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3.8. Constraints

There are two types of constraints in business processes: code constraints and rule constraints.
  • Code constraints are boolean expressions evaluated directly whenever they are reached; these constraints are written in either Java or MVEL. Both Java and MVEL code constraints have direct access to the globals and variables defined in the process.
    Here is an example of a valid Java code constraint, person being a variable in the process: 
    return person.getAge() > 20;
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    Here is an example of a valid MVEL code constraint, person being a variable in the process: 
    return person.age > 20;
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  • Rule constraints are equal to normal Drools rule conditions. They use the Drools Rule Language syntax to express complex constraints. These rules can, like any other rule, refer to data in the working memory. They can also refer to globals directly. Here is an example of a valid rule constraint: 
    Person( age > 20 )
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    This tests for a person older than 20 in the working memory.
Rule constraints do not have direct access to variables defined inside the process. However, it is possible to refer to the current process instance inside a rule constraint by adding the process instance to the working memory and matching for the process instance in your rule constraint. Logic is included to make sure that a variable processInstance of type WorkflowProcessInstance will only match the current process instance and not other process instances in the working memory. Note, it is necessary to insert the process instance into the session. If it is necessary to update the process instance, use Java code or an on-entry, on-exit, or explicit action in the process. The following example of a rule constraint will search for a person with the same name as the value stored in the variable name of the process: 
processInstance : WorkflowProcessInstance()
Person( name == ( processInstance.getVariable("name") ) )
# add more constraints here ...
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