Spring Boot service-level ID counter conflicts with MongoDB persisted documents on application restart

Context I have a Spring Boot application with three interchangeable persistence layers: in-memory, PostgreSQL, and MongoDB. The active persistence is chosen via app.storage in application.properties . My TreeService manages node creation with a simple in-memory ID counter: java @Service public class TreeService { private final TreeRepository repo; private final TreeAlgorithmStrategy strategy; private Long idCounter = 1L; public Node createRoot(String value) { Node root = new Node(idCounter++, value, null); return repo.save(root); } public Node addChild(Long parentId, String value) { Node child