Caching problem w/ primary-foreign key mapping

Question

I have seen this a couple of times now. It is not consistent enough to
create a simple reproducible test case, so I will have to describe it to you
with an example and hope you can track it down. It only occurs when caching
is enabled.
Here are the classes:
class C1 { int id; C2 c2; }
class C2 { int id; C1 c1; }
Each class uses application identity using static nested Id classes: C1.Id
and C2.Id. What is unusual is that the same value is used for both
instances:
int id = nextId();
C1 c1 = new C1(id);
C2 c2 = new C2(id);
c1.c2 = c2;
c2.c1 = c1;
This all works fine using optimistic transactions with caching disabled.
Although the integer values are the same, the oids are unique because each
class defines its own unique oid class.
Here is the schema and mapping (this works with caching disabled but fails
with caching enabled):
table t1: column id integer, column revision integer, primary key (id)
table t2: column id integer, column revision integer, primary key (id)
<jdo>
<package name="test">
<class name="C1" objectid-class="C1$Id">
<extension vendor-name="kodo" key="jdbc-class-map" value="base">
<extension vendor-name="kodo" key="table" value="t1"/>
</extension>
<extension vendor-name="kodo" key="jdbc-version-ind"
value="version-number">
<extension vendor-name="kodo" key="column" value="revision"/>
</extension>
<field name="id" primary-key="true">
<extension vendor-name="kodo" key="jdbc-field-map" value="value">
<extension vendor-name="kodo" key="column" value="id"/>
</extension>
</field>
<field name="c2">
<extension vendor-name="kodo" key="jdbc-field-map" value="one-one">
<extension vendor-name="kodo" key="column.id" value="id"/>
</extension>
</field>
</class>
<class name="C2" objectid-class="C2$Id">
<extension vendor-name="kodo" key="jdbc-class-map" value="base">
<extension vendor-name="kodo" key="table" value="t2"/>
</extension>
<extension vendor-name="kodo" key="jdbc-version-ind"
value="version-number">
<extension vendor-name="kodo" key="column" value="revision"/>
</extension>
<field name="id" primary-key="true">
<extension vendor-name="kodo" key="jdbc-field-map" value="value">
<extension vendor-name="kodo" key="column" value="id"/>
</extension>
</field>
<field name="c1">
<extension vendor-name="kodo" key="dependent" value="true"/>
<extension vendor-name="kodo" key="inverse-owner" value="c2"/>
<extension vendor-name="kodo" key="jdbc-field-map" value="one-one">
<extension vendor-name="kodo" key="table" value="t1"/>
<extension vendor-name="kodo" key="ref-column.id" value="id"/>
<extension vendor-name="kodo" key="column.id" value="id"/>
</extension>
</field>
</class>
</package>
</jdo>
Because the ids are known to be the same, the primary key values are also
used as foreign key values. Accessing C2.c1 is always non-null when caching
is disabled. With caching is enabled C2.c1 is usually non-null but sometimes
null. When it is null we get warnings about dangling references to deleted
instances with id values of 0 and other similar warnings.
The workaround is to add a redundant column with the same value. For some
reason this works around the caching problem (this is unnecessary with
caching disabled):
table t1: column id integer, column id2 integer, column revision integer,
primary key (id), unique index (id2)
table t2: column id integer, column revision integer, primary key (id)
<jdo>
<package name="test">
<class name="C1" objectid-class="C1$Id">
<extension vendor-name="kodo" key="jdbc-class-map" value="base">
<extension vendor-name="kodo" key="table" value="t1"/>
</extension>
<extension vendor-name="kodo" key="jdbc-version-ind"
value="version-number">
<extension vendor-name="kodo" key="column" value="revision"/>
</extension>
<field name="id" primary-key="true">
<extension vendor-name="kodo" key="jdbc-field-map" value="value">
<extension vendor-name="kodo" key="column" value="id"/>
</extension>
</field>
<field name="c2">
<extension vendor-name="kodo" key="jdbc-field-map" value="one-one">
<extension vendor-name="kodo" key="column.id" value="id2"/>
</extension>
</field>
</class>
<class name="C2" objectid-class="C2$Id">
<extension vendor-name="kodo" key="jdbc-class-map" value="base">
<extension vendor-name="kodo" key="table" value="t2"/>
</extension>
<extension vendor-name="kodo" key="jdbc-version-ind"
value="version-number">
<extension vendor-name="kodo" key="column" value="revision"/>
</extension>
<field name="id" primary-key="true">
<extension vendor-name="kodo" key="jdbc-field-map" value="value">
<extension vendor-name="kodo" key="column" value="id"/>
</extension>
</field>
<field name="c1">
<extension vendor-name="kodo" key="dependent" value="true"/>
<extension vendor-name="kodo" key="inverse-owner" value="c2"/>
<extension vendor-name="kodo" key="jdbc-field-map" value="one-one">
<extension vendor-name="kodo" key="table" value="t1"/>
<extension vendor-name="kodo" key="ref-column.id" value="id2"/>
<extension vendor-name="kodo" key="column.id" value="id"/>
</extension>
</field>
</class>
</package>
</jdo>
Needless to say, the extra column adds a lot of overhead, including the
addition of a second unique index, for no value other than working around
the caching defect.

Answer 1

Tom-
The first thing that I think of whenever I see a problem like this is
that the equals() and hashCode() methods of your application identity
classes are not correct. Can you check them to ensure that they are
written in accordance to the guidelines at:
http://docs.solarmetric.com/manual.html#jdo_overview_pc_identity_application
If that doesn't help address the problem, can you post the code for your
application identity classes so we can double-check, and we will try to
determine what might be causing the problem.
In article <[email protected]>, Tom Landon wrote:
I have seen this a couple of times now. It is not consistent enough to
create a simple reproducible test case, so I will have to describe it to you
with an example and hope you can track it down. It only occurs when caching
is enabled.
Here are the classes:
class C1 { int id; C2 c2; }
class C2 { int id; C1 c1; }
Each class uses application identity using static nested Id classes: C1.Id
and C2.Id. What is unusual is that the same value is used for both
instances:
int id = nextId();
C1 c1 = new C1(id);
C2 c2 = new C2(id);
c1.c2 = c2;
c2.c1 = c1;
This all works fine using optimistic transactions with caching disabled.
Although the integer values are the same, the oids are unique because each
class defines its own unique oid class.
Here is the schema and mapping (this works with caching disabled but fails
with caching enabled):
table t1: column id integer, column revision integer, primary key (id)
table t2: column id integer, column revision integer, primary key (id)
<jdo>
<package name="test">
<class name="C1" objectid-class="C1$Id">
<extension vendor-name="kodo" key="jdbc-class-map" value="base">
<extension vendor-name="kodo" key="table" value="t1"/>
</extension>
<extension vendor-name="kodo" key="jdbc-version-ind"
value="version-number">
<extension vendor-name="kodo" key="column" value="revision"/>
</extension>
<field name="id" primary-key="true">
<extension vendor-name="kodo" key="jdbc-field-map" value="value">
<extension vendor-name="kodo" key="column" value="id"/>
</extension>
</field>
<field name="c2">
<extension vendor-name="kodo" key="jdbc-field-map" value="one-one">
<extension vendor-name="kodo" key="column.id" value="id"/>
</extension>
</field>
</class>
<class name="C2" objectid-class="C2$Id">
<extension vendor-name="kodo" key="jdbc-class-map" value="base">
<extension vendor-name="kodo" key="table" value="t2"/>
</extension>
<extension vendor-name="kodo" key="jdbc-version-ind"
value="version-number">
<extension vendor-name="kodo" key="column" value="revision"/>
</extension>
<field name="id" primary-key="true">
<extension vendor-name="kodo" key="jdbc-field-map" value="value">
<extension vendor-name="kodo" key="column" value="id"/>
</extension>
</field>
<field name="c1">
<extension vendor-name="kodo" key="dependent" value="true"/>
<extension vendor-name="kodo" key="inverse-owner" value="c2"/>
<extension vendor-name="kodo" key="jdbc-field-map" value="one-one">
<extension vendor-name="kodo" key="table" value="t1"/>
<extension vendor-name="kodo" key="ref-column.id" value="id"/>
<extension vendor-name="kodo" key="column.id" value="id"/>
</extension>
</field>
</class>
</package>
</jdo>
Because the ids are known to be the same, the primary key values are also
used as foreign key values. Accessing C2.c1 is always non-null when caching
is disabled. With caching is enabled C2.c1 is usually non-null but sometimes
null. When it is null we get warnings about dangling references to deleted
instances with id values of 0 and other similar warnings.
The workaround is to add a redundant column with the same value. For some
reason this works around the caching problem (this is unnecessary with
caching disabled):
table t1: column id integer, column id2 integer, column revision integer,
primary key (id), unique index (id2)
table t2: column id integer, column revision integer, primary key (id)
<jdo>
<package name="test">
<class name="C1" objectid-class="C1$Id">
<extension vendor-name="kodo" key="jdbc-class-map" value="base">
<extension vendor-name="kodo" key="table" value="t1"/>
</extension>
<extension vendor-name="kodo" key="jdbc-version-ind"
value="version-number">
<extension vendor-name="kodo" key="column" value="revision"/>
</extension>
<field name="id" primary-key="true">
<extension vendor-name="kodo" key="jdbc-field-map" value="value">
<extension vendor-name="kodo" key="column" value="id"/>
</extension>
</field>
<field name="c2">
<extension vendor-name="kodo" key="jdbc-field-map" value="one-one">
<extension vendor-name="kodo" key="column.id" value="id2"/>
</extension>
</field>
</class>
<class name="C2" objectid-class="C2$Id">
<extension vendor-name="kodo" key="jdbc-class-map" value="base">
<extension vendor-name="kodo" key="table" value="t2"/>
</extension>
<extension vendor-name="kodo" key="jdbc-version-ind"
value="version-number">
<extension vendor-name="kodo" key="column" value="revision"/>
</extension>
<field name="id" primary-key="true">
<extension vendor-name="kodo" key="jdbc-field-map" value="value">
<extension vendor-name="kodo" key="column" value="id"/>
</extension>
</field>
<field name="c1">
<extension vendor-name="kodo" key="dependent" value="true"/>
<extension vendor-name="kodo" key="inverse-owner" value="c2"/>
<extension vendor-name="kodo" key="jdbc-field-map" value="one-one">
<extension vendor-name="kodo" key="table" value="t1"/>
<extension vendor-name="kodo" key="ref-column.id" value="id2"/>
<extension vendor-name="kodo" key="column.id" value="id"/>
</extension>
</field>
</class>
</package>
</jdo>
Needless to say, the extra column adds a lot of overhead, including the
addition of a second unique index, for no value other than working around
the caching defect.
Marc Prud'hommeaux [email protected]
SolarMetric Inc. http://www.solarmetric.com