In Slony 2.2 we have decided to disallow cluster configurations like

  Node 100
     set 1, set 2----->Node 102
      |                  set 2
      |                   |
      V                   V
   Node 101----------->Node 103
      set 1              set 1, set 2


A node that receives multiple sets from the same origin will now need to 
receive those sets from the same provider.  Jan wrote about this
http://lists.slony.info/pipermail/slony1-general/2011-December/011997.html

A question that has come up is how should the subscribe set command 
behave when you reshape a cluster.

Say you have

Node 100----------------------->Node 102
   set 1,set 2                  set 1, set 2
    |
    |
    V
Node 103
   set 1, set 2

then you want to make node 103 receive from 102 instead of from 100.

Today you would do
subscribe set(id=1,provider=102,receiver=103,forward=yes);
subscribe set(id=2,provider=102,receiver=103,forward=yes);

that won't work anymore because it leaves the cluster in an invalid 
intermediate state.

We can instead

1)  Make the first subscribe set automatically also move set 2.  This 
will preserve existing slonik scripts but will mean that the old slonik 
scripts do something different than they used to (a subscribe set to 
reshape set 1 might also change the subscription for set 2).

2) Make slonik fail if you try to resubscribe a set with subscribe set 
that would leave you in an invalid state after the first command. 
Instead you would need to use a new 'reshape subscription' command that 
was like 'reshape subscriptions(receiver=103, old provider=101, new 
provider=101);

3) Make slonik fail if you ever try the subscribe set command in cases 
where the receiver is already subscribed to the set but from a different 
provider.  All reshaping would need to go through the 'reshape 
subscriptions' command described above?


I am leaning towards (3).   What do others think?

Steve