The caching docs are embarrassingly out of date. It's been requested a
number of times to fix them. Maybe finally I'll do it...
As for the cache strategy, you'd usually pick one of LOCAL_CACHE or
SHARED_CACHE (or NO_CACHE for no caching, but that's the default
already). Javadocs for QueryCacheStrategy explain each one of the
strategies. Let me point to the differences between "LOCAL" and
"SHARED" here. "Local" means attached to your ObjectContext, "Shared"
- shared by all ObjectContext produced by a given Cayenne stack
(usually this means shared by all contexts in a given JVM). Accessing
shared cache is somewhat slower than local, so if you have a singleton
DataContext (so DataContext is already shared by itself), select local
cache.
Andrus
On Apr 11, 2009, at 3:31 AM, Joe Baldwin wrote:
Lawrence,
I am still struggling to understand Andrus' setCacheStrategy()
approach in his previous email (he claims it is simple and I am all
for that :) ). I am attempting some black box testing to figure out
exactly where my data object is getting cached (and not updated
properly). In my case I am initializing a class variable with DB
data via Cayenne into a singleton when the class is initially
loaded. I am not sure whether it is my design or execution of my
design that is at fault.
strategies, but a simple Refresh All would get me across the line
for the moment. Is there any info on this?
WRT your issue, I found this class in the 3.0 API:
http://cayenne.apache.org/doc/api/org/apache/cayenne/query/RefreshQuery.html
The docs assert:
"A query that allows to explicitly clear both object and list
caches either via refetch (eager refresh) or invalidate (lazy
refresh)."
This may be the ticket for you.
Unfortunately, my singleton appears to be attached to the Tomcat app
and not the session so I can't find an elegant way for it to refresh.
Please let me know if you get this working.
Joe
On Apr 10, 2009, at 5:28 PM, Lawrence Gerstley wrote:
So, in my knowledge-gaining journey with this topic, I ran across
this page: http://cayenne.apache.org/doc/refreshquery.html, which
looks like a list of items yet to be done or at least yet to be
documented (and boy, when I have things really understood, I want
to volunteer some documentation time to the project). There is a
topic headline of "Refresh All", and an indication in the links
posted as to the "RefreshQuery", but the pertinent part of the
links are broken, and I can't track down the resolution of the
items. However, this is exactly what I need to do for a first step.
My application's environment will be (mandated by the customer), a
thick client running on a Citrix instance, and some of the
challenges posed by JGroups will take me awhile to understand. In
the meantime, I want to provide a simple "Refresh All" button that
will provide for a dumb refresh without leaving the application.
I'm struggling with different caching strategies, but a simple
Refresh All would get me across the line for the moment. Is there
any info on this?
Cheers,
Lawrence
On Apr 10, 2009, at 6:09 AM, Andrus Adamchik wrote:
As mentioned in the quoted docs, there are ways to receive
immediate notifications on the individual objects updates (if they
are updated via Cayenne). This approach, while the most powerful
on the surface, is least practical, especially across the VM. It
suffers from a number of shortcomings (as also have been mentioned
here):
* It has a potential to generate too much network traffic
* As all update events are broadcast, it has a potential to DDOS
the apps who may not care about 90% of the updates (as all
incoming events incur processing overhead), so some manual event
channel filtering may be needed.
* It does not correctly refresh cached query lists. E.g. if you
have a cached fetch for "documents that are in draft mode", and
then received an event saying that one of the drafts has changed
to "not a draft", the object will be refreshed, the list will
become stale, as its composition no longer matches the search
criteria.
* Finally, the data can change in DB by non Cayenne clients...
So I am very much in favor of the Query Cache approach that is not
documented that well, but is really simple to use:
query.setCacheStrategy(QueryCacheStrategy.LOCAL_CACHE); // or
SHARED_CACHE...
query.setCacheGroups("g1", "g2", ...);
Once you start doing that for your queries, you can perform
further cache configuration in a semi-declarative manner. E.g. I
am successfully using OSQueryCacheFactory:
dataDomain.setQueryCacheFactory(new OSQueryCacheFactory());
This ties Cayenne query cache to OSCache which allows time based
expiration of entries, cron like expressions, and forced
invalidation, including remote invalidation via JGroups. All of
that incurs nearly zero overhead, as the entries are not actively
purged from cache, but rather marked as invalid by "group" (see
'setCacheGroups' above). Cross-VM events are also sent as the
names of the groups to invalidate, not full object snapshots. This
is very powerful and easy to use stuff.
Andrus
On Apr 10, 2009, at 10:17 AM, Andrey Razumovsky wrote:
The proposed way is to use JGroups or JMS for synchronization:
http://cayenne.apache.org/doc/configuring-caching-behavior.html
2009/4/10 Lawrence Gerstley <[email protected]>
So, I have the same question here--multiple thick clients
(desktop RCP
applications), each with a DataContext tied to the same backend,
and
potential database access (direct or otherwise) from other
toolsets out of
my control. Is there a recommended strategy for refreshing each
applications
singleton DataContext to stay in synch, or manually a supplying
refresh
command to the DataContext to periodically update (and, if so,
with
what/how)?
Kind regards,
Lawrence
===================================
Lawrence Gerstley, Ph.D.
PSMI Consulting
[email protected]
Cel: (415) 694-0844
On Apr 8, 2009, at 4:22 PM, Malcolm Edgar wrote:
Hi Joe,
Your singleton cache is going to need to be update periodically
if
there are changes to the under lying database from other sources.
regards Malcolm Edgar
On Thu, Apr 9, 2009 at 7:45 AM, Joe Baldwin <[email protected]
>
wrote:
I *think* this is a life-cycle question, but there may be more
to it.
Proposed Design:
1. Standard Web page JSP using Tomcat server.
2. One of the JSP's accesses a singleton.
3. The singleton accesses and stores a database field via
Cayenne
(presumably when the class is initially loaded) and should
never need to
access the field again.
4. I would prefer it if the database field change would be
propagated to
the
singleton upon the next new client-Session.
Problem
1. Here is the odd bit: the database field can be modified via
direct
access
to the database (SQL, etc).
2. Cayenne appears not to see this change even when a new
client-Session
is
initialized.
3. I can *force* the singleton to recognize the change by
restarting
Tomcat
(but that is totally lame :) )
4. Unless I have made a mistake (which is possible), the
singleton should
be
only associated with JSP session scope. But if I am wrong,
this could be
the problem.
Obviously, I have a misunderstanding about either Cayenne or
Tomcat
caching
or perhaps its a combo of the two. It appears from my tests
that the
singleton class may be constructed the first time after Tomcat
is
restarted
and then remains persistent even across different sessions.
Are there any suggestions as to a simple design in which my
singleton
forces
re-initialized (i.e. refresh the Cayenne object from the DBMS
data) upon
each new session?
Thanks,
Joe
