Implementation Guide › Case Study Examples › Business Logic Scripting Examples › Case Study 12: Using Counter Logic to Calculate Number of Failures

Case Study 12: Using Counter Logic to Calculate Number of Failures

The following example calculates the number of failures that occurred within a given calculation period. This formula and the methods used to implement it can be taken as an example of a formula that is required wherever it is necessary to calculate something.

The following calculation assumptions are used:

• Input Events:
  • Availability Event, Status (0/1)
  • The availability Event arrives every few minutes. It is not possible to assume the frequency of the Events (the Event may occur every five minutes or once an hour) and there may also be redundant Events (for example: 0, 0, 0, 1, 1, 0, etc.).
• Timeslot- Business Hours, Failures that occurred out of timeslot are not counted.
• Required Result is the count of how many failures have occurred during the period.

In order to tally the failures that occurred during the calculation period it is necessary to store a periodic counter variable and also a variable that stores the system status. Since redundant Event information may be received (i.e. An 'Up' event followed by another 'Up' event), it is also necessary to count the number of locations where there was a change of the system status from "Up" to "Down" and not to count every time a "Down" event is received as this may be a redundant Event that represents a failure that has already been counted.

The following figure graphically depicts counting system Up and Down times.

Important points that should be considered:

• Constants-the use of constant definitions, rather than that of constant values within the code, is recommended. In this way, if the value needs changing, it is easy to change the value in the constant definition only and not to have to search for it throughout the code. In addition, it is easy to change it to a parameter use if required. In the above example, the values that represent the system status received in the event are defined as constants in order to make the code more understandable, and also to differentiate when the number zero is used as a number for counting purposes rather than where it represents a system status.
• Global variables:
  • Counter-the definition of the counter variable is a global definition. In the formula it is declared at the top of the code, and outside of any subroutines or functions. It is important to define the counter variable in this case as a global variable. This allows it to be used in several subroutines/functions within the code, and also allows the system to keep its value throughout the calculation period and provide its result at the end of the period.

    In this example the counter variable needs to be used in three places in the code:

    • To be initialized at the beginning of a period.
    • To be advanced in case of a failed event in the event handler.
    • To be outputted by the result function at the end of the period.
  • System previous status-this variable holds the status of the system and is updated whenever a new event with the system status is received. This variable also needs to be global because it is used in several subroutines/functions in the code, such as:
    • To be initialized in the beginning of the calculation.
    • To be updated when a new event is received.
• Timeslot considerations-in order to verify whether a failure occurs within a timeslot period, it is possible to use the value of the context.IsWithinTimeSlot property. The context is a global object that can be used from anywhere in the code, and in this case, is used to indicate when a failure is received and whether it is within or outside of the timeslot period. If at the timestamp of the event the flag returns TRUE, then the event occurs within a timeslot period, however if it returns FALSE, this indicates that the event took place outside of the timeslot. According to the required logic, any failure that occurs outside of the timeslot is ignored and therefore not counted.
• Initialize variables:
  • Counter variable-holds a periodic value and is therefore initialized in the OnPeriodstart handler to make sure that each calculation period counts its failures separately. Each period starts with zero and outputs only the number of failures within this period.

    In cases where it is required to calculate the accumulated failures within each period, (meaning that the result of each period is all of the failures that have occurred up until the end of this period, and including all periods before that), then it is required to initialize the counter only in the OnLoad handler and remove it from the OnPeriodStart handler. Thus, the counter continues counting and accumulating between the periods as shown below:

    Sub OnLoad(time)
        FingerprInted=0
    End Sub
    

  • System status variable-should be initialized once at the beginning of the calculation, and from that point on, be updated on each Status event. This variable retains its value throughout the calculation and is not restricted to a certain calculation period. It must also retain its value between the calculation periods. Since the status of the system before receiving the first Event is unknown, an assumption must be made as to the system status. The common assumption is that the system is "Up". This assumption should be agreed upon and checked as it can lead to the following:

    If when the calculation started and the system status was in fact "Down" and the first event arrives to indicate this "Down" status, this will be counted as a failure because of the assumed status was "Up". This failure will be counted towards the first calculation period even if it did not necessarily occur during that period.

    Option Explicit
    
    'Constants definitions
    Const UP=1
    Const DOWN=0
    
    'Global variable for counting failures
    Dim FingerprInted
    Dim SystemStatus
    
    Sub OnRegistration(dispatcher)
       ' The parameters of the method are: <procedure name>, <Event Type>
       dispatcher.RegisterByContractPartyAndService "OnAvailabilityEvent","AvailabilityEvent"
    End Sub
    
    Sub OnLoad(time)
       SystemStatus = UP 'assume the first system status
    End Sub
    
    Sub OnPeriodStart(time)
       FingerprInted = 0
    End Sub
    
    Sub OnAvailabilityEvent(eventDetails)
       ' In case it's a failure and within the timeslot then it is counted
       If context.IsWithinTimeSlot and SystemStatus=UP and _
             eventDetails("Status")=DOWN Then
          FingerprInted = FingerprInted + 1
       End If
       ' update the system status for next comparison
       SystemStatus = eventDetails("Status") 
    End Sub
    
    Function Result
       Result = FingerprInted
    End Function