DataNucleus Object Store

Apache Isis documentation is broken out into a number of user and reference guides.

The user guides available are:

The reference guides are:

The remaining guides are:

These configuration properties are typically stored in WEB-INF/persistor_datanucleus.properties. However, you can place all configuration properties into WEB-INF/isis.properties if you wish (the configuration properties from all config files are merged together).

DataNucleus will for itself also and read the META-INF/persistence.xml. In theory it can hold mappings and even connection strings. However, with Apache Isis we tend to use annotations instead and externalize connection strings. so its definition is extremely simply, specifying just the name of the "persistence unit".

Here’s the one provided by the SimpleApp archetype:

Normally all one needs to do is to change the persistence-unit name.

See DataNucleus' documentation on persistence.xml to learn more.

Both Apache Isis and DataNucleus have their own metamodels of the domain entities. Apache Isis builds its metamodel by walking the graph of types of the domain services. The JDO/DataNucleus objectstore then takes these types and registers them with DataNucleus.

In some cases, though, not every entity type is discoverable from the API of the service actions. This is especially the case if you have lots of subtypes (where the action method specifies only the supertype). In such cases the Isis and JDO metamodels is built lazily, when an instance of that (sub)type is first encountered.

Apache Isis is quite happy for the metamodel to be lazily created, and - to be fair - DataNucleus also works well in most cases. In some cases, though, we have found that the JDBC driver (eg HSQLDB) will deadlock if DataNucleus tries to submit some DDL (for a lazily discovered type) intermingled with DML (for updating). In any case, it’s probably not good practice to have DataNucleus work this way.

The framework thus provide mechanisms to search for all @PersistenceCapable entities under specified package(s), and registers them all eagerly. In fact there are two:

Further discussion on specifying the package(s) in integration testing (for either approach) can be found in the user guide.

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 Isis' RepositoryService#persist(.) or RepositoryService#persistAndFlush(.) methods.

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

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 Isis' 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 persistor_datanucleus.properties:

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

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.

Isis' JDO objectstore can be configured either to connect to the database using its own connection pool, or by using a container-managed datasource.

Consider a bidirectional one-to-many association between two entities; a collection member in the "parent" and a property member on the "child".

We can tell DataNucleus about the bidirectionality using @Persistent(mappedBy=…​), or we can take responsibility for this aspect ourselves.

In addition, the two entities can be associated either without or with a join table (indicated by the @Join annotation):

Testing (as of 1.13.0, against dn-core 4.1.7/dn-rdbms 4.1.9) has determined there are two main rules:

In the examples that follow, we use two entities, Batch and FermentationVessel (from a brewery domain). In the original example domain the relationship between these two entities was optional (a FermentationVessel may have either none or one Batch associated with it); for the purpose of this article we’ll explore both mandatory and optional associations.

If you have a hierarchy of classes then you need to decide which inheritance strategy to use.

In the first "rollup" case, we can have a situation where - logically speaking - the property is mandatory in the subtype - but it must be mapped as nullable in the database because it is n/a for any other subtypes that are rolled up.

In this situation we must tell JDO that the column is optional, but to Apache Isis we want to enforce it being mandatory. This can be done using the @Property(optionality=Optionality.MANDATORY) annotation.

For example:

JDO/DataNucleus supports the ability to map the entity that is mapped to a view rather than a database table. Moreover, DataNucleus itself can create/maintain this view.

One use case for this is to support use cases which act upon aggregate information. An example is in the (non-ASF) Estatio application, which uses a view to define an "invoice run": a representatoin of all pending invoices to be sent out for a particular shopping centre. (Note that example also shows the entity as being "non-durable", but if the view is read/write then — I think — that this isn’t necessary required).

For more on this topic, see the DataNucleus documentation.

JDO/DataNucleus does not automatically create these schema objects, but it does provide a listener callback API on the initialization of each class into the JDO metamodel.

Therefore Apache Isis attaches a listener, CreateSchemaObjectFromClassMetadata, that checks for the schema’s existence, and creates the schema if required.

The guts of its implementation is:

where MetaDataListener is the DataNucleus listener API:

Although not formal API, the default CreateSchemaObjectFromClassMetadata has been designed to be easily overrideable if you need to tweak it to support other RDBMS'. Any implementation must implement org.datanucleus.metadata.MetaDataListener:

The implementation provided has has been tested for HSQLDB, PostgreSQL and MS SQL Server, and is used automatically unless an alternative implementation is specified (as described in the section below).

An alternative implementation can be registered and used through the following configuration property:

Because this pertains to the JDO Objectstore we suggest you put this configuration property in WEB-INF/persistor_datanucleus.properties; but putting it in isis.properties will also work.

Any implementation must implement org.datanucleus.metadata.MetaDataListener. In many cases simply subclassing from CreateSchemaObjectFromClassMetadata and overriding buildSqlToCheck(…​) and buildSqlToExec(…​) should suffice.

If you do need more control, your implementation can also optionally implement org.apache.isis.objectstore.jdo.datanucleus.DataNucleusPropertiesAware:

This provides access to the properties passed through to JDO/DataNucleus.

The JDO Objectstore (or rather, the underlying DataNucleus implementation) builds its own persistence metamodel by reading both annotations on the class and also by searching for metadata in XML files. The metadata in the XML files takes precedence over the annotations, and so can be used to override metadata that is "hard-coded" in annotations.

In fact, JDO/DataNucleus provides two different XML files that have slightly different purposes and capabilities:

If you want to use the first approach (the .jdo file), you’ll find that you can download the effective XML representation of domain entities using the downloadJdoMetadata mixin action available in prototyping mode. This then needs to be renamed and placed in the appropriate location on the classpath; see the DataNucleus documentation for details.

However, using this first approach does create a maintenance effort; if the domain entity’s class structure changes over time, then the XML metadata file will need to be updated.

The second approach (using an .orm file) is therefore often more useful than the first, because the metadata provided overrides rather than replaces the annotations (and annotations not overridden continue to be honoured).

A typical use case is to change the database schema for an entity. For example, as of 1.9.0 the various (non-ASF) Incode Platform modules use schemas for each entity. For example, the AuditEntry entity in the (non-ASF) Incode Platform's audit module is annotated as:

This will map the AuditEntry class to a table "IsisAddonsAudit"."AuditEntry"; that is using a custom schema to own the object.

Suppose though that for whatever reason we didn’t want to use a custom schema but would rather use the default. Also suppose we are using SQL Server as our target database.

We can override the above annotation using a AuditEntry-sqlserver.orm file, placed in the same package as the AuditEntry entity. For example:

It’s also necessary to tell JDO/DataNucleus about which vendor is being used (sqlserver in the example above). This is done using a configuration property:

Taken from this thread on the Apache Isis users mailing list…​

If it seems that Apache Isis (or rather DataNucleus) isn’t fully populating domain entities (ie leaving some properties as null), then check that your actions are not accessing the fields directly. Use getters instead.

Why so? Because DataNucleus will potentially lazy load some properties, but to do this it needs to know that the field is being requested. This is the purpose of the enhancement phase: the bytecode of the original getter method is actually wrapped in code that does the lazy loading checking. But hitting the field directly means that the lazy loading code does not run.

This error can be subtle: sometimes "incorrect" code that accesses the fields will seem to work. But that will be because the field has been populated already, for whatever reason.

One case where you will find the issue highlighted is for subtype tables that have been mapped using an inheritance strategy of NEW_TABLE, eg:

and then:

This will generate two tables in the database, with the primary key of the supertype table propagated as a foreign key (also primary key) of the subtype table (sometimes called "table per type" strategy). This means that DataNucleus might retrieve data from only the supertype table, and the lazily load the subtype fields only as required. This is preferable to doing a left outer join from the super- to the subtype tables to retrieve data that might not be needed.

On the other hand, if the SUPERCLASS_TABLE strategy (aka "table per hierarchy" or roll-up) or the SUBCLASS_TABLE strategy (roll-down) was used, then the problem is less likely to occur because DataNucleus would obtain all the data for any given instance from a single table.

Final note: references to other objects (either scalar references or in collections) in particular require that getters rather than fields to be used to obtain them: it’s hopefully obvious that DataNucleus (like all ORMs) should not and will not resolve such references (otherwise, where to stop…​ and the whole database would be loaded into memory).

In summary, there’s just one rule: always use the getters, never the fields.

DataNucleus 4.x supports Java 7, but can also be used with Java 8, eg for streams support against collections managed by DataNucleus.

Just include within <dependencies> of your dom module’s pom.xml:

(As of DN 4.1) set a break point in FetchRequest#execute(…​):

The "Variables" pane will tell you which field(s) are being loaded, and the stack trace should help explain why the field is required.

For example, it may be that an object is being loaded in a table and the initial query did not eagerly load that field. In such a case, consider using fetch groups in the initial repository query to bring the required data into memory with just one SQL call. See this hint/tip for further details.

Fetch groups provide a means to hint to DataNucleus that it should perform a SQL join when querying. A common use case is to avoid the n+1 issue.

(So far as I could ascertain) it isn’t possible to specify fetch group hints using JDOQL, but it is possible to specify them using the programmatic API or using typesafe queries.

For example, here’s a JDOQL query:

which normally would be used from a repository:

This can be re-written as a type-safe query as follows:

Now the IncomingInvoice has four fields that require eager loading. This can be specified by defining a named fetch group:

This fetch group can then be used in the query using q.getFetchPlan().addGroup(…​). Putting this all together, we get: