11.7.1 Exclusive Gateway

Exclusive Gateways (Decisions) are used to create alternative paths within a Process or a Choreography. For details of how Exclusive Gateways are used within an Orchestration Process see page 289.

Exclusive Gateways are used in Choreography, but they are constrained by the lack of a central mechanism to store the data that will be used in the Condition expressions of the Gateway’s outgoing Sequence Flows. Choreographies MAY contain natural language descriptions of the Gateway’s Conditions to document the alternative paths of the Choreography (e.g., “large orders” will go down one path while “small orders” will go down another path), but such Choreographies would be underspecified and would not be enforceable. To create an enforceable

Choreography, the Gateway Conditions MUST be formal Condition Expressions. However:

The data used for Gateway Conditions MUST have been in a Message that was sent prior to (upstream from) the Gateway.

More specifically, all Participants that are directly affected by the Gateway MUST have either sent or received the Message(s) that contained the data used in the Conditions.

Furthermore, all these Participants MUST have the same understanding of the data. That is, the actual values of the data cannot selectively change after a Participant has seen a Message. Changes to data during the course of the Choreography MUST be visible to all the Participants affected by the

Gateway.

These constraints ensure that the Participants in the Choreography understand the basis (the actual value of the data) for the decision behind the Gateway.