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From Andrus Adamchik <and...@objectstyle.org>
Subject Re: caching documentation
Date Wed, 05 Mar 2008 11:20:07 GMT

On Mar 4, 2008, at 1:26 AM, Marcin Skladaniec wrote:

> Hi
>
> The documentation on caching (http://cayenne.apache.org/doc/caching-and-fresh-data.html

>  and http://cayenne.apache.org/doc/object-caching.html) isn't very  
> comprehensive,

Agreed. There are lots of new features related to caching in 3.0, and  
we do not communicate them well to the users as of yet.

> it does not answer questions like:
>
> - what is actually stored in cache pks? datarows ? objectIds ?

There are two types of cache: object cache [1] and query cache.

* Object cache (stored at ObjectContext): Map<ObjectId, Persistent>  
(it may not be declared as such, but this is what it is).
* Object cache (stored at DataDomain... so really a snapshot cache):  
Map<ObjectId, DataRow>
* Query cache (stored at ObjectContext, aka LOCAL_CACHE): Map<String,  
List<Persistent|DataRow>
* Query cache (stored at DataDomain, aka SHARED_CACHE): Map<String,  
List<DataRow>


> - does caching change when paging is on ?

Yes, there are some caveats, and a few things were tweaked recently.  
LOCAL_CACHE works (both ROP and two tier). There is no SHARED_CACHE  
support (and I want to make this more formal - throw an  
IllegalStateException if pagination and SHARED_CACHE are used  
together). One reason why I want to do that is that it appeared under  
ROP as if SHARED_CACHE worked, when it fact things worked differently,  
as a side effect of the special handling of paginated lists on the ROP  
server (see below).


> - does caching require special measures when used with ROP ?  
> (meaning the propagation of changes between contexts)

Not really, maybe an understanding of how it is implemented. Paginated  
list is always cached in the *server* local cache, regardless of the  
query cache settings. I.e. "LOCAL_CACHE + paginated list + ROP" means  
caching on both server and client; "NO_CACHE + paginated list + ROP"  
still means caching on the server. This is done in order to avoid  
transferring unresolved ID's to the client.


> - how to properly use SelectQuery.setCacheGroups()?

Cache groups are ignored unless you use advanced implementations of  
QueryCache on the server (e.g. OSCache). RefreshQuery can also target  
cache groups (see below). "cache group" is a mechanism to allow  
backend code to perform smart cache invalidation without knowing  
anything about the nature of the queries. E.g. you can have two groups  
"objects_that_change_often" and "objects_that_rarely_change",  
corresponding to 2 OSCache invalidation rules, "once per minute" vs.  
"once per day"... Now when you add new queries, you do not need to  
change configuration, if they fall into one of the existing "groups"...

So the trick with cache groups is to find common data invalidation  
patterns in your app. Each repeating pattern becomes a group. This is  
a logical task, with very little code involved.

> what happens when a query has more than one cache group specified?

Invalidation rules for all groups are combined. I rarely used that in  
practice, but still think this allows some extra flexibility, e.g. if  
the same query falls in a broad category and also in a very specific  
one. E.g. "objects_that_rarely_change" and  
"objects_that_change_when_event_X_occurs".

> - how long the cache entries sit in the memory, is there a way to  
> invalidate all cache from time to time ?

Query cache (both shared and local): default mechanism is LRU and no  
expiration. OSCache allows to configure size and advanced expiration  
rules per cache group.

Snapshot cache: LRU. Size configurable in the Modeler.

Object cache (server): Unlimited size map with weak references.


> - how to invalidate cache using RefreshQuery, the http://cayenne.apache.org/doc/refreshquery.html

>  is just a list of suggestions on how it might work in the future.

Yeah, this is not documented properly. I need to poke around a bit  
more to provide accurate information on RefreshQuery behavior. It was  
an early idea of cache handling, but I stopped using it in my own  
apps, as OSCache works beautifully, supports clustering, etc., etc.  
And rather importantly - it removes cache management logic from the  
code (i.e. explicit invalidation vs. configuration-based one).

> Me and Ari are willing to document the caching feature, but we would  
> need some help.

Awesome! I'd imagine the trick here is to separate everything  
discussed here into "internal-design-not-relevant-to-the-user" part  
and "cache-user-guide" part to avoid confusing people and exposing too  
many implementation details that will likely change over time.

Andrus


[1] http://cayenne.apache.org/doc/object-caching.html


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