Decoupling Pattern 6: External Rule [editor]

This pattern uses External rules to call a web service that is defined in another project. The great thing of this pattern is that it is triggered by the rule engine and not the flow.

Characteristics

Type	Description
Implicit vs. Explicit call	This is an implicit call that is triggered by the rule engine each time when an input value changes.
User set vs. System set	The values that are set by an external rule are system set.
Input and Output parameters vs. Complex model	The implementation of an external rule has access to the complete profile. It is therefore important to document what values are needed by the external rule. You can set what values should trigger the external rule mechanism. Most likely are those identical to the input values. An external rule can be used as default on an attribute, but in general it could change the value of any attribute. There are no additional attributes required in your domain, keeping it clean.
Simple Parameters vs. Complex Instances	The complete profile can be used as input, but the Action attributes of the External rules are most likely to be the input parameters.
Internal vs. External	This type of decoupling is mainly for internal and external use. As the function is in a web service, any party could call it.
Automated vs. User Interaction	This type of decoupling is only for automatic tasks.
Complexity of the Decoupling pattern vs Gain	External rules are a standard element in studio. Creating the external rule that calls a web service is not very complex.
Out-of-the-box vs. Custom Code	A new external rule that calls a specific web service has to be created by a developer.
Part of Application vs. Separate Deployment	The decoupled function is partly in the model, and partly in code. You should make sure that the code is deployed together with the project.
Maintainability	This pattern does not clutter your domain with unnecessary attributes, which is an advantage. A disadvantage is that it is not clear to the business engineer what the external rule exactly does. That is all hidden away in the service. For example, you can not use the dependency function in studio to find which external rule might change its value if it is not placed as default value on the attribute.