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
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