Switch default value of Neo4j open session in view

[michael-simons]

The current default of "open session in view" for Spring Data Neo4j hurts performance in cluster scenarios.

By opening the session on the request, the session will be a general purpose session ("read write") and always go to the leader of a cluster, leaving all followers and read replicas unused. I have written down an analysis of the problem here Spring Data Neo4j, Neo4j-OGM and OSIV.

Neo4j-OGM doesn't have the issue of lazy loading and keeping the session up and running through the request only helps caching inside OGM.

If such a change is unappropriated for 2.3, I would like to see a better warning, for example

logger.warn("spring.data.neo4j.open-in-view is enabled by default."
    + "Therefore, database queries may be performed during view "
    + "rendering. It also prevent's chosing anything else than the leader in a cluster scenario. Explicitly configure "
   + "spring.data.neo4j.open-in-view to disable this warning");

Or, the best might even check if one or more bolt+routing respectively neo4j:// Uris have been configured and than according to that, choose a sane default.

So, I basically see three options:

• Change the warning to be more drastic or
• Change the default or
• Change the default conditionally when there's one or more bolt+routing:// or neo4j:// uris or more than one bolt:// URI.

I could implement each one of them, but wanted to hear your opinion on that.

[snicoll]

@michael-simons thanks for the report and such a change for the reason you've described seems appropriate to me.

I don't really have an opinion besides the fact that the last option makes it harder to resonate what the default value of the flag is. If we keep a boolean flag, I'd rather keep a straight true/false rather than true if the transport is configured in a certain way.

My vote would go for #2. Flagging for team attention to see what the rest of the team thinks.

[michael-simons]

Completely agree with you on this one.

[wilkinsona]

Neo4j-OGM doesn't have the issue of lazy loading

In that case, +1 for changing the default. The pattern's only really of value when it reduces the learning curve of lazy loading problems. Without that problem to solve, it feels like a bad default to me.