On May 7, 2009, at 9:14 AM, Aristedes Maniatis wrote:
On the object cache page you discuss Memory Management with the
shared and local caches.
1. Should this section be moved to the parent page since it applies
both to query and object caches? That is, is the memory space of
10,000 shared between object and query cache?
Actually no. It only applies to the object cache. Query cache may have
its own memory management considerations, but with a variety of
options with it, I didn't delve into details. In any event until we
start using a single cache mechanism for object and query cache, I'd
stay the memory management is separate for both as well.
2. If an object is stored in the shared cache by one context and
another context uses it, will localObject() need to be called on it
to move it to the correct context or is that all handled
automatically?
This is transparent. Shared cache actually stores DataRows, but since
one of my goals with this doc was to avoid too many low level details,
I omitted that mention.
3. If objects are always synchronised between local caches is
refreshing ONLY needed when thinking about multiple instances of the
application in separate JVMs? If so, this could be made even clearer
in the refreshing section.
"Object caching happens behind the scenes, so the only case when
users need to worry about object cache is when an object gets stale
and needs to get refreshed"
maybe instead:
"Object cache refreshing is done automatically for you be Cayenne,
so the only case users need to worry about refreshing manually is
when you are running multiple instances of the application in
separate JVMs. In that case you have the following options:"
We can clarify this, but then we should also mention that the data in
DB can get updated independently from Cayenne or Java apps (e.g. via a
SQL load). I.e. there's more than one way for it to get stale, so
again I omitted the details, but we can add them if we want.
4. I'd like to understand a little better the relationship between
query cache and object cache. For example, will Cayenne every
perform a query against the database and then throw away the results
it gets because it sees that the objectIds are already in the object
cache? That is, does a query which hits the db always result in a
refresh of the object cache?
Since 3.0 the answer to the last question is definite yes. It used to
be configurable via 'setRefreshingObjects(..)', still with the
advances in the query cache, this option became redundant and got
deprecated.
And, if the query is cached and therefore no SELECT is issued, will
the results sometimes include hollow objects which will then be
faulted? Perhaps the object cache was full and those objects had
been expired.
Unless I am missing something, I don't think this scenario is
possible. With the shared object cache (that uses LRU map) even if the
DataRow is expired, the cached query list will provide its own
DataRows (DataRows are not subject to uniqing unlike DataObjects);
with a local object cache (that uses weak reference map) the fact that
an object is cached in the list will prevent it from being garbage
collected.
So, whether the object cache is refreshed will depend on whether a
query hits the db or comes from cache?
Correct.
Andrus
