A sequence flow is the connector between two elements of a process. After an element is visited during process execution, all outgoing sequence flows are followed. This means that the default nature of BPMN 2.0 is to be parallel: two outgoing sequence flows will create two separate, parallel paths of execution.

Camunda Extensions

Conditional Sequence Flow

A sequence flow can have a condition defined on it. When a BPMN 2.0 activity is left, the default behavior is to evaluate the conditions on the outgoing sequence flows. When a condition evaluates to ‘true’, that outgoing sequence flow is selected. When multiple sequence flows are selected that way, multiple executions will be generated and the process is continued in a parallel way. Note: This is different for gateways. Gateways will handle sequence flows with conditions in specific ways, depending on the gateway type.

A conditional sequence flow is represented in XML as a regular sequence flow, containing a conditionExpression sub-element. Note that at the moment only tFormalExpressions are supported. Omitting the xsi:type=“” definition will simply default to this type of expression.

<sequenceFlow id="flow" sourceRef="theStart" targetRef="theTask">
  <conditionExpression xsi:type="tFormalExpression">
    <![CDATA[${order.price > 100 && order.price < 250}]]>
  </conditionExpression>
</sequenceFlow>

Currently, conditionalExpressions can be used with UEL and scripts. The expression or script used should resolve to a boolean value, otherwise an exception is thrown while evaluating the condition.

The example below references data of a process variable, in the typical JavaBean style through getters.

<conditionExpression xsi:type="tFormalExpression">
  <![CDATA[${order.price > 100 && order.price < 250}]]>
</conditionExpression>

This example invokes a method that resolves to a boolean value.

<conditionExpression xsi:type="tFormalExpression">
  <![CDATA[${order.isStandardOrder()}]]>
</conditionExpression>

In this example a simple groovy script is used to evaluate a process variable status.

<conditionExpression xsi:type="tFormalExpression" language="groovy">
  <![CDATA[status == 'complete']]>
</conditionExpression>

Similar to a script task, an external script resource can also be specified (see the documentation on script source for more information).

<conditionExpression xsi:type="tFormalExpression" language="groovy"
  camunda:resource="org/camunda/bpm/exampe/condition.groovy" />

Extensions for conditionExpression

Default Sequence Flow

All BPMN 2.0 tasks and gateways can have a default sequence flow. This sequence flow is only selected as the outgoing sequence flow for that activity if none of the other sequence flows could be selected. Conditions on a default sequence flow are always ignored.

A default sequence flow for a certain activity is defined by the default attribute on that activity. The following example shows an exclusive gateway with a default sequence flow. Only when x is neither 1 nor 2 it will be selected as outgoing sequence flow for the gateway.

Note the ‘slash’ marker at the beginning of the default sequence flow. The corresponding XML snippet shows how flow4 is configured as a default sequence flow.

<exclusiveGateway id="exclusiveGw" name="Exclusive Gateway" default="flow4" />

<sequenceFlow id="flow2" sourceRef="exclusiveGw" targetRef="theTask1" name="${x==1}">
  <conditionExpression xsi:type="tFormalExpression">${x == 1}</conditionExpression>
</sequenceFlow>

<sequenceFlow id="flow3" sourceRef="exclusiveGw" targetRef="theTask2" name="${x==2}">
  <conditionExpression xsi:type="tFormalExpression">${x == 2}</conditionExpression>
</sequenceFlow>

<sequenceFlow id="flow4" sourceRef="exclusiveGw" targetRef="theTask3" name="else">
</sequenceFlow>