TopLink caching behaving strangly?

Hi folks,
Realizing that my previous post titled 'Session#acquireUnitOfWork() resets identiy maps' at Session#acquireUnitOfWork() resets identiy maps may be a bit muddleheaded I'll take a different approach at the same problem.
I'm not yet very familiar with TopLink caching (links to good articles greatly appreciated) but the behaviour we see here still does not make sense to me. Caching is provided by identity maps referenced by the server session. That much I understand. So, every time an application reads/writes data from/to the database TopLink would update those identity maps - if necessary. Based on that I assume that those maps remain untouched as long as I do not modify a certain object and commit those modifications.
[Pseudo code]
1: TopLink.save(obj1)
2: obj2 = TopLink.read(queryForObj1)
3: obj1 == obj2
It all boils down to the simple question why obj1 and obj2 are two different instances? I would expect them to be one and the same instance.
I would greatly appreciate if someone could provide some clarification.
Regards,
Marcel

Hello Marcel,
Your question in this and the other thread touch on how the UnitOfWork and TopLink caching works. Basically what you are seeing is probably because you are reading instances from different UnitOfWork objects. Each UOW has its own cache to maintain transactional issolation. Using a UOW will return a cloned copy of the object contained in the central cache so that you can modify it without changes being seen outside the transaction. On commit, the changes in the clone are put in the database and merged into the object in the main cache. Only changes made to the UnitOfWork working copy of an object will be persisted, but you can read from the session or ClientSession directly instead of the UnitOfWork - this will return the copy from the central cache.
When you commit/release a UnitOfWork, all working copies become detached; changes will no longer be persisted to the database.
I would recommend looking over the white paper on the UnitOfWork at:
http://www.oracle.com/technology/products/ias/toplink/technical/unitOfWorkWP.pdf
Best Regards,
Chris

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How do I sync Toplink Cache with my Database?

Hey guys,
We are running macro's through an excel sheet that connects to the database and performs Updates and Inserts.
Since this database update is NOT taking place through TopLink (in a Unit of Work) - we do not see the database changes through the web app on the front end unless we bounce our webserver. Presumably the Toplink cache is re-built on start up...so then we can see the changes.
My question is, what can I do to make sure the TopLink cache is aware of the database changes we have made through the macro without having to bounce the server? Is there a re-fresh or sync command that can be run?
This task is sort of a one time thing, so I don't want a solution that involves the cache going to sync itself on a schedule or anything like that. Maybe bouncing is the best solution?
Thoughts?
We are using Toplink 9.0.4
Thanks.

Hello,
Because it is a one time thing, if you can make sure no other TopLink process are going on, you can probably get away with initializeAllIdentityMaps() or the intitializeIdentityMap(Class) methods on the session. These will clear the identity maps, having the obvious draw back of removing all object identity which will cause problems with running processes though. It might be better than bouncing the server - it depends on the application. Logging out of the session and logging back in has the same effect of clearing the cache, but with a bit more overhead. Benifit is that running processes will get errors if they continue to use the session, rather than strange behaviior if they continue to use objects after identity is lost.
Another alternative is to run refresh queries on the data you know might be in the cache or that might have been affected. Drawbacks are that this brings them into the cache if they are not already there.
TopLink 9.0.4 is a quire a few versions back, and in newer versions there is cache invalidation. An object marked as invalid is not removed, so object identity is maintained, but on the next query the data will be refreshed - ensuring subsequent queries will get results from the database without having to be told explicietly to refresh or being set to always refresh. Invalidation can be triggered on particular objects, classes or the entire cache or different policies can be set to set a time to live etc.
Except for bouncing the server or logging out of the session, all of the above leave some possibility that a concurrent user will still have a reference to stale data and continue to use it after the process has run on the database. So I hope you use optimistic locking and that your batch process updates the version to avoid other process from overwriting with stale data.
Best Regards,
Chris

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Toplink Cache not refreshed

In my project, TOPLINK is used. To carry out the CRUD operation, Stored procedures are used on all objectcs.
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Hi
We are using toplink as ORM.we are using default cache option (soft cache weak identity)provided by toplink, need to optimize the object retrival from cache, is there any option to create Index ( like database index) on the toplink cache.
Message was edited by:
[email protected]

The TopLink cache is indexed by primary key only. It does not support additional indexes.
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Armed with this better knowledge of how TopLink queries the cache, we do have a few objects that we
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CheckCacheThenDatabase will check the entire cache for a match using a linear search. This can be efficient if the cache is very large. Typically it is more efficient to access the database if your cache is large and the field you are querying in the database is indexed in the table.
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C1 to Card ( one - many mapping )
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Hello,
Its not that the cache isn't working, its that you are updating relationships in the database but not in the cache. Take the case of a 1:M from B->C used in a previous response. It looks like the foreign key used in this relationship (in the C table) is mapped to the C object either as a direct to field mapping, or through a 1:1 to the A object. When you change this so that it now somehow points to the B object, you haven't updated the B object to include the C in its collection. Because B has a 1:M mapping collection of Cs, when you next get that B object from the cache, its collection will not contain the newly referenced C (assuming indirection is not used or has been previously triggered). This is a result of you mapping the 1:M in the first place - since it is storing the collection in the object.
If you are going to make changes that affect relationships and yet not update the relationships directly I would suggest either:
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B) Not mapping these relationships. Instead, when you need to find C objects referenced from B, query for them.
Both options are less efficient as they cause more hits to the database. This is why it would be preferred if you updated the relationships when you change fields that affect those relationships, but it is up to you to decide which will work best for your application.
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Best Regards,
Chris

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