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From Simone Gianni <s.gia...@thebug.it>
Subject Re: Flow and hibernate sessions
Date Fri, 10 Mar 2006 01:14:59 GMT
Hi Mark,
don't want to start a thread on which solution is better than another, 
they all try to get there :) . I find your solution much more 
complicated, the hibernate adapter is installed with 2 lines in 
cocoon.xconf, a "hibernate" word added in the sitemap where you declare 
flows, and explicit flush in the DAOs (which someone says is a good 
practice anyway).

Detaching and retaching objects was the solution of 
HibernateflowAdapter1 (the one i made before that i sent you), but 
brought us only problems : a lot of work in DAOs to merge eventually 
preloaded and then modified objects, hibernate not retaching objects or 
lazy not working after retach (expecially on collections), duplicated 
objects, basically impossible to avoid the situation where an exception 
occurrs because of the state of an object that is retached every time, 
hibernate persisting partially modified objects between two different 
flow steps.

Some of this problems were caused by the hibernate-spring interaction, 
so maybe in a pure hibernate configuration this would not happen.

Simone



Mark H wrote:

> Hi Simone,
>
>  
>
> It looks interesting and complicated! For the moment I've decided to 
> detach the object from the session by calling Hibernate.initialize() 
> on the collections and getters on the associations. This is 
> implemented in the object's DAO. This can be used for objects that 
> span a web session as well as a flow. It would be handy if Hibernate 
> provided a Hibernate.detach() method. I know this will introduce 
> unnecessary database fetching for associations/collections that are 
> not needed, though it's probably better than non-lazy loading.
>
>  
>
> I hope to spend sometime trying to implement your "flow" wrapper - it 
> looks interesting. Though I'm a little uncomfortable having open 
> sessions hanging around, though as you said it doesn't hold onto 
> database connections.
>
>  
>
> Mark H
>
> ------------------------------------------------------------------------
>
> *From:* 
> users-return-85662-mailinglists=markhorgan.com@cocoon.apache.org 
> [mailto:users-return-85662-mailinglists=markhorgan.com@cocoon.apache.org] 
> *On Behalf Of *Simone Gianni
> *Sent:* 06 March 2006 18:31
> *To:* users@cocoon.apache.org
> *Subject:* Re: Flow and hibernate sessions
>
>  
>
> Hi Mark,
> we had the same problem with Hibernate. I solved it with a "flow" 
> wrapper. This wrapper wraps the flow interpreter of your choice 
> (tested with javaflow a lot, simple tests with javascript, but should 
> not give big problems), opens a new session when the flow is first 
> accessed (map:call function), detaches the session from the DB when 
> the flow sends a page, then retaches the same session when the 
> continuation is invoked (map:call continuation). It schedules a task 
> to recollect and close sessions connected to continuations which has 
> been marked as invalid.
>
> This magically gives you a coherent hibernate session inside your 
> flow, but it has some drawbacks :
> - Sessions are left there, open (but disconnected from the database, 
> so the DB collection is free) for all the time it takes for the 
> continuation tree to be marked invalid, so potentially much more than 
> needed, but there isn't currently a notion of "flow finished", so i 
> don't think there can be any other way. This will increase memory usage.
> - It's written for a hibernate + spring configuration, since it uses 
> spring transaction manager to set the session as "current session" 
> while executing the flow.
> - Since the DB connection is detached, it forces a "flush never" to 
> avoid spring/hibernate to flush the connection and thus persist object 
> while they are, for example, being edited in a form or manipulated by 
> the flow. This means you must manually flush the connection (or the 
> spring hibernate template in DAOs, or whatever else) to persist your 
> changes where needed (you can access the session inside the flow with 
> request.getAttribute("HIBERNATE_SESSION") to do whatever you want with 
> it). This is again needed since there is no way of knowing 
> automatically when it is ok to flush the session and when we are just 
> in an intermediate sendpageandwait.
> - Since the DB connection is detached, this can create potential 
> problems in database transaction for databases which does not support 
> transactions spanning different connections (which one does?) when not 
> using another external transaction system.
>
> But also brings many advantages :
> - You can use lazy loading everywhere, since the session will be there 
> as long as the continuation will be there.
> - No more worries about objects being persisted in the middle of a 
> flow-form interaction (thus causing hibernate exceptions, or even 
> worse data corruption)
> - Hibernate persistence will now be "horizontal and transparent", at 
> least in your flow (DAO, or backend services, will always need to know 
> something about hibernate)
> - No problems of merging, stale objects, duplicate objects ... that 
> would arise with other tecniques (object detach, retach; multiple 
> sessions etc..)
>
> We are currently developing a lot of stuff using cocoon + hibernate + 
> spring with this component.
>
> You can find javadoc in the main HibernateFlowAdapter2 class, 
> explaining how to use it, feel free to ask everything you don't 
> understand.
>
> Unfortunately this code will not enter in the cocoon repository cause 
> it would include dependencies on hibernate, so i think we can use this 
> thread to keep it up to date in case you improve it.
>
>
> Hope it helps!
> Simone
>
> Mark H wrote:
>
> What is the best way to handle hibernate sessions in flow? At the 
> moment I'm using a servlet filter to close sessions after each request 
> but this makes it awkward when dealing with objects that span a number 
> of requests but are within one flow function (I'm using flowscript). I 
> could have the session span the flowscript function but if the user 
> never finishes the flow the session will never be closed.
>
>  
>
> Mark H
>
>  
>
> -- 
> Simone Gianni
>
-- 
Simone Gianni

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