[[disabling-persistence-by-reachability]]
= Persistence by Reachability

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By default, JDO/DataNucleus supports the concept of link:http://www.datanucleus.org/products/datanucleus/jdo/persistence.html#persistence_by_reachability[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.

[WARNING]
====
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' xref:refguide:applib:index/util/ObjectContracts.adoc[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`:

[source,ini]
----
datanucleus.persistenceByReachabilityAtCommit=false
----

This change has been made to both the xref:docs:starters:helloworld.adoc[HelloWorld] and xref:docs:starters:simpleapp.adoc[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 (http://yuml.me/edit/b8681268[yuml.me/b8681268]):

image::runtime/configuring-datanucleus/disabling-persistence-by-reachability/party-agreementrole-agreement.png[width="750px"]



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

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

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

while ``Agreement``'s is implemented as:

[source,java]
----
public class Agreement {
    ...
    public int compareTo(Agreement other) {
        return ObjectContracts.compareTo(this, other, "reference");
    }
}
----

and ``Party``'s is similarly implemented as:

[source,java]
----
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.
