Explaining Gateways

Gateways are used in Business Process Model and Notation (BPMN) to control the direction(s) of the flow throughout the process. Gateways are represented as diamond figures in BPMN notation.

A gateway is a type of flow object in the process that shows a merge or a split of a process flow. Create a gateway to indicate that a decision must be made or that the process flow must be split or merged depending on the given conditions.

Gateway Type	Symbol	Properties
Exclusive Choice		Only one path can be taken. Each Gate has a Boolean data expression.
Parallel Split (also AND gateway)		Splits sequence flow in parallel stream.
Event Based Choice		Only one path can be taken. Decision is based on events.
Uncontrolled Merge		Merge exclusive alternative incoming sequence flows to one outgoing sequence flow.
Parallel Join		Merge parallel alternative in-coming sequence flows to one outgoing sequence flow.
Inclusive Choice		Offers the possibility, that more than one path can be taken.

The exclusive split gateway allows the control flow to take one path dependent on the conditions that are defined. Multiple paths can leave from an exclusive split gateway. Although not mandatory in all cases, divergent control paths from exclusive split gateways should converge at uncontrolled merge gateways.

Every exclusive split gateway has a default path that has no conditions set. Given conditions must return a boolean value. If the conditions for the other paths are not met, then the control flow follows the default path. The default path is notated in the business process with a slash mark.

With the parallel split gateway, you can create a parallel flow. You do not need to use the parallel split gateway for parallel flow but with this gateway, you can clarify the behavior of complex situations where a parallel flow is required. Each gate of the parallel split gateway must have an associated sequence flow. You cannot define any conditions for this gateway or the sequence flows and must use a parallel join to merge them together later in the process.

Remember the memory consumption when updating the same data object at the same time as you might encounter locks, poor performance and data inconsistencies.

A parallel split gateway ensures that all paths diverging out of the split must be taken. The parallel join gateway ensures that all paths leaving the gateway are taken. All paths eventually meet at the parallel join gateway where they merge back into one flow. The parallel join gateway will not allow a process to continue until all paths arrive at the parallel join gateway. The figure shows an example of the use of a parallel split and a parallel join.

The figure shows an example of the use of a parallel split and a parallel join.

The event-based choice gateway controls the flow direction based on events. An event-based choice gateway is immediately followed by an intermediate timer event and an intermediate message event. The event that is triggered first is the path that is followed by the process flow. The event-based choice gateway only controls flow direction. There is no configuration specific to the event-based choice; only the intermediate events that follow the event-based choice gateways are configured to allow the activity or process to continue once all conditions are met.

The inclusive choice gateway controls the sequence flow between activities. Similar to the exclusive choice gateway, it is also like a question in the process with a defined set of answers. But unlike the exclusive choice gateway, the inclusive choice gateway offers the possibility that more than one answer is chosen - the process can take more than one sequence flow out of the inclusive choice gateway. The inclusive choice gateway has a number of gates that are potential paths out of the gateway. Each gate has a condition based on a boolean expression. The expressions use the values of process data to determine which path has to be taken. Each gate has an outgoing sequence flow.

The figure illustrates an example of an Inclusive Choice Gateway with an Uncontrolled Merge. Note: the highlighted area indents the default path.