 provides only an initial overview about the central class structures. Details
of further classes are introduced later in this section by further UML class diagrams. The
following overview is discussed in groups according to the openETCS meta model structure.
EVC State Machine The EVC state machine is the basis in all openETCS implementations 1 .
Thus, exact one object of CEVCStateMachine is used for building an EVC. Since the EVC
is a state machine or rather an ETCS Mode machine, a general state design pattern [32,
pp. 305-313] was specialised for openETCS: CEVCState is the only class representing ETCS
Modes, accordingly no base class for states is provided. Furthermore, CEVCState is nested [79]
in CEVCStateMachine to provide its instances direct access to its parent object via the
m_pParent aggregation [64]. The current active state is indicated by the m_pCurrentState
aggregation end in CEVCStateMachine. Additionally, it holds all available states in the
m_AvailbleStates composition [64]. The current ETCS Application Level is stored in a string
as a m_CurrentApplicationLevel composition.
The CEVCTransition class can be interpreted as the Transition relationship in the meta model.
Each of its object is associated with a start state m_pStartState, a target state m_pTargetState,
and a condition m_pCondition, which it is activated in. The m_TransitionStack aggregation
stores enabled transition as a stack to support situations, which more than one transition is
enabled in. In such cases, the transition to be executed should be selected by the highest
priority.
The CEVCCondition class is part of data flows and therefore inherits from CFunctionBlock
and only takes a Boolean input as condition for the corresponding CEVCTransition object.
CLevelCondition objects are used similarly to switch to a new Application Level depending on
their m_TargetApplicationLevel composition.
Data Flows For each available ETCS Application Level in CEVCState, at least one CDataFlow
object must be available. Those are combined in the m_DataFlows composition of CEVCState
while the currently executed data flows are indicated by the m_CurrentDataFlow aggregation.
If more than one CDataFlow object is used in a certain CEVCState and a certain Application
Level m_CurrentApplicationLevel, this means that those data flows are independent and can
be executed in parallel to gain performance on multi-core systems. Anyway, how independent
data flows are identified is an issue of the code generator and will be discussed in Chapter 9.
Each CDataFlow object holds an aggregation m_pParent to its parent CEVCState object.
Again, to provide direct access to members in CEVCState and CEVCStateMachine, CDataFlow
is nested in the state class.
A CDataFlow object holds a set of CFunctionBlock objects in the m_FunctionBlocks
aggregation, which are executed in the order as they are stored in the aggregation. The cyclic
execution described in Section 7.6 is actually implemented in the CEVCState class, which
starts the execution of m_CurrentDataFlow with a fixed sample time.
1 in the meaning of the generated code
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