On 5/21/2010 10:42 AM, Cyril Scetbon wrote:
> 
> Jan Wieck a écrit :
>> I don't care much about that one old event. It does no harm other than 
>> currently confusing test_slony_state. What I worry about is attempting 
>> to failover in the case of emergency with an only half functioning path 
>> network.
>>   
> I don't really understand the issue you're talking about... Certainly 
> I've a weak knowledge of your code :)
> You're talking about missing errors in network cause there are no SYNC 
> generated on a receiver ? If yes, if it confirms events from others it's 
> not enough to say that everything works ?

Let me try to explain the problem.

In a multi node cluster, not every node necessarily needs to be able to 
talk to every other node. Let us just look at a cascaded 3 node cluster:

     1 - 2 - 3

This setup requires 4 sl_path entries to work:

     server=1, client=2
     server=2, client=1
     server=2, client=3
     server=3, client=2

And it is supposed to generate the following sl_listen rows:

     origin=1, receiver=2, provider=1
     origin=1, receiver=3, provider=2
     origin=2, receiver=1, provider=2
     origin=2, receiver=3, provider=2
     origin=3, receiver=1, provider=2
     origin=3, receiver=2, provider=3

It does not matter which node is currently the origin of any set at all. 
All these paths and connections are important for the health and well 
being of the Slony cluster. If for example the listening for events from 
2, receiver=3 would be broken, then node 3 would still perfectly fine 
replicate data originating from 1. But as soon as you move set to node 
2, it would start falling behind and you effectively lose your second 
level backup.

This is why Slony originally created a SYNC on EVERY node at least every 
10 seconds. Just so there is some harmless event passing going on to 
have something to monitor and keep sl_status looking good.

That is what got removed and that is what I think we should put back.


Jan

-- 
Anyone who trades liberty for security deserves neither
liberty nor security. -- Benjamin Franklin