why we should have integration layer and data layer separately?
your data layer may have multiple data sources and / or your data sources for a data model can change over time from one source to another. If you have the data layer in the same rulesets as the integration layer, it is harder to swap that out. When deploying a ruleset, all the rules within that ruleset are deployed. If you separate these, then you have more flexibility in deploying both the data model, which may have a different lifecycle than the integrations, and those integrations.
Hope this helps…
Thomas
The other main benefit is you can design your case type data model to be composed of the objects in your data layer that are defined specifically with your application and business needs in mind, completely independent of how and where that data is stored.
For example, a Loan Application case type has an “Applicant” page property of type “Customer”. By defining the Customer data type in the data layer according to the needs of the application (and future needs of the business), future changes to how and where the Customer data is stored affect only the integration layer.
Mapping rules between the integration layer and the corresponding data objects that serve the application help assure that the instances of Customer data served up to the application behave as they were always intended. Your application is less at risk of breaking when changes to where and how Customer data are stored inevitably come.