How does the 2nd level cache work ?

samfra · **Posted:** Tue May 25, 2004 4:26 am

I would like to load the structure at once !

core_usergroup(id:int, name:varchar)
core_usermembership(usergroup_child:int, usergroup_parent:int)
core_userpassword(usergroup_id:int,data:datetime,value:varchar)

how can I do ?? ok fetch join I was answered... but how ?
session.find("From CoreUsergroup as user left join fetch user.children left join fetch user.parents left join fetch user.coreUserPasswords") ?

once in cache, I would like to have the list of all usergroups to handle it by myself... how to ?
iterate("From CoreUsergroup") ? it still does a sql request to retreive IDs... I don't want that ! And if I delete an object, the next time I call iterate, it still gives me the deleted ID and it crashes in IterateImpl)

I don't understand how the cache work ! Could someone explain ? Or is there any article to read on this ?

This is what I have :

I have a usergroup table that can contains other usergroups and be contained in other usergroups.
A usergroup also has a list of password (don't ask why)

Here is my xml :

Quote:

<hibernate-mapping>
<class name="test.CoreUsersgroup"
table="core_usersgroups" dynamic-update="false" dynamic-insert="false">

<cache usage="read-write" />

<id name="id" column="id" type="java.lang.Integer" unsaved-value="-1" >
<generator class="hilo">
<param name="table">hibernate_unique_key</param>
<param name="column">core_usersgroups</param>
<param name="max_lo">0</param>
</generator>
</id>

<property name="name" type="java.lang.String" update="true" insert="true" column="name" length="50" not-null="true"/>

<set name="children" lazy="false" inverse="true" cascade="delete" sort="unsorted">
<cache usage="read-write" />
<key column="usergroup_child" />
<one-to-many class="test.CoreUserMembership" />
</set>

<set name="parents" lazy="false" inverse="true" cascade="delete" sort="unsorted">
<cache usage="read-write" />
<key column="usergroup_parent" />
<one-to-many class="test.CoreUserMembership" />
</set>

<set name="coreUserPasswords" lazy="false" inverse="false" cascade="delete" sort="unsorted">
<cache usage="read-write" />
<key column="usergroup_id" />
<one-to-many class="test.CoreUserPassword" />
</set>
</class>

</hibernate-mapping>

Thank you very much

anthony · **Posted:** Tue May 25, 2004 6:09 am

1- only one collection can be fetch
2- what you need in addition of second level cache is a query cache.

By doing this, if you call twice exactly the same query, it will not hit the db.

Try it, all is explained is the reference doc

samfra · **Posted:** Tue May 25, 2004 11:22 am

Hello,

thank you for your answer...

I'm very disappointed of your answer, it's a very basic approach of caching...

Am I the only one on earth ;o) to use the cache so much ??
I would appreciate if someone that often uses caching technics could contact me...

I can not load in memory all my data with hibernate, because I have more than one fetch to do...

What could I do ? Before discovering Hibernate, I made my own framework to handle persistence and cache. I don't like to reinvent the wheel, so I would like to find a way to use Hibernate... even if I just use it to load each table one by one... to avoid creating sql request...

thank you anthony !

anthony · **Posted:** Tue May 25, 2004 11:35 am

my answer was unclear,
only one fetch is possible to load in one shot one of your objects' collection

so i'll try to explain better, what you can do:
1- 1rst query (must be defined as cacheable, see the doc to know how it works) from Obj as myObj fetch join myObj.collectionA
2- query.iterate() for example
3- 2nd query (must also be cacheable) from Obj as myObj fetch join myObj.collectionB
4- query.iterate()
5- At this point, normally your objects and their 2 collections are filled in only 2 queries
6- from now, every time you execute query 1 or query 2, it should not hit the since the queries, the class and the collections are cached too

don't forget to defined the 2 collections cacheable.

larry · **Joined:** Fri Jan 23, 2004 4:29 pm **Posts:** 39

I have the same problem.

When I delete an object it still shows up in a collection and fails on lazy loading.

Did you manage to fix that?

anthony · **Posted:** Wed Jun 23, 2004 2:48 am

how is it related with this post?

larry · **Joined:** Fri Jan 23, 2004 4:29 pm **Posts:** 39

anthony wrote:

once in cache, I would like to have the list of all usergroups to handle it by myself... how to ?
iterate("From CoreUsergroup") ? it still does a sql request to retreive IDs... I don't want that ! And if I delete an object, the next time I call iterate, it still gives me the deleted ID and it crashes in IterateImpl)

After I delete an object that is in a collection. The cache still contains the deleted object's key. And tries to lazy load it from the database. When I access the collection.

anthony · **Posted:** Wed Jun 23, 2004 4:25 am

have you removed the object from the collection?

larry · **Joined:** Fri Jan 23, 2004 4:29 pm **Posts:** 39

anthony wrote:

have you removed the object from the collection?

I have not done that. Should not Hibernate do that if i specify cascade="all"?

anthony · **Posted:** Wed Jun 23, 2004 4:47 am

read about parent/child relationship please and if you cannot find a way to parameter the association ... come back ;)

larry · **Joined:** Fri Jan 23, 2004 4:29 pm **Posts:** 39

I read the Parent/Child example twice. But it does not clearly state what happens in my case.

It says among other things:
<i>
To tighten things up a bit, we could create an addChild() method of Parent.
public void addChild(Child c) {
c.setParent(this);
children.add(c);
}
</i>

In my case I delete an object not through the collection of the parent object. But directly through session.delete()
So I guess I have to call parent.getChildren().remove() also.