Re: caching documentation

[Borut Bolčina <bo...@gmail.com>]

Marcin,

will you be including examples with OSCache?

Thanks, looking forward to read the cache docs!

Cheers,
Borut

2008/3/5 Marcin Skladaniec <ma...@ish.com.au>:

> Thank you very much for the reply, I need to read through one more time
> and I'll try to make use of it as well as write some documentation down for
> the website.
>
> Marcin
>
>
> On 05/03/2008, at 9:20 PM, Andrus Adamchik wrote:
>
>
> > 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
> >
> >
>