Jan Wieck wrote:
>> Is there a reason for not generating all "sensible" sl_listen entries?
>> I didn't find any documentation on the performance overhead a
>> sl_listen entry causes.

> Exactly the "sensible" part of that all is important. Problems arise 
> when a node receives an event from any set-origin, which has not yet 
> been processed by its data provider for that set. For example
> 
>     1 -> 2
> 
>       3
> 
> 1 being origin, 2 is subscriber, 3 is a new node not subscribed yet. 3 
> has paths for 1 and 2, so naturally it would listen on each of them for 
> their events. If we now subscribe 3 as a cascaded node with 2 as its 
> data provider, the ENABLE_SUBSCRIPTION event that will follow from node 
> 1, on which node 3 will start copy_set, must be received by 3 from 2. 
> That is the only way that 2 at the moment where 3 starts to copy data 
> actually has data itself. It could still be busy with it's own copy_set, 
> meaning that not only the data in the tables is missing, the tables 
> themself aren't in sl_table either yet.
Ahhh... That explains alot...

So, the following should find "sensible" sl_listen entries for
node X, shouldn't it?
1) If a node Y is origin of a set that X subscribes, use
the data-provider of that subscription as the sole event-provider
for events from Y.
2) Otherwise, check for all neighbours of X if they can receive
events from Y without traversing node X. (Taking (1) into account).
If they can, use the neighbour as a provider for events from Y.

> And to spice this up a little more, reading the events is done async in 
> the remote_listen thread. They are queued and the remote_worker thread 
> will process them from the queue. At the moment where node 3 gets the 
> SUBSCRIBE_SET event, it will have a lot of stuff already queued, so it 
> better restart ASAP to throw that away and listen again, this time for 
> all 1-events on 2.
Hm.. but this doesn't influence the sl_listen logic, does it? It's more
a matter of calling RebuildListenEntries at the right time, either
from some other plpgsql function or from slon.

BTW, is there a way to disable the automatic generation of sl_listen entries?

greetings, Florian Pflug