Building a Decision Matrix

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Building aDecision Matrix
Pete Sykes
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Background

• During a contract with NATS (National Air Traffic
  Services) I was responsible, amongst other
  things, for the design and development of the
  user interface of the system.
• This GUI could exist in a number of different
  states but only certain states were valid for
  particular types of user.
• Controls were enabled depending on the
  functionality valid for each particular state.
• The business rules for this were easily set down
  in tabular form but were likely to change a
  number of times before the project was complete.
• The initial code to implement the rules was a
  mess - although I wrote it I didnt want to be
  responsible for maintaining it!

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The problem

• Many business rules are easily represented in a
  tabular form
• Implementing those rules in code is not
  necessarily straightforward
• and often involves complex if..then..else
  structures
• which can obscure the business logic and make
  the code difficult to maintain

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For example
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Could be coded as

• AllowAction False
• if UserType Guest then
• if UserAction Read Data then
• AllowAction True
• if UserType Data then
• if UserAction Sign In then
• AllowAction True
• else
• if UserAction Read Data then
• AllowAction True
• else
• if UserAction Update Data then
• AllowAction True
• and so on

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To achieve the same in DecisionMatrix

• UFMatrix TDecisionMatrix.Create(nil)
• UFMatrix.DefaultResult False
• UFMatrix.SetResult(Guest, Read Data, True)
• UFMatrix.SetResult(Data, Sign In, True)
• UFMatrix.SetResult(Data, Read Data, True)
• and so on
• then
• AllowAction UFMatrix.CheckResult(Admin,
  Update Data)

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With DecisionMatrix

• All the complicated stuff is done in the setup
  phase
• which lends itself to use of a design-time
  editor if the class is implemented as a component
• or to streaming in from any external source
• Checking the result of a comparison is a simple
  one line effort.
• Code is much more readable (and hence
  maintainable)
• Changes in business rules require no code changes
• The concept is infinitely extendable to allow for
  n-dimensional matrices