[hibernate-dev] Session and carrying 3rd party state

[Emmanuel Bernard]

I took some more time to think about our conversation from 2 IRC meeting ago
about offering the ability to carry session bound state not related to
ORM per se.
Below is a sum and a potential solution.
If you are short on time, read Goals, then the SessionSessionEventListener
approach and ignore the rest.

## Goals

The goal is to be able to carry session bound state for non-ORM projects
like search and OGM.
We want to avoid ThreadLocal use, in particular when it cannot be
protected by a try / catch for proper resource cleaning.
We want to avoid a structure that would be shared across threads concurrently
i.e. using ConcurrentHashMap with a Weak reference to the session.
It needs to be informed of a call to session.clear()
It needs to be informed of a call to session.close()
The state needs to be accessed from event listener implementations and custom
persister / loader implementations i.e. SessionImplementor and maybe
EventSource?

## Approaches

I'll discuss the approaches we explored in the meeting and then offer an
alternative one that I think is pretty interesting and fit better with
the current Session model.

### Map

This is essentially sticking a map on SessionImpl and use it to carry
state.
The following is a pseudo implementation


    /**
     * interface implemented by SessionImpl and the like
     */
    interface SessionCompanion {
        Object getCompanion(String key);
        void addCompanion(String key, Object companion);
        void removeCompanion(String key);
    }
    
    
    /**
     * adds a map to the SessionImpl
     */
    SessionImpl {
        private Map<String, Object> companions;
        public Object getCompanion(String key) { return companions.get(key); }
        public void addCompanion(String key, Object value) { companions.add(key, companion); }
        public void removeCompanion(String key) { companions.remove(key) }
    }

The persister or event listener would call SessionCompation.*Companion method
to put and retrieve its state.

There is no clear / close event listener loop and it would need to be added.

### Delegator

Gunnar and teve discussed an approach where the delegator would be passed to
the underlying session and be accessible via an `unwrap` method.
I have not followed the details but this approach has one major flaw: the
delegator (OgmSession, FullTextSession etc) is not always created and thus
would not be necessarily available.
A somewhat similar idea involving passing the session owner has the same
drawback. And another one described by Gunnar in
https://hibernate.atlassian.net/browse/OGM-469

### The type-safe map approach

This approach is vaguely similar to the Map approach except that the payload is
represented and looked up by Class. This has the benefit of not having
namespace problems and is generally less String-y.


    /**
     * interface implemented by SessionImpl and the like
     */
    interface SessionCompanion {
        T getCompanion(Class<T> type);
        void addCompanion(Object companion);
        void removeCompanion(Class<?> type)
        
    }
    
    
    SessionImpl {
        //could also use an array or an ArrayList
        private Map<Class<?>, Object> companions;
        public T getCompanion(Class<T> type) { return (T) companions.get(type); }
        public void addCompanion(Object companion) { companions.add(companion.getClass(), type); }
        public void removeCompanion(Class<T> type) { companions.remove(type); }
    }

Like in the Map approach, the persister or custom event listener would interact
with SessionCompanion.
There are open issues like what should be done when two objects of the same
type are added to the same session.
Likewise the clear / close hook issues need to be addressed.

### the SessionEventListener approach

I did not know but there is a concept of `SessionEventListener` which can be
added to a `SessionImplementor`. It has hooks that are addressing most of the
goals.


    //interface already exists
    interface SessionImplementor {
        public SessionEventListenerManager getEventListenerManager();
    }
    
    //interface already exists
    public interface SessionEventListenerManager extends SessionEventListener {
        // add this method to be able to retrieve a specific listener holding some state for a 3rd party project
        List<SessionEventListener> getSessionEventListeners();
    }

OGM or Search would implement a `SessionEventListener` and attach an instance to a session via `Session.addEventListeners()`.
It would require to add a method to retrieve the list of `SessionEventListener`s attached to a `SessionEventListenerManager`.

    List<SessionEventListeners> listeners = sessionImplementor.getSessionEventListenerManager().getEnlistedListeners();
    OgmSessionEventListener ogmListener = findOrAddOgmListener(sessionImplementor, listeners);
    ogmListener.someStuff();

## What about clear and close?

We have a few ways to react to these.
SessionEventListener is already called back when a flush begins / ends as well as when Session closes.
We need to either:

- add a clear begins / ends callback
- have the third party project add a ClearEventListener which would access the SessionEventListeners and do some magic.

The first approach has my preference and would do:


    public interface SessionEventListener {
        [...]
        void clearStart();
        void clearEnd();
    }

What do you guys think? The SessionEventListener approach feels more natural.

Emmanuel