Hi, Neo4j today is made up of a core service that persists nodes, relationships and properties. In addition to that there are a number of additional services, where index-util is probably the most used and most important at the moment. index-util is also a good example of the problem I would like to discuss in this email.
With the current architecture there is no way of introspecting which additional services have been initialized with a particular Neo store, for example there is no way of telling which (if any) IndexService has been used. This causes problems with the transaction recovery process, where the recovery mechanism today has to know about all possible extension services without having a compile time dependency on them. Needless to say this causes a mess. The code for this is ugly and borderline-buggy. It also causes a problem when introspecting a store, or even restarting your software, since it is up to the programmer to remember to restart all of the same services as last time. I would like to propose an addition to the store, in a separate metadata file in the store directory, where we store a simple list of all started additional services. Does anyone have any suggestions to what this should look like to be reasonable future proof (i.e. be able to handle some service that is not implemented yet as well as the current index-utils). What comes to mind is something similar to the Java ServiceLoader API [1], but simplified with the fact that we can require all classes referenced in the file to implement one specific interface, and we know where the file will be, in effect taking out all the complications involving ClassLoaders. What I'm worrying about is that storing Java classnames in a file will make this metadata unusable to any future implementations of the NeoStore format outside of the Java platform. Any suggestions to how information about loaded services can be stored is welcome, my idea of a clear text file might not even be good (since people have a tendency to think they can patch those manually). Since we have a community of smart and entrepreneurial individuals I thought I'd throw the question out here and see if I got any good responses. Cheers, Tobias [1] http://java.sun.com/javase/6/docs/api/java/util/ServiceLoader.html Simple explanation of how it works: * You have an interface with fully qualified classname: com.somedomain.someproduct.ServiceInterface * In each jar that provides implementations of the ServiceInterface you add a file called META-INF/services/com.somedomain.someproduct.ServiceInterface * In this file you put the fully qualified classnames of each implementing class separated by newline * In your code you do: ServiceLoader<ServiceInterface> impls = ServiceLoader.load(ServiceInterface.class); -- Tobias Ivarsson <tobias.ivars...@neotechnology.com> Hacker, Neo Technology www.neotechnology.com Cellphone: +46 706 534857 _______________________________________________ Neo mailing list User@lists.neo4j.org https://lists.neo4j.org/mailman/listinfo/user
