Persistence by Reachability

By default, JDO/DataNucleus supports the concept of persistence-by-reachability. That is, if a non-persistent entity is associated with an already-persistent entity, then DataNucleus will detect this and will automatically persist the associated object. Put another way: there is no need to call Apache Causeway' RepositoryService#persist(.) or RepositoryService#persistAndFlush(.) methods.

However, convenient though this feature is, you may find that it causes performance issues.

DataNucleus' persistence-by-reachability may cause performance issues. We strongly recommend that you disable it.

One scenario in particular where this performance issues can arise is if your entities implement the java.lang.Comparable interface, and you have used Apache Causeway' ObjectContracts utility class. The issue here is that ObjectContracts implementation can cause DataNucleus to recursively rehydrate a larger number of associated entities. (More detail below).

We therefore recommend that you disable persistence-by-reachability by adding the following to application.properties:

datanucleus.persistenceByReachabilityAtCommit=false

This change has been made to both the HelloWorld and SimpleApp starter apps.

If you do disable this feature, then you will (of course) need to ensure that you explicitly persist all entities using the RepositoryService#persist(.) or RepositoryService#persistAndFlush(.) methods.

The issue in more detail

Consider these entities (yuml.me/b8681268):

In the course of a transaction, the Agreement entity is loaded into memory (not necessarily modified), and then new AgreementRoles are associated to it.

All these entities implement Comparable using ObjectContracts, and the implementation of AgreementRole's (simplified) is:

public class AgreementRole {
    ...
    public int compareTo(AgreementRole other) {
        return ObjectContracts.compareTo(this, other, "agreement","startDate","party");
    }
}

while Agreement's is implemented as:

public class Agreement {
    ...
    public int compareTo(Agreement other) {
        return ObjectContracts.compareTo(this, other, "reference");
    }
}

and Party's is similarly implemented as:

public class Party {
    ...
    public int compareTo(Party other) {
        return ObjectContracts.compareTo(this, other, "reference");
    }
}

DataNucleus’s persistence-by-reachability algorithm adds the AgreementRole instances into a SortedSet, which causes AgreementRole#compareTo() to fire:

the evaluation of the "agreement" property delegates back to the Agreement, whose own Agreement#compareTo() uses the scalar reference property. As the Agreement is already in-memory, this does not trigger any further database queries
the evaluation of the "startDate" property is just a scalar property of the AgreementRole, so will already in-memory
the evaluation of the "party" property delegates back to the Party, whose own Party#compareTo() requires the uses the scalar reference property. However, since the Party is not yet in-memory, using the reference property triggers a database query to "rehydrate" the Party instance.

In other words, in figuring out whether AgreementRole requires the persistence-by-reachability algorithm to run, it causes the adjacent associated entity Party to also be retrieved.