Creating Rules

The example presented is a very simple use case: determining the discount percentage based on a customer category. To start, click the Create decision button.

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Provide a decision table name and a unique decision table key and select the Create new decision model button.

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You are now ready to define your decision table. Let us describe what is presented within the editor.

Hit Policy

In the top left corner, you can select a hit policy.

There are seven hit policies available:

single hit

  • FIRST: multiple (overlapping) rules can match, with different output entries. The first hit by rule order is returned and evaluation halts.

  • UNIQUE: no overlap is possible and all rules are disjoint. Only a single rule can be matched.

  • ANY: there may be overlap, but all of the matching rules show equal output entries for each output so that any match is used. If the output entries are non-equal, the hit policy is incorrect and the result is empty and marked as failed. When strict mode is disabled the last valid rule is the result. (The violation is presented as a validation message.)

  • PRIORITY: multiple rules can match, with different output entries. This policy returns the matching rule with the highest output priority. Output priorities are specified in the ordered list of output values, in decreasing order of priority. When strict mode is disabled and there are no output values defined the first valid rule be the result. (The violation is presented as a validation message.)

multiple hits

  • RULE ORDER: returns all hits in rule order.

  • OUTPUT ORDER: returns all hits in decreasing output priority order. Output priorities are specified in the ordered list of output values in decreasing order of priority.

  • COLLECT: returns all hits in arbitrary order. An operator ('+', '<', '>', '#') can be added to apply a simple function to the outputs. If no operator is present, the result is the list of all the output entries.

    • + (sum): the result of the decision table is the sum of all the distinct outputs.

    • < (min): the result of the decision table is the smallest value of all the outputs.

    • > (max): the result of the decision table is the largest value of all the outputs.

    • # (count): the result of the decision table is the number of distinct outputs.

Input and Output Expressions

The header of the decision table itself is divided into two sections; blue (on the left) and green (on the right). In the blue section are the input expressions; the green section are the output expressions.

Click on New Input and the following dialog is presented:

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Within an input expression, you can define the variable that is used in the expression of the rule input entries (explained below). It is possible to define multiple input expressions by selecting Add input (right click the option menu or by clicking the plus icon).

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Within an output expression, you can define what variable is created to form the result of a decision table execution (the output entry expression determines the value of the variable; explained below). It is possible to define multiple output expressions by selecting Add output (right click the option menu or by clicking the plus icon).

Rules

Each rule consists of one or more input entries and one or more output entries. An input entry is an expression that is evaluated against the input variable (of that 'column'). When all input entries are evaluated to be true, the rule is considered true, and the output entry is evaluated.

The DMN specification defines an expression language: (S)-FEEL. Currently, we do not support this part of the specification. Within Flowable DMN, we use Java Unified Expression Language (JUEL) as the expression language.

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To enter an expression, first select the condition that is applied to the input variable from the drop down box in the first column. Next double-click on the corresponding cell and enter BRONZE. Combined with the variable defined in the corresponding input expression (column header), this results at runtime in the full expression customerCat == "BRONZE".

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To enter an output expression, double-click the corresponding cell. In this example, the expression 5 entered. This is actually more like an implicit assignment. The value 5 is assigned to the variable in the corresponding output entry (column) when all input entries of that rule are evaluated true.

We can then continue completing the decision table by adding more rules (by selecting Add rule below).

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In our example, rule 4 has an empty input entry. The engine evaluates empty input entries as true. This means that if none of the other rules are valid, the outcome of rule 4 is the output of this decision table. In this case, variable discountPerc has the value 0.

The decision table can now be saved and linked to other process and case models as needed.