|
In this article, we will be highlighting the advantages of hierarchical state
machine design over conventional state machine design.
In conventional state machine design, all states are considered at the same
level. The design does not capture the commonality that exists among states. In
real life, many states handle most messages in similar fashion and differ
only in handling of few key messages. Even when the actual handling differs,
there is still some commonality. Hierarchical state machine design captures the
commonality by organizing the states as a hierarchy. The states at the higher
level in hierarchy perform the common message handling, while the lower level
states inherit the commonality from higher level ones and perform the state
specific functions. The table given below shows the mapping between conventional
states and their hierarchical counterparts for a typical call state machine.
| Conventional States |
Hierarchical States |
| Awaiting First Digit |
Setup.CollectingDigits.AwaitingFirstDigit |
| Collecting Digits |
Setup.CollectingDigits.AwaitingSubsequent Digits |
| Routing Call |
Setup.RoutingCall |
| Switching Path |
Setup.SwitchingPath |
| Conversation |
Conversation |
| Awaiting Onhook |
Releasing.AwaitingOnhook |
| Releasing Path |
Releasing.ReleasingPath |
A conventional state machine is designed as a two dimensional array
with one dimension as the state and the other dimension specifying the message
to be handled. The state machine determines the message handler to be called by
indexing with the current state and the received message. In real life scenario,
a task usually has a number of states along with many different types of input
messages. This leads to a message handler code explosion. Also, a huge two
dimensional array needs to be maintained. Hierarchical state machine design
avoids this problem by recognizing that most states differ in the handling of
only a few messages. When a new hierarchical state is defined, only the state
specific handlers need to be specified.
Conventional State Machine Example
The figure below describes the state transition diagram for an active standby
pair. The design here assumes that the active and standby are being managed by
an external entity.
The different states for the state machine are Active, Standby, Suspect and
Failed. The input messages to be handled are Switchover, Fault Trigger,
Diagnostics Passed, Diagnostics Failed and Operator Inservice. Thus the handler
two dimensional array is 4 x 5 i.e. 20 handlers need to be managed.
The code below shows the handlers that need to be defined. A dummy "do
nothing" handler should be specified for all other entries of the two
dimensional state table. This simple example clearly illustrates the problem
with conventional state design. There is a lot of code repetition between
handlers. This creates a maintenance headache for state machine designers. We
will see in the following section that hierarchical state machine design
exploits these very similarities to implement a more elegant state structure.
| Conventional
State Machine Implementation |
/* == Active State Handlers == */
void ActiveStateFaultTriggerHandler(Msg *pMsg)
{
PerformSwitchover(); // Perform Switchover, as active failed
NextState = SUSPECT; // Run diagnostics to confirm fault
SendDiagnosticsRequest();
RaiseAlarm(LOSS_OF_REDUNDANCY); // Report loss of redundancy to operator
}
void ActiveStateSwitchoverHandler(Msg *pMsg)
{
PerformSwitchover(); // Perform Switchover on operator command
CheckMateStatus(); // Check if switchover completed
SendSwitchoverResponse(); // Inform operator about switchover
NextState = STANDBY; // Transition to standby
}
/* == Standby State Handlers == */
void StandbyStateFaultTriggerHandler(Msg *pMsg)
{
NextState = SUSPECT; // Run diagnostics to confirm fault
SendDiagnosticsRequest();
RaiseAlarm(LOSS_OF_REDUNDANCY); // Report loss of redundancy to operator
}
void StandbyStateSwitchoverHandler(Msg *pMsg)
{
PerformSwitchover(); // Perform switchover on operator command
CheckMateStatus(); // Check if switchover completed
SendSwitchoverResponse(); // Inform operator about switchover
NextState = ACTIVE; // Transition to active
}
/* == Suspect State Handlers == */
void SuspectStateDiagnosticsFailedHandler(Msg *pMsg)
{
SendDiagnosticsFailureReport(); // Inform operator about diagnostics
NextState = FAILED; // Move to the failed state
}
void SuspectStateDiagnosticsPassedHandler(Msg *pMsg)
{
SendDiagnosticsPassReport(); // Inform operator about diagnostics
ClearAlarm(LOSS_OF_REDUNDANCY); // Clear loss of redundancy alarm
NextState = STANDBY; // Move to standby state
}
void SuspectStateOperatorInservice(Msg *pMsg)
{
// Operator has replaced the card, so abort the current diagnostics
// and restart new diagnostics on the replaced card.
AbortDiagostics();
SendDiagnosticsRequest(); // Run diagnostics on replaced card
SendOperatorInserviceResponse(); // Inform operator about diagnostics start
}
/* == Failed State Handlers == */
void FailedStateOperatorInservice(Msg *pMsg)
{
SendDiagnosticsRequest(); // Run diagnostics on replaced card
SendOperatorInserviceResponse(); // Inform operator about diagnostics start
NextState = SUSPECT; // Move to suspect state for diagnostics
}
|
Hierarchical State Machine Example
The following state transition diagram recasts the state machine by
introducing two levels in the hierarchy. Inservice and Out_Of_Service are the high
level states that capture the common message handling. Active and Standby states
are low level states inheriting from Inservice state. Suspect and Failed are low
level states inheriting from Out_Of_Service state.
The following diagram clearly illustrates the state hierarchy. Even the
Inservice and Out_Of_Service, high level states inherit from the Unit_State that is
at the highest level.
Hierarchical State Machine Source Code
The C++ implementation details of the hierarchical state machine are given
below. It is apparent that all the commonality has moved to the high level
states viz. Inservice and Out_Of_Service. Also, contrast this with the
conventional state machine implementation.
The code below contains hyperlinks to more detailed information about the
classes, methods and variables in this information.
Header File
The header file below declares the Unit state machine using the Hierarchical_State_Machine
class. Important points to note are:
- The state classes are nested private classes within the state machine
class. Thus they are not visible to other classes.
- The state machine declares all states to be friend classes. This does not
break the encapsulation as only a private class is being declared as a friend.
- The base class (Unit_State)
provides a "do nothing" implementation for all handlers. Thus an inheriting
state has to provide an implementation only for that methods it supports.
- State objects are declared static. Thus multiple instances of the state
machine will share the same state objects. Due to this, the Hierarchical_State_Machine
class has a small memory footprint.
- Only the main message handler,
On_Message, is declared public. All helper functions are private.
- A pointer to the current state is maintained in p_Current_State variable.
This variable gets initialized using the
Next_State method.
| Hierarchical_State_Machine.h |
00001
00002 class Message;
00009
00010
00011
00031
00032 class Hierarchical_State_Machine
00033 {
00037
00038 class Unit_State
00039 {
00040 public:
00041
00043 virtual void On_Switchover(Hierarchical_State_Machine &u,
const Message *p_Message) {}
00044
00046 virtual void On_Fault_Trigger(Hierarchical_State_Machine &u,
const Message *p_Message) {}
00047
00049 virtual void On_Diagnostics_Failed(Hierarchical_State_Machine &u,
const Message *p_Message) {}
00050
00052 virtual void On_Diagnostics_Passed(Hierarchical_State_Machine &u,
const Message *p_Message) {}
00053
00055 virtual void On_Operator_Inservice(Hierarchical_State_Machine &u,
const Message *p_Message) {}
00056 };
00057 friend Unit_State;
00058
00059
00062
00063 class Inservice : public Unit_State
00064 {
00065 public:
00066 void On_Switchover(Hierarchical_State_Machine &u,
const Message *p_Message);
00067 void On_Fault_Trigger(Hierarchical_State_Machine &u,
const Message *p_Message);
00068 };
00069 friend Inservice;
00070
00075
00076 class Active : public Inservice
00077 {
00078 public:
00079 void On_Switchover(Hierarchical_State_Machine &u,
const Message *p_Message);
00080 void On_Fault_Trigger(Hierarchical_State_Machine &u,
const Message *p_Message);
00081 };
00082 friend Active;
00083
00088
00089 class Standby : public Inservice
00090 {
00091 public:
00092 void On_Switchover(Hierarchical_State_Machine &u,
const Message *p_Message);
00093 };
00094 friend Standby;
00095
00098
00099 class Out_Of_Service : public Unit_State
00100 {
00101 public:
00102 void On_Operator_Inservice(Hierarchical_State_Machine &u,
const Message *p_Message);
00103 };
00104 friend Out_Of_Service;
00105
00109 class Suspect : public Out_Of_Service
00110 {
00111 public:
00112 void On_Diagnostics_Failed(Hierarchical_State_Machine &u,
const Message *p_Message);
00113 void On_Diagnostics_Passed(Hierarchical_State_Machine &u,
const Message *p_Message);
00114 void On_Operator_Inservice(Hierarchical_State_Machine &u,
const Message *p_Message);
00115 };
00116 friend Suspect;
00117
00121 class Failed : public Out_Of_Service
00122 {
00123 public:
00124
00125 };
00126 friend Failed;
00127
00128 private:
00129 static Active Active_State;
00130 static Standby Standby_State;
00131 static Suspect Suspect_State;
00132 static Failed Failed_State;
00133
00134 void Next_State(Unit_State &r_State);
00135
00136
00137
00138
00139 virtual void Send_Diagnostics_Request();
00140 virtual void Raise_Alarm(int reason);
00141 virtual void Clear_Alarm(int reason);
00142 virtual void Perform_Switchover();
00143
00144 virtual void Send_Switchover_Response();
00145 virtual void Send_Operator_Inservice_Response();
00146 virtual void Send_Diagnostics_Failure_Report();
00147 virtual void Send_Diagnostics_Pass_Report();
00148 virtual void Abort_Diagnostics();
00149 virtual void Check_Mate_Status();
00150 Unit_State *p_Current_State;
00151
00152 public:
00153 void On_Message(const Message *p_Message);
00154 };
00155
00159
00160 void Hierarchical_State_Machine::Next_State(Unit_State &r_State)
00161 {
00162 p_Current_State = &r_State;
00163 }
00164
00165
|
Source File
Important things to note about the source file:
-
On_Message, the main message handler invokes the appropriate handler based
on the type of the message. The message is passed to the current state object.
- The Out_Of_Service
and Inservice
base states handle most of the message processing. In some cases, the
inheriting states perform some additonal action and call the handler for the
base state for the common part of the handling.
| Hierarchical_State_Machine.cpp |
00007
00008 #include "Hierarchical_State_Machine.h"
00009 #include "Unit_Messages.h"
00010 #include "assert.h"
00011
00016
00017 void Hierarchical_State_Machine::On_Message(const Message *p_Message)
00018 {
00019 switch (p_Message->GetType())
00020 {
00021 case Message::FAULT_TRIGGER:
00022 p_Current_State->On_Fault_Trigger(*this, p_Message);
00023 break;
00024
00025 case Message::SWITCHOVER:
00026 p_Current_State->On_Switchover(*this, p_Message);
00027 break;
00028
00029 case Message::DIAGNOSTICS_PASSED:
00030 p_Current_State->On_Diagnostics_Passed(*this, p_Message);
00031 break;
00032
00033 case Message::DIAGNOSTICS_FAILED:
00034 p_Current_State->On_Diagnostics_Failed(*this, p_Message);
00035 break;
00036
00037 case Message::OPERATOR_INSERVICE:
00038 p_Current_State->On_Operator_Inservice(*this, p_Message);
00039 break;
00040
00041 default:
00042 assert(false);
00043 break;
00044 }
00045 }
00046
00055
00056 void Hierarchical_State_Machine::Inservice::On_Fault_Trigger(
Hierarchical_State_Machine &u,
const Message *p_Message)
00057 {
00058 u.Next_State(u.Suspect_State);
00059 u.Send_Diagnostics_Request();
00060 u.Raise_Alarm(LOSS_OF_REDUNDANCY);
00061 }
00062
00070
00071 void Hierarchical_State_Machine::Inservice::On_Switchover(
Hierarchical_State_Machine &u,
const Message *p_Message)
00072 {
00073 u.Perform_Switchover();
00074 u.Check_Mate_Status();
00075 u.Send_Switchover_Response();
00076 }
00077
00088
00089 void Hierarchical_State_Machine::Active::On_Fault_Trigger(
Hierarchical_State_Machine &u,
const Message *p_Message)
00090 {
00091 u.Perform_Switchover();
00092 Inservice::On_Fault_Trigger(u, p_Message);
00093 }
00094
00100
00101 void Hierarchical_State_Machine::Active::On_Switchover(
Hierarchical_State_Machine &u,
const Message *p_Message)
00102 {
00103 Inservice::On_Switchover(u, p_Message);
00104 u.Next_State(u.Standby_State);
00105 }
00106
00112
00113 void Hierarchical_State_Machine::Standby::On_Switchover(
Hierarchical_State_Machine &u,
const Message *p_Message)
00114 {
00115 Inservice::On_Switchover(u, p_Message);
00116 u.Next_State(u.Active_State);
00117 }
00118
00127
00128 void Hierarchical_State_Machine::Out_Of_Service::On_Operator_Inservice(
Hierarchical_State_Machine &u,
const Message *p_Message)
00129 {
00130
00131
00132 u.Send_Diagnostics_Request();
00133 u.Send_Operator_Inservice_Response();
00134 u.Next_State(u.Suspect_State);
00135 }
00136
00142
00143 void Hierarchical_State_Machine::Suspect::On_Diagnostics_Failed(
Hierarchical_State_Machine &u,
const Message *p_Message)
00144 {
00145 u.Send_Diagnostics_Failure_Report();
00146 u.Next_State(u.Failed_State);
00147 }
00148
00155
00156 void Hierarchical_State_Machine::Suspect::On_Diagnostics_Passed(
Hierarchical_State_Machine &u,
const Message *p_Message)
00157 {
00158 u.Send_Diagnostics_Pass_Report();
00159 u.Clear_Alarm(LOSS_OF_REDUNDANCY);
00160 u.Next_State(u.Standby_State);
00161 }
00162
00167
00168 void Hierarchical_State_Machine::Suspect::On_Operator_Inservice(
Hierarchical_State_Machine &u,
const Message *p_Message)
00169 {
00170 u.Abort_Diagnostics();
00171 Out_Of_Service::On_Operator_Inservice(u, p_Message);
00172 }
|
|
