Thursday, 27 October 2011

Neo4J - 2nd Look - Setting a Primary Key on Nodes

Primary Key

In my last post I considered the lack of Primary Key like Id's as something I need to solve. My use case is

The application I will be building out will have, after all is said and done a really simple Web Interface with REST type URLs. So .. for example, I will be able to do.
The 55 there will result in a query to Neo4J to locate the "MySpecialThing" object with ID of type 55 and display it.

I also considered using a UUID across objects which is also good, but not really what I was after. I want a class of objects to all share an identifier. It has a lot of use. To solve the problem I arrived at the following solution.

Solution Outline

  • All Domain Objects extend fro super type of AbstractLongDomain which has getId()/setId() (Long)
  • An Aspect wrapped around the getId() looks for a null value and if no Id is found. It creates one
  • In the aspect creation of an ID involves talking to a singleton IdManager for a "nextId()"
  • nextId() on the manager looks to it's cache HashMap to see if it has an IdObject that knows what the next Id is
  • IdObject self persists to the repository after each call (** this could be slow.. see how we go)

To the Code

All my Objects extend the AbstractLongDomain The ID Object holds a "per" class Long Id, so each time an ID is needed, one of these objects gives one out. Next we have the IdManager that is managed as a Spring Singleton Bean. Its job is to return an id based on the "class" that needs an Id via getNextId(Class klass). The idRepository you see there is a simple Spring Data Neo4J Repository which has aspect-magic dust sprinkled on it to make the actual implementation.

Because I am not sure if neo4j is the "best" place to store the Id's (though it is the logical) I created a simple idGenerator interface which is simply what the aspect will call and talk to. One implementation (the only) is the Neo4JBackedIdGenerator which uses the id objects and idmanager above.

So first the interface for the IdGenerator

And then the actual Neo4JBackedIdGenerator which is created and managed as a spring bean.

I will have to play with the transactional semantics on this one. I recall a horrid situation which a similar design but via stored procs many moons ago where we had the sproc that generate Id's wrapped in transactions. They needed to be in their own transaction to ensure that mass object thread creation would not get stuck on a lock. (just an area I know can be sticky and bite.. so I put the @Transactional in there and commented out to remind me.

So last, we have the AspectJ which wraps our getId(). All the domain objects extend org.soqqo.luap.model.AbstractLongDomain which means we will get the Id creation and generation for free each time getId is called. (technically a catch here is that setId doesn't get checked if called manually on setting an Id. It could I guess look into the repo to see if the Id is already used.

And finally the Unit Test code to show that it all works

Note the use of @DirtiesContext, because the Neo4J is transactional, after each test the contents are dumped, which means that the idManager which has the HashMap cache becomes stale and it is singleton and has a lifecycle of the test class, not just the method. So the fix is either..manually flush() the cache or use @DirtiesContext which tells spring to re-build the context file. Both work but manually flushing my HashMap (new() ) is 2 seconds faster (0.037s for the test) than spring is at rebuilding.

The 2nd last piece to show is my test context file - model-test-context.xml

The very last piece is what my Maven POM looks like because a lot of people like to see that... Hopefully these are the correct relevant bits. I am happy to provide all this as a ZIP or push it up to github if people want to see more of it.

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