I have an existing Java Appengine standard project. I'm writing some new functionality using the beta NodeJS standard environment. The two need to share a common Cloud Datastore database.
Can a single project host both a Java and Node service? If not, how can I share the database between projects?
Yes, you can have any mix of standard and flexible environment services, written in any languages, see also:
You don't have to do anything special to share the datastore, it's automatically shared by all app's services.
But you may need to pay extra care to keep the datastore index definitions coherent across all services (the datastore index configuration is an app-level configuration, not a service-level one). See also: Do I need to define datastore-indexes in every microservice(module) which uses it or just in root application?