Or we just tell the two solution and let's user decide what he wants ?

For me, i prefer to extract as i find it more robust and painless. 
I see also your point as you maybe do not want to change backend in the same time as you change the varnish (need to do a sync migration)

> i maybe won't notice that i start mixing up the hash between different groups

I'm certainly misunderstanding, but i don't see how this can happen ?