Business rule tasks
A business rule task is used to model the evaluation of a business rule; for example, a decision modeled in Decision Model and Notation (DMN).
Camunda 8 supports alternative task implementations for the business rule task. If you want to use your own implementation for a business rule task, see the job worker implementation section below. The sections before this job worker implementation apply to the DMN decision implementation only.
If you only want to evaluate a DMN decision, you can use the
EvaluateDecision API.
When the process instance arrives at a business rule task, a decision is evaluated using the internal DMN decision engine. Once the decision is made, the process instance continues.
If the decision evaluation is unsuccessful, an incident is raised at the business rule task. When the incident is resolved, the decision is evaluated again.
Defining a called decision
A called decision links the business rule task to a DMN decision, either to
a decision table or to
a decision literal expression
. It can be defined using the
zeebe:calledDecision extension element.
A business rule task must define the DMN decision id of the
called decision as decisionId. Usually, the decisionId is defined as a static value (e.g. shipping_box_size), but
it can also be defined as an expression (
e.g. = "shipping_box_size_" + countryCode). The expression is evaluated on activating the business rule task (or when
an incident at the business rule task is resolved) after input mappings have been applied. The expression must result in
a string.
A business rule task must define the process variable name of the decision result as
resultVariable. The result of the decision is stored in this variable. The resultVariable
is defined as a static value.
Variable mappings
By default, the variable defined by resultVariable is merged into the process instance. This
behavior can be customized by defining an output mapping at the business rule task.
All variables in scope of the business rule task are available to the decision engine when the decision is evaluated. Input mappings can be used to transform the variables into a format accepted by the decision.
Input mappings are applied on activating the business rule task (or when an incident at the business rule task is resolved), before the decision evaluation. When an incident is resolved at the business rule task, the input mappings are applied again before evaluating the decision. This can affect the result of the decision.
For more information about this topic, visit the documentation about input/output variable mappings.
Job worker implementation
A business rule task does not have to evaluate a decision modeled with DMN. Instead, you can also use job workers to implement your business rule task.
A job worker implementation can be defined using the zeebe:taskDefinition extension element.
Business rule tasks with a job worker implementation behave exactly like service tasks. The differences between these task types are the visual representation (i.e. the task marker) and the semantics for the model.
When a process instance enters a business rule task with alternative task implementation, it creates a corresponding job and waits for its completion. A job worker should request jobs of this job type and process them. When the job is completed, the process instance continues.
A business rule task must define a job type the same way as a service task does. This is used as reference to specify which job workers request the respective business rule task job. For example, order-items. Note that type can be specified as any static value (myType) or as a FEEL expression prefixed by = that evaluates to any FEEL string; for example, = "order-" + priorityGroup.
Use task headers to pass static parameters to the job worker (e.g. the key of the decision to evaluate).
Define variable mappings the same way as a service task does to transform the variables passed to the job worker, or to customize how the variables of the job merge.
Additional resources
XML representation
A business rule task with a called decision:
<bpmn:businessRuleTask id="determine-box-size" name="Determine shipping box size">
<bpmn:extensionElements>
<zeebe:calledDecision decisionId="shipping_box_size" resultVariable="boxSize" />
</bpmn:extensionElements>
</bpmn:businessRuleTask>
A business rule task with a job worker implementation and a custom header:
<bpmn:businessRuleTask id="calculate-risk" name="Calculate risk">
<bpmn:extensionElements>
<zeebe:taskDefinition type="calculate_risk" />
<zeebe:taskHeaders>
<zeebe:header key="decisionRef" value="risk" />
</zeebe:taskHeaders>
</bpmn:extensionElements>
</bpmn:businessRuleTask>