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From Laurent Marchal <lmarc...@smaeur.com>
Subject Thread-safe Write DataContext
Date Tue, 25 Mar 2008 18:30:55 GMT
Hi all !

I spent some time searching documentation about the thread-safety status 
of DataContext, i found some answers on this mailing list but i would 
like to know some details :

I have a big application (Eclipse RCP based) which monitors a database, 
so i have to poll the database each 10 seconds.
To be simple i have a single DataContext in my App in : 
Session->getDataContext() and i created an API that look like

public static List<User> getAll()
{
    SelectQuery q = new SelectQuery(User.class);
   try {
            return (List<User>)Session.getDataContext().performQuery(q);
         } catch(CayenneRuntimeException e  {
            //manage exception
        }
}

So everywhere the API use a single DataContext no bound to a thread.

Naturally to be reactive the application fetch the data from the 
database in a background thread (actually there is one background thread 
for each view in the application), and what i've seen is that read 
operations are thread-safe within a DataContext so i don't care and all 
threads use the API and so the same DataContext.

Here comes the complex part : I need this single DataContext to have a 
good cache in my app, and because the application is 90% visualization 
and only 10% modifications, so i just had to find a thread-safe 
workaround for writing data.
I tried some solutions :
- Bind a DC per thread is not a good solution because all my fetching is 
in background threads so the main Session DC (in the UI thread) is never 
updated.
- I have tried to create a childDataContext bound to the current thread 
for writing, but i had some strange behaviors, and i don't know if the 
flushToParent() is thread safe ?

So i would like to know if the new LifecycleListener can be used to 
"lock" the DataContext while writing, to have a single thread-safe R/W 
DataContext like :

dataContext.getEntityResolver().getCallbackRegistry().addDefaultListener(new 
LifecycleListener(){

                Lock lock = new ReentrantLock();

                public void postPersist(Object entity) {
                    lock.unlock();
                }
                public void postRemove(Object entity) {
                    lock.unlock();
                }
                public void postUpdate(Object entity) {
                    lock.unlock();
                }
                public void prePersist(Object entity) {
                    lock.lock();
                }
                public void preRemove(Object entity) {
                    lock.lock();
                }
                public void preUpdate(Object entity) {
                    lock.lock();
                }
});

Do you think it can be a good solution ?

Thanks.

Laurent Marchal.

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