Precondition 
Preconditions help in reusing business rules.
As you create business rules, many rules may have common conditions and actions. You can define a rule with the common condition and use it as a precondition to other rules. This way maintaining the rules becomes easy.
For example, Rule A must be satisfied is a precondition to Rule B; Rule B is satisfied only when conditions in both rule A and rule B are satisfied.
A rule can have more than one precondition rule, in which case the rule is satisfied only if all the precondition rules are satisfied.
A rule used as a precondition may also contain actions. In such a case, when the precondition rule is satisfied, the actions in the precondition rule will be fired.
Example
Rule D has a precondition Rule C must not be satisfied. This implies that if Rule C is satisfied, Rule D will not be satisfied.
Typical Uses
When many rules have the same conditions, it is advisable to declare a rule that contains the common conditions and use it as a precondition in all the rules. This makes changing and maintaining rules easier.
Example
Rule A |
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|---|---|
If |
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Condition 1 |
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and Condition 2 |
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and Condition 3 |
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Then |
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... |
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Rule B |
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|---|---|
If |
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Condition 1 |
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and Condition 2 |
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and Condition 4 |
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Then |
|
... |
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You can re-organize these rules as:
Pre Rule |
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If |
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Condition 1 |
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and Condition 2 |
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Then |
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... |
Rule A |
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|---|---|
Precondition Pre Rule |
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If |
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and Condition 3 |
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Then |
|
... |
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Rule B |
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|---|---|
Precondition Pre Rule |
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If |
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and Condition 4 |
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Then |
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... |
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You can see that the common conditions have been placed in a precondition rule, making it easier to change and manage these rules.