Member AbstractMachine::exportDotIdentifier ($str, $prefix= 's_'): Extract Graphviz library to keep graphs and parts of the graphs contained.
Member AbstractMachine::getView ($id, $view, &$properties_cache=null, &$view_cache=null, &$persistent_view_cache=null): Override or call handlers?
Member AbstractMachine::isTransitionAllowed (Reference $ref, $transition_name, $state=null): Transition should not have full definition of the policy, only its name. Definitions should be in common place.
Member AbstractMachine::performSelfCheck (): Extend this method with more checks.
Class FlupdoBackend: Make this dumb and provide it from somewhere else.
Member FlupdoBackend::createQueryBuilder ($type): This should be machine listing, not general query builder.
Member FlupdoMachine::$user_id_auth_method: Review this.
Member FlupdoMachine::checkAccessPolicy ($access_policy_name, Reference $ref): Caching ? Reference object has property cache. It would be nice to pass it here.
Member FlupdoMachine::queryAddPropertiesSelect ($query): Skip some properties in lisings.
Member JsonDirBackend::createListing ($filters, $filtering_flags=0): : Support complex filtering over multiple machine types and make 'type' filter optional.
Member Reference::__construct (AbstractMachine $machine, $id=null): Check $id to be made of scalar values only.
Class Utils: Refactor this to standalone classes or libraries.