The Inclusive Gateway can be seen as a combination of an exclusive and a parallel gateway. Like an exclusive gateway, you can define conditions on outgoing sequence flows and the inclusive gateway will evaluate them. However, the main difference is that the inclusive gateway can receive more than one sequence flow, like a parallel gateway.

The functionality of the inclusive gateway is based on the incoming and outgoing sequence flows:

• fork: all outgoing sequence flow conditions are evaluated and for the sequence flow conditions that evaluate to ‘true’, the flows are followed in parallel, creating one concurrent execution for each sequence flow.
• join: all concurrent executions arriving at the inclusive gateway wait at the gateway until an execution has arrived for each of the incoming sequence flows that have a process token. This is an important difference to the parallel gateway. So in other words, the inclusive gateway will only wait for the incoming sequence flows that are executed. After the join, the process continues past the joining inclusive gateway.

Note that an inclusive gateway can have both fork and join behavior, if there are multiple incoming and outgoing sequence flows for the same inclusive gateway. In that case, the gateway will first join all incoming sequence flows that have a process token, before splitting into multiple concurrent paths of executions for the outgoing sequence flows that have a condition that evaluates to ‘true’.

Defining an inclusive gateway needs one line of XML:

<inclusiveGateway id="myInclusiveGateway" />

The actual behavior (fork, join or both) is defined by the sequence flows connected to the inclusive gateway. For example, the model above comes down to the following XML:

<startEvent id="theStart" />
<sequenceFlow id="flow1" sourceRef="theStart" targetRef="fork" />

<inclusiveGateway id="fork" />
<sequenceFlow sourceRef="fork" targetRef="receivePayment" >
<conditionExpression xsi:type="tFormalExpression">${paymentReceived == false}</conditionExpression>
</sequenceFlow>
<sequenceFlow sourceRef="fork" targetRef="shipOrder" >
<conditionExpression xsi:type="tFormalExpression">${shipOrder == true}</conditionExpression>
</sequenceFlow>

<userTask id="receivePayment" name="Receive Payment" />
<sequenceFlow sourceRef="receivePayment" targetRef="join" />

<userTask id="shipOrder" name="Ship Order" />
<sequenceFlow sourceRef="shipOrder" targetRef="join" />

<inclusiveGateway id="join" />
<sequenceFlow sourceRef="join" targetRef="archiveOrder" />

<userTask id="archiveOrder" name="Archive Order" />
<sequenceFlow sourceRef="archiveOrder" targetRef="theEnd" />

<endEvent id="theEnd" />

In the above example, after the process is started, two tasks are created if the process variables paymentReceived == false and shipOrder == true. In case only one of these conditions evaluates to true, only one task will be created. If no condition evaluates to true, an exception is thrown. This can be prevented by specifying a default outgoing sequence flow. In the following example one task will be created, the ship order task:

HashMap<String, Object> variableMap = new HashMap<String, Object>();
variableMap.put("receivedPayment", true);
variableMap.put("shipOrder", true);

ProcessInstance pi = runtimeService.startProcessInstanceByKey("forkJoin");

TaskQuery query = taskService.createTaskQuery()
                         .processInstanceId(pi.getId())
                         .orderByTaskName()
                         .asc();

List<Task> tasks = query.list();
assertEquals(1, tasks.size());

Task task = tasks.get(0);
assertEquals("Ship Order", task.getName());

When this task is completed, the second inclusive gateway joins the two executions and, since there is only one outgoing sequence flow, no concurrent paths of execution are created and only the Archive Order task is active.

Note that an inclusive gateway does not need to be ‘balanced’ (i.e., a matching number of incoming/outgoing sequence flows for corresponding inclusive gateways). An inclusive gateway will simply wait for all incoming sequence flows and create a concurrent path of execution for each outgoing sequence flow, not influenced by other constructs in the process model.

Camunda-specific behavior

Note that in Camunda’s implementation of the inclusive gateway, the following holds:

• If the join waits for a token, but that token takes a different turn in the process so that it can no longer reach the join (e.g. because of a boundary event interrupting the flow), then the join will not trigger.
• The join will trigger when:
  • it received a number of tokens greater or equal to the number of incoming sequence flows. The tokens do not necessarily need to reach the gateway through different sequence flows.
  • it received a number of tokens smaller than the number of incoming sequence flows and there are no more tokens that can arrive at the gateway.

The following examples show under which conditions an inclusive gateway will trigger a join:

1. In the following scenario, Parallel Gateway 1 creates three execution tokens, but only two sequence flows join in the inclusive gateway. In this scenario, the inclusive gateway will trigger even with only two tokens since the tokens from Task 1 and Task 2 were joined in a single token by Parallel Gateway 2.

2. In this scenario, Parallel Gateway 1 creates two execution tokens, and three sequence flows join in the inclusive gateway. In this scenario, the inclusive gateway will trigger with three tokens since Parallel Gateway 2 splits the single token from Task 1 into two separate tokens for Task 3 and Task 4.

3. In the diagram below, the parallel gateway creates two execution tokens. The first execution token will wait at User Task 1, and the second will reach the Inclusive Gateway. The Inclusive Gateway will trigger immediately for the first token, and a second time, for the second token, as both tokens arrive on the same sequence flow. As a result, there will be two instances of User Task 2 that will need to be completed.

4. In the last scenario, the parallel gateway creates two execution tokens. The first execution token will wait at User Task 1, and the second will reach the Inclusive Gateway 2 and wait for the gateway to trigger. However, the Inclusive Gateway 2 will not trigger a join until User Task 1 is completed and the second token arrives at the gateway. As a result, the Inclusive Gateway 2 will trigger only once instead of two times. According to the BPMN 2.0 specification, since both tokens pass the same sequence flow (true), the inclusive gateway should trigger twice. Finally, due to this behavior, only one instance of User Task 2 will need to be completed instead of the expected two. In cases like this one, it is recommended to use an Exclusive Gateway instead of the Inclusive Gateway 1.

Camunda Extensions

Additional Resources

• Inclusive Gateways in the BPMN 2.0 Modeling Reference
• Conditional and Default Sequence Flows