On Fri, Feb 5, 2010 at 4:52 PM, Andrew Sullivan <ajs at crankycanuck.ca> wrote:
> On Fri, Feb 05, 2010 at 04:33:49PM -0800, Tory M Blue wrote:
>
>> However if I lose a Slave node which is a provider to a Query node, I
>> don't only lose the Slave, but in all reality I lose the Slave and the
>> Qslave (since it's getting it's data from the Slave node). Unless you
>> are citing that I would actually do a switchover/failover against the
>> Slave nodes which are talking to the Qnodes.. I may need to change the
>> subject or create another thread. But I'm close, but I continue to
>> have to rebuild my QSLAVE host after failing the SLAVE host (it's
>> provider).
>
> In general, your life will be better if you stop thinking of nodes as
> having particular roles, and think of them as having one or more
> functions:
>
>  - replica
>  - forwarder
>  - origin
>
> A replica can receive data in a given set from some source, which can
> either be the data origin strictly conceived (i.e. where the data
> comes from, the active data-write source of that set), or it can be
> some forwarder of that set.
>
> In your graph, you have a single data origin for all sets (1), and you
> have two forwarders; those forwarders are also replicas (you're
> calling them "slaves": 2,3).  They become data providers for other
> replicas (which you're calling qslaves, 4,5).
>
> Every node in this graph _knows_ about the data flow.  So, if 4
> receives from 2 and 5 receives from 3 and 2 dies, 4 can be told, "Go
> get this from 3" and it will pick up where it left off last from 2.
>
> This is why Slony overhead is so great: you get the benefit of
> resilience, but it comes at the cost of nodes knowing about things
> they're not actively using.
>
> Does that help make sense of some of this?
>
> A

HAHA yes, thanks, every once in a while a slap is a good thing. Yes
putting things into the correct terms does seem to make it easier for
me to grasp.

I'm still working on when a forwarder dies, how I get that information
to the replica. Seems I just need to change paths and change the
subscription, but I guess failover does some of that for you. However
when I'm actually just dropping a node, that seems to be causing me
some issues in terms of getting the replica to know that it now needs
to talk to the forwarder that is available.

Thanks Anthony, your patience is appreciated

Tory