On Nov 3, 2007, at 4:20 PM, Craig Russell (JIRA) wrote:
[ https://issues.apache.org/jira/browse/OPENJPA-370?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel
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Craig Russell commented on OPENJPA-370:
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Joe Grassel commented:
I'm wondering what the intent of the LoadFetchGroup function was
when it was designed. The manual states:
"A field can also declare a load fetch group. When you access a
lazy loaded field for the first time, OpenJPA makes a datastore
trip to fetch that field's data. Sometimes, however, you know that
whenever you access a lazy field A, you're likely to access lazy
fields B and C as well. Therefore, it would be more efficient to
fetch the data for A, B, and C in the same datastore trip. By
setting A's load fetch group to the name of a fetch group
containing B and C, you can tell OpenJPA to load all of these
fields together when A is first accessed."
I guess I have a question about the function I'd like clarified:
What does it mean when B and C are co-fetched in the same datastore
trip? Is the data just loaded into the entitymanager's datacache
and held there until a hit is made on it (when the application
finally reads the entity persistable property for the first time,
this would save an additional hit to the database) or is it
genuinely considered eagerly fetched (entity persistable property
field is populated when the entity object is constructed by the
find/query operation?)
It's useful to highlight when the load fetch group behavior is
activated: when you access a lazy loaded field for the first time.
This is not done during query or find, but only when the lazy loaded
field is accessed (from the application) and it's not already loaded.
So, for B and C to be loaded, the lazy-loaded field that has the load
fetch group annotation has to be accessed while the entity is still
managed. When A is faulted in, it faults B and C in automatically,
even if they are never accessed while the entity is managed.
So given the entity FGEmployee (using this since the code is already
available as an attachment, and keeps my message shorter), which (all,
some, or none) of the following LoadFetchGroup scenarios are valid:
Scenario A:
Starting environment: No fetch groups other then "default" are
active, persistence context is clear of any managed entities
1) Start a transaction
2) Find/Query FGEmployee
3) Read rating (this causes addressGroup to be faulted in, due to the
@LoadFetchGroup on rating)
4) Commit the transaction
5) Clear the persistence context (not necessary for TS-CM PCs)
6) Read rating, since it was read while the entity was managed in step
3, its data should be available
7) Read addressGroup, although it was not read while the entity was
managed, it was faulted in when addressGroup was accessed in step 3,
because of the @LoadFetchGroup, so it should be available. Other lazy
loaded fields, like description, dept, manager, are not available at
this point since they were never accessed while the entity was
managed, and since default was the only active fetch group, they would
have observed lazy loading behavior.
Scenario B:
Starting environment: Active Fetch Groups: "default",
"RatingFetchGroup". Persistence context is clear of any managed
entities
1) Start a transaction
2) Find/Query FGEmployee
3) Commit the Transaction
4) Clear the persistence context
(Note, the first four steps can be condensed to just 1 for a TS-CM PS
if performed outside of a transaction)
5) Read rating, since it was targeted by the RatingFetchGroup, it
should be available even though it was never accessed while the entity
was managed. This has been tested and proven that this is the
observed behavior for persistable attributes identified by fetch groups.
6) Read addressGroup. This is the rub. Is it available? ratings was
loaded in because it was part of an active fetch plan. I would expect
that since rating was loaded, and since rating has declared that
addressGroup should also be loaded via its @LoadFetchGroup annotation,
then addressGroup should be available as well.
Other lazy loaded fields, like description, dept, manager, are not
available at this point since they were never accessed while the
entity was managed, and were never a member of any of the active fetch
groups, they would have observed lazy loading behavior.
The documentation as it is written makes it sound like both scenarios
are valid -- any situation where rating's data is available,
addressGroup's data is also supposed to be available, thanks to the
@LoadFetchGroup. Which scenarios are valid interpretations of the
function? Scenario A, Scenario B, both, or neither?
To your other point, if fields B and C are in field A's load fetch
group, then accessing field A makes is available in the detached
instance, and fields B and C are also available in the detached
instance as if they were accessed at the same time as field A was
accessed.
This makes a big difference in what an application programmer
should expect. If the former, then LoadFetchGroup is just a
datastore optimization that doesn't really make B and C eagerly
loaded. It just saves a datastore trip should they ever be
loaded. That means that if the entity becomes detached, B and C
are not available because they were never accessed when the entity
was managed by the persistence context.
Yes, this does affect the fields that are detached. If fields B and
C are loaded, then they are also loaded in the detached instance.
But I'd be careful calling this behavior eager loading. Eager
loading is done for fields based on the fetch plan in effect for a
find or query that first loads the instance into memory. The load
fetch group isn't considered here. The load fetch group is only
activated when you access a field that wasn't eagerly loaded.
So, this means that Scenario A is the only scenario where
@LoadFetchGroup will work? Since rating, in Scenario B, would not be
considered a lazy loaded field because it is a member of an active
fetch plan?
The latter, and the function behavior I expected, if data is
acquired from the datastore hit, then I'd expect it to be available
for reading from the entity object, even if the field was not
access prior to becoming detached, since active fetch groups (or
those referenced by a load fetch group) effectively nullify the
LAZY loading setting on an affected persistable attribute. Knowing
what behavior to expect is especially important, especially in the
situation where entities are acquired with a transaction-scoped
persistence context when then find/query occurs outside of a
transaction. I'd expect A to be loaded because it was referenced
in an active fetch group, and B and C to be loaded (and
referenceable in the entity) due to the load fetch group setting.
No, here's the difference between active fetch groups and load fetch
groups. If you want fields B and C to be loaded when the instance is
first accessed via find or query, then you need to include B and C
in one of the active fetch groups when you execute find or query. If
you want fields B and C to be loaded only when some lazy loaded
field is accessed, then put B and C into a fetch group and define
that fetch group as the load fetch group of the lazy loaded fields
that you want to trigger the fetch of fields B and C.
A slightly different slant on this is that if field A is in some
fetch group FG1, and use of field A requires fields B and C, then
any fetch group that includes A (e.g. FG1) should also include B and
C. There's no need for a load fetch group here.
So, in the situation where we and both B and C to be loaded if A is
loaded, we would either:
1) Do Scenario A
2) Have fetch groups that include A, B, and C active when the find/
query is performed.
If that is the case, then I think the documentation really needs to be
updated to make that crystal clear. Otherwise, it will be easy to
assume (as I did) that @LoadFetchGroups will always be triggered if
the persistable attribute annotated with it is loaded, be it
approached in Scenario A or B. By saying "you can tell OpenJPA to
load all of these fields together when A is first accessed." that
sounds like B and C will always be loaded along with A, no matter what
the loading circumstance (A loaded by a getter method while the entity
is managed, or A loaded in by fetch group).
Also, I noticed that some of the examples closed the entitymanager
in order to test loadfetchgroup behavior -- what about when an
entity is just detached from the persistence context, em.close() is
one way to approach it, but that only works in JSE and JEE:
Application Managed Persistence Contexts. That's not going to work
in Container Managed Persistence Contexts, and detachment is
probably going to be frequently seen by Transaction Scoped
persistence contexts, and situations where entities are serialized
across the wire to distinct application components (say, to an
application client, a web service, or via RMIIIOP to a remote
application server's ejb/web container.) I would expect that data
to be available due to the fetchgroup/loadfetchgroup configuration.
The test cases use em.clear() or em.close() to detach the instances,
but any operation that detaches instances, including serialization,
should exhibit the behavior.
This includes both non-relational and relational lazy-loaded fields.
I don't understand this comment. Are you referring to relationship
fields?
Yes, making sure that both attributes that make sense to be lazy
loaded (Strings, blobs, etc.) and relationships (any, although the
collection based relationships are lazy by default) are included under
consideration here.
LoadFetchGroup annotation was not recognized during the fetch1
--------------------------------------------------------------
Key: OPENJPA-370
URL: https://issues.apache.org/jira/browse/OPENJPA-370
Project: OpenJPA
Issue Type: Bug
Components: kernel
Affects Versions: 1.0.1, 1.0.2, 1.1.0
Reporter: Teresa Kan
Assignee: Teresa Kan
Fix For: 1.0.2, 1.1.0
Attachments: OPENJPA_370_2.patch, smime.p7s,
TestFetchGroup.zip, TestJIRA370.zip
Employee class has a LoadFetchGroup annotation defined on the
Rating field, when getRating was called, the address should be
returned also. However, openjpa did not handle the LoadFetchGroup
correctly, therefore, address was not eargly fetched.
public class FGEmployee{
@Id
private int id;
@OneToOne(fetch=FetchType.LAZY)
private FGAddress address;
@Basic(fetch=FetchType.LAZY)
@LoadFetchGroup("AddressFetchGroup")
private String rating;
@ManyToOne(fetch=FetchType.LAZY)
private FGManager manager;
..
}
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