Chapter 5. Hit policies for guided decision tables

Hit policies determine the order in which rules (rows) in a guided decision table are applied, whether top to bottom, per specified priority, or other options.

The following hit policies are available:

  • None: (Default hit policy) Multiple rows can be executed and the verification warns about rows that conflict. Any decision tables that have been uploaded (using a non-guided decision table spreadsheet) will adopt this hit policy.
  • Resolved Hit: Only one row at a time can be executed according to specified priority, regardless of list order (you can give row 10 priority over row 5, for example). This means you can keep the order of the rows you want for visual readability, but specify priority exceptions.
  • Unique Hit: Only one row at a time can be executed, and each row must be unique, with no overlap of conditions being met. If more than one row is executed, then the verification produces a warning at development time.
  • First Hit: Only one row at a time can be executed in the order listed in the table, top to bottom.
  • Rule Order: Multiple rows can be executed and verification does not report conflicts between the rows since they are expected to happen.

Figure 5.1. Available hit policies

hit policies image 1

5.1. Hit policy examples: Decision table for discounts on movie tickets

The following is part of a decision table for discounts on movie tickets based on customer age, student status, or military status, or all three.

Table 5.1. Decision table for available discounts on movie tickets

Row NumberDiscount TypeDiscount

1

Senior citizen (age 60+)

10%

2

Student

10%

3

Military

10%

The total discount to be applied in the end will vary depending on the hit policy specified for the table, as follows.

  • None/Rule Order: With both None and Rule Order hit policies, all applicable rules are incorporated, in this case allowing discounts to be stacked for each customer.

    Example: A senior citizen who is also a student and a military veteran will receive all three discounts, totaling 30%.

    Key difference: With None, warnings are created for multiple rows applied. With Rule Order, those warnings are not created.

  • First Hit/Resolved Hit: With both First Hit and Resolved Hit policies, only one of the discounts can be applied.

    For First Hit, the discount that is satisfied first in the list is applied and the others are ignored.

    Example: A senior citizen who is also a student and a military veteran will receive only the senior citizen discount of 10%, since that is listed first in the table.

    For Resolved Hit, a modified table is required. The discount that you assign a priority exception to in the table, regardless of listed order, will be applied first. To assign this exception, include a new column that specifies the priority of one discount (row) over others.

    Example: If military discounts are prioritized higher than age or student discounts, despite the listed order, then a senior citizen who is also a student and a military veteran will receive only the military discount of 10%, regardless of age or student status.

    Consider the following modified decision table that accommodates a Resolved Hit policy:

    Table 5.2. Modified decision table that accommodates a Resolved Hit policy

    Row NumberDiscount TypeHas Priority over RowDiscount

    1

    Senior citizen (age 60+)

     

    10%

    2

    Student

     

    10%

    3

    Military

    1

    10%

    In this modified table, the military discount is essentially the new row 1 and therefore takes priority over both age and student discounts, and any other discounts added later. You do not need to specify priority over rows "1 and 2", only over row "1". This changes the row hit order to 3 → 1 → 2 → …​ and so on as the table grows.

    Note

    The row order would be changed in the same way if you actually moved the military discount to row 1 and applied a First Hit policy to the table instead. However, if you want the rules listed in a certain way and applied differently, such as in an alphabetized table, the Resolved Hit policy is useful.

    Key difference: With First Hit, rules are applied strictly in the listed order. With Resolved Hit, rules are applied in the listed order unless priority exceptions are specified.

  • Unique Hit: A modified table is required. With a Unique Hit policy, rows must be created in a way that it is impossible to satisfy multiple rules at one time. However, you can still specify row-by-row whether to apply one rule or multiple. In this way, with a Unique Hit policy you can make decision tables more granular and prevent overlap warnings.

    Consider the following modified decision table that accommodates a Unique Hit policy:

    Table 5.3. Modified decision table that accommodates a Unique Hit policy

    Row NumberIs Senior Citizen (age 65+)Is StudentIs MilitaryDiscount

    1

    yes

    no

    no

    10%

    2

    no

    yes

    no

    10%

    3

    no

    no

    yes

    10%

    4

    yes

    yes

    no

    20%

    5

    yes

    no

    yes

    20%

    6

    no

    yes

    yes

    20%

    7

    yes

    yes

    yes

    30%

    In this modified table, each row is unique, with no allowance of overlap, and any single discount or any combination of discounts is accommodated.

5.1.1. Types of guided decision tables

Two types of decision tables are supported in Red Hat Decision Manager: Extended entry and Limited entry tables.

  • Extended entry: An Extended Entry decision table is one for which the column definitions specify Pattern, Field, and Operator but not value. The values, or states, are themselves held in the body of the decision table.

    Extended entry
  • Limited entry: A Limited Entry decision table is one for which the column definitions specify value in addition to Pattern, Field, and Operator. The decision table states, held in the body of the table, are boolean where a positive value (a marked check box) has the effect of meaning the column should apply, or be matched. A negative value (a cleared check box) means the column does not apply.

    Limited entry