Regarding your current error that errors occurs when you are missing the JDBC driver on your classpath. The error is misleading due do an issue within Spring or Spring Boot. There is spring-projects/spring-boot#13222 issue logged for Boot.
Which driver do you use? How did you add the library?
I would love to use these servers, but I haven’t figured out a way to add my own JavaDelegate subclasses.
Why not build your own Spring Boot application with your own JavaDelegates and use the flowable-spring-boot-starter-rest? That would create the same dependency war / jar as the flowable-rest.war
A description of how the various modules (wars) interact. I suspect that they all interact ONLY thru the database. Is that true?
Yes they interact through the database. However, for logging in to the flowable-idm.war, flowable-task.war, flowable-admin.war and flowable-modeler.war they use the flowable-idm.war as a SSO server.
The flowable-modeler.war also has a publish button that publishes the application over REST to the configured rest endpoint.
The flowable-admin.war actually uses REST to fetch the information for the display.
All this means that you can use your own custom rest application and configure the flowable-modeler.war and flowable-admin.war to talk with it.
The flowable-task.war has the UI and an engine. It and the flowable-rest.war are actually the same, so you should not use it if you are adding custom JavaDelegates and build your own REST application.