JDO2.2 : Dynamic fetch groups
-----------------------------
Key: JDO-554
URL: https://issues.apache.org/jira/browse/JDO-554
Project: JDO
Issue Type: New Feature
Components: api2, api2-legacy, specification
Reporter: Andy Jefferson
>From the Apache JDO jdo-dev mailing list, to register the issue for a
>subsequent JDO release, and holder for any further discussion
Below is a proposal that could possibly be included in a JDO2.2 (or in JDO2.1
if feedback is positive for that, and JPOX already implements it).
========================================
Problem : fetch groups are static, defined in metadata (XML/annotations).
Sometimes it would be more convenient to be able to define fetch groups
dynamically, for example based on user interaction in a web system.
========================================
Proposal :
We add a new interface defining a FetchGroup, where a FetchGroup has a symbolic
name and is for a class defining the fields of that class that are in the fetch
group.
public interface FetchGroup
{
String getName(); // Symbolic name (as also used in MetaData)
String getClassName(); // Class to which this group refers
FetchGroup add(String fieldName); // Add a field
FetchGroup remove(String fieldName); // Remove a field
boolean hasField(String fieldName);
String[] getFieldNames();
void setPostLoad(boolean postLoad);
boolean getPostLoad();
}
We allow users to register/deregister their FetchGroups with the PMF
PersistenceManagerFactory
{
...
void addFetchGroup(FetchGroup grp);
void removeFetchGroup(String name, Class cls);
FetchGroup createFetchGroup(String name, Class cls);
FetchGroup getFetchGroup(String grpName, Class cls);
FetchGroup[] getFetchGroups();
void clearFetchGroups();
}
========================================
Usage:
FetchGroup grp1 = pmf.createFetchGroup("myGroup1", MyClass.class);
grp1.add("field1").add("field2").add("field4");
pmf.addFetchGroup(grp1); // FetchGroup registered
pm.getFetchPlan().setGroup("myGroup1"); // FetchGroup used in this plan
// FetchPlan now has MyClass {field1, field2, field4}
We can then also allow dynamic changes like
pmf.getFetchGroup("myGroup1", MyClass.class).add("field7");
and this is directly reflected in the FetchPlan
Possible changes:-
1. PMF has createFetchGroup and addFetchGroup and we could merge these so when
creating a FetchGroup it is added
2. Doesnt support "recursion-depth" specification when adding a field to a
FetchGroup, so we could add a method "add(String fieldName, int depth)"
Comments by Erik Samson
JDO 2 needs alternate ways to define fetch plans. Some food for thoughts here:
- possibility to add the DFG of a class into a fetch group
- possibility to add all fields of a class into a fetch group
- possibility to add all primitive fields of a class into a fetch group
- possibility to add all reference fields of a class into a fetch group
- possibility to add all collections fields of a class into a fetch
group
- possibility to remove all primitive fields of a class into a fetch
group
- possibility to remove all reference fields of a class into a fetch
group
- possibility to remove all collections fields of a class into a fetch
group
- possibility to create a global fetch plan without fetch groups at all
pm.setFetchPlan( "Person(name,age, address( {dfg} , country( {all} , -flagIMG )
), accounts( {simple} , +{references} ) )" ) ;
Person actually references the candidate class, so I suppose it could be
optional.
This method will load name and age from a Person, then will load the configured
DFG from the reference to Address, then will load all fields but flagIMG from
the reference to Country into address, and finally will load simple fields and
unary references to other objects from the collection of Accounts. We should
also probably support depth in that mechanism.
Having this "SSFP" (Single String Fetch Plan) will allow to tune the system
externally, from JMX or a configuration file for instance.
Comments by Matthew Adams
You might also consider overloaded methods on interface FetchGroup, just for
completeness:
// (importing java.lang.reflect.Field)
FetchGroup add(Field field);
FetchGroup remove(Field field);
boolean hasField(Field field); // or has(Field) -- I'd consider better verb
Field[] getFields();
The add & remove methods should throw if the Field isn't contained in the class.
Comments by Christiaan:
May be also think about an option to restore to a fetchGroup to a state before
you start changing it (possibly via supporting clone()) or reset to
configuration defined in JDO file.
Comments by Craig Russell:
Also, I think that we should consider ways to manipulate FetchPlans as well,
both in programmatic as well as declarative approaches. Specifically, I'd like
to be able to specify in my configuration the FetchPlan to use in a specific
application context, e.g. the first time a PersistenceManager is used to
getObjectById or newQuery, the FetchPlan for that use case is looked up from
configuration and set as the current FetchPlan.
Further, if the application wants a specific FetchPlan, they should be able to
call a method setFetchPlan with either the name of a configured FetchPlan or a
FetchPlan to use.
And then, assuming that the FetchPlan must change during some interval of
application processing, and then reset to the previous settings,
public void pushFetchPlan(FetchPlan);
public void pushFetchPlan(String fetchPlanName)
public FetchPlan popFetchPlan()
would allow a temporary override of the FetchPlan without the application
having to preserve the settings and update the FetchPlan to restore it.
In this light, it might make sense to be able to register FetchPlans by name
with the PersistenceManagerFactory.
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