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From Daniel Dekany <ddek...@freemail.hu>
Subject Re: Getting "custom" objects from the repository?
Date Sat, 16 Apr 2005 18:17:07 GMT
Saturday, April 16, 2005, 3:58:11 PM, Julien Viet wrote:

> invalidate the cache :
>
> - time based invalidation, you say that an entry is valid for 1 minute
> for instance
> - notification, you can use JCR observations mechanism to remove entries
> from the cache

Neither works. As I said "Of course no hazards and unpredictable delays
are allowed after the repository was modified", and it can't be achieved
with asynchronous event notifications and time based invalidation. It's
so obvious that I guess it was not clear what I meant with "no hazards
and unpredictable delays", so let me tell a simple example: You have an
MVC web application, and for each page visit you use script (like a
Jython script) and a HTML template. These are from the Web application
point of view mycms.Script objects (stores the pre-parsed (compiled)
script) and mycms.Template objects (stores the pre-parsed template), and
from the JCR point of with nodes (maybe of type mycms:template and
mycms:script). These object are seldom changed, but if they are, you
have certain expectations. Assume I start transaction, modify a script
and a template in the repository, and then commit the transaction. Now,
when you start a new transaction later (e.g. for satisfying a HTTP
request), you want to see the new objects, not the old ones, especially
if you want to check what you have just committed. Also, you don't want
to face with inconsistent states, where for example the script object is
in its old state, and the template object is in new state (since in
general the old template works correctly only with the old script, and
the new template works correctly only with the new script, and mixing
the two will result in error 500 pages and other malfunctions). It's
basically the same as the ACID thing of the RDBMS-es, and surely I don't
need to explain why is this absolutely important for server side
applications. Now, on the JCR level everything is "ACID" (well, it can
be if transactions are supported etc). But you will seldom use raw JCR
without some kind of object mapping behind it, right? And if all the
"ACID" loses with the object mapping (as the cache can't be kept
precisely in sync with the content of the repository), then in practice
you have just trashed the ACIDity of the JCR repository. Does anybody
get what I'm saying? You have to modify sites while they are running...
think about a big "portal", so you need that ACID very much.

> cheers
>
> Daniel Dekany wrote:
>
>>I suppose most applications want to work with objects like mycms.User,
>>mycms.PageTemplate, myapp.Book, etc., rather than directly with the tree
>>of items that JCR shows. I don't see what's the intended way of doing
>>this. With a concrete example: I would like to store page templates in
>>the repository, which are complex and frequently used objects, so they
>>shouldn't be recreated from a binary property (or from string string
>>property, whatever) every time they are read from the repository (that
>>is, for each page hits). I could use cache in front of JCR that caches
>>the template objects, but how to ensue that the cache is in sync with
>>the repository? Of course no hazards and unpredictable delays are
>>allowed after the repository was modified.
>>
>>What's the planned way of doing things like this?
>>
>>(I know this is not a Jackrabbit but a JCR question and I apologize for
>>that, but I didn't found a more suitable public list.)

-- 
Best regards,
 Daniel Dekany


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