Shibboleth logout suboptimal and logout implementation for RedisI

The main problem here is that logout has been implemented only for file or db session.

We are using Redis sessions. So I wanted to implement the logout for redis.

I dived a little bit into it. Here are my findings : 

When an idp wants to logout it call a soap web service in moodle passing to it a shibbolethid.

Moodle has then to find the session with that shibboleth id and destroy it. The problem is that this shibbolethid is stored in the session itself. So moodle has to iterate over all the active sessions decode it and check if shibbolethid is the good one. This is suboptimal and can take a long time in a big platform  (we have platforms with more than 8000 active sessions).

What I propose to do is the following :

• Stores the association of shibloethid and sessionid at connection (auth/shibboleth/auth.php)
• At logout use that correspondence to find sessionid

With that correspondence it is then very easy to implements logout for Redis, Memcache, etc ...

My question is what is  the better choice :

• create a new table withe shibid - sid correspondence
• store that that shibid directly in mdl_sessions table

Myself I have a preference for the second option even if it is modifying a core table. 

What do you guys think?

Noel Dieschburg