A.8. Gateways

A.8.1. Gateways

“Gateways are used to control how Sequence Flows interact as they converge and diverge within a Process.[4]”

Gateways are used to create or synchronize branches in the workflow using a set of conditions which is called the gating mechanism. Gateways are either converging (multiple Flows into one Flow) or diverging (One Flow into multiple Flows).

One Gateway cannot have multiple incoming and multiple outgoing Flows.

Depending on the gating mechanism you want to apply, you can use the following types of gateways:

  • Parallel (AND): in a converging gateway, waits for all incoming Flows. In a diverging gateway, takes all outgoing Flows simultaneously.
  • Inclusive (OR): in a converging gateway, waits for all incoming Flows whose condition evaluates to true. In a diverging gateway takes all outgoing Flows whose condition evaluates to true.
  • Exclusive (XOR): in a converging gateway, only the first incoming Flow whose condition evaluates to true is chosen. In a diverging gateway only one outgoing Flow is chosen.
  • Event-based: used only in diverging gateways for reacting to events. See Section A.8.2.1, “Event-based Gateway”.
  • Data-based Exclusive: used in both diverging and converging gateways to make decisions based on available data. See Section A.8.2.4, “Data-based Exclusive Gateway”.

A.8.2. Gateway types

A.8.2.1. Event-based Gateway

“The Event-Based Gateway has pass-through semantics for a set of incoming branches (merging behavior). Exactly one of the outgoing branches is activated afterwards (branching behavior), depending on which of Events of the Gateway configuration is first triggered. [5]”

The Gateway is only diverging and allows you to react to possible Events as opposed to the Data-based Exclusive Gateway, which reacts to the process data. It is the Event that actually occurs that decides which outgoing Flow is taken. As it provides the mechanism to react to exactly one of the possible Events, it is exclusive, that is, only one outgoing Flow is taken.

The Gateway might act as a Start Event, where the process is instantiated only if one the Intermediate Events connected to the Event-Based Gateway occurs.

A.8.2.2. Parallel Gateway

“A Parallel Gateway is used to synchronize (combine) parallel flows and to create parallel flows.[6]”

Diverging
Once the incoming Flow is taken, all outgoing Flows are taken simultaneously.
Converging
The Gateway waits until all incoming Flows have entered and only then triggers the outgoing Flow.

A.8.2.3. Inclusive Gateway

Diverging
Once the incoming Flow is taken, all outgoing Flows whose condition evaluates to true are taken. Connections with lower priority numbers are triggered before triggering higher priority ones; priorities are evaluated but the BPMN2 specification doesn’t guarantee this. So for portability reasons it is recommended that you do not depend on this.
Important

Make sure that at least one of the outgoing Flow evaluates to true at runtime; otherwise, the process instance terminates with a runtime exception.

Converging
The Gateway merges all incoming Flows previously created by a diverging Inclusive Gateway; that is, it serves as a synchronizing entry point for the Inclusive Gateway branches.

Attributes

Default gate
The outgoing Flow taken by default if no other Flow can be taken

A.8.2.4. Data-based Exclusive Gateway

Diverging
The Gateway triggers exactly one outgoing Flow: the Flow with the constraint evaluated to true and the lowest Priority is taken. After evaluating the constraints that are linked to the outgoing Flows: the constraint with the lowest priority number that evaluates to true is selected.
Possible runtime exception

Make sure that at least one of the outgoing Flows evaluates to true at runtime: if no Flow can be taken, the execution returns a runtime exception.

Converging
The Gateway allows a workflow branch to continue to its outgoing Flow as soon as it reaches the Gateway; that is, whenever on of the incoming Flows triggers the Gateway, the workflow is sent to the outgoing Flow of the Gateway; if it is triggered from more than one incoming connection, it triggers the next node for each trigger.

Attributes

Default gate
The outgoing Flow taken by default if no other Flow can be taken

[4] Business Process Model and Notation (BPMN). Version 2.0, OMG Document Number: formal/2011-01-03 http://www.omg.org/spec/BPMN/2.0
[5] Business Process Model and Notation (BPMN). Version 2.0, OMG Document Number: formal/2011-01-03 http://www.omg.org/spec/BPMN/2.0
[6] Business Process Model and Notation (BPMN). Version 2.0, OMG Document Number: formal/2011-01-03 http://www.omg.org/spec/BPMN/2.0