In Process Engine, I can restore the legacy behavior

AT

In the Process Engine, I can restore the DMN legacy behavior to keep my current DMN models as-is.

Scope

• Minimum: legacy mode can be turned on for all DMN models or none
• Maximum: legacy mode can be turned on/off per DMN model

Implementation outline

• Different factories for different versions of the engine (Legacy & Scala)
• Flag for enabling legacy behavior to generate the legacy engine or Scala engine
• DecisionInvocation uses the DmnEngine from the config
• ProcessEngineConfigurationImpl#initDmnEngine should look for a flag for legacy behavior and initialize the corresponding engine
• Maximum Scope
  • Gradual update of single models could be achieved by
    • Register two DMN deployers (legacy and Scala) that receive the deployment unit and need to decide based on the config if they parse the DMN deployment or not
    • Allow to configure which models to parse as legacy by either
      • Provide an engine config option with a list of DMN model keys that should be parsed as legacy OR
      • Allow to define a marker in the respective models to be parsed as legacy or not
    • Get the right engine for the model by either
      • introduce new getter in engine config “getDmnEngineByKey” that receives the engine respecting the DMN model key OR
      • change the current getter “getDmnEngine” to require a key parameter so people notice right away something changed in that behavior (it is private API)

See
https://docs.google.com/document/d/1fvp2qf75W7VBSFjfHVz3iRN4Ynq349R6qtq3eP62opw/edit#bookmark=id.w7tx54oviqyj