Hit Policies

The hit policy controls how the decision table handles multiple matching rules. Set it on the <decisionTable> element via the hitPolicy attribute.

UNIQUE

Exactly one rule must match. If zero rules match, or more than one rule matches, the engine throws an error.

Use when your rules are exhaustive and mutually exclusive — every valid input has exactly one answer.

<decisionTable hitPolicy="UNIQUE">

When to use: Well-defined classification rules where overlap is a modeling mistake.

FIRST

The first matching rule wins. Rules are evaluated in the order they appear in the XML (top to bottom in the table UI). Only the first match’s output is returned.

Use when you want priority-by-position: put the most specific rules at the top and a catch-all at the bottom.

<decisionTable hitPolicy="FIRST">

When to use: Tiered pricing, fallback logic, override hierarchies.

ANY

All matching rules must agree on the output value. If they disagree, the engine throws an error. If they agree, that value is returned.

Use when rules may overlap by design but must all produce the same answer (consistency check).

<decisionTable hitPolicy="ANY">

When to use: Redundant rules that encode the same policy from different angles — disagreement signals a modeling error.

COLLECT

All matching rules are collected. Returns a list of outputs. Supports optional aggregation operators:

<decisionTable hitPolicy="COLLECT">   <!-- list -->
<decisionTable hitPolicy="COLLECT" aggregation="SUM">

When to use: Fee calculations (add all applicable fees), validation (collect all failing rule messages).

RULE ORDER

All matching rules are returned in the order they appear in the table. Returns a list.

Similar to COLLECT but explicitly preserves declaration order without aggregation.

<decisionTable hitPolicy="RULE ORDER">

When to use: Ordered recommendation lists, ranked alternatives.

PRIORITY

The rule with the highest-priority output wins. Priority is determined by the output column’s allowed values list — values listed earlier have higher priority.

<decisionTable hitPolicy="PRIORITY">
  <output name="level" typeRef="string">
    <outputValues>
      <text>"critical","high","medium","low"</text>
    </outputValues>
  </output>

In the above, "critical" has the highest priority. Among all matching rules, the one whose output appears earliest in the list wins.

When to use: Severity classification, alert prioritization.

OUTPUT ORDER

All matches sorted by output priority. Returns a list ordered by the output column’s priority list (same as PRIORITY, but returns all matches instead of just the winner).

<decisionTable hitPolicy="OUTPUT ORDER">

When to use: Generating a ranked list of options where order matters.

Choosing a Hit Policy