Re: Logical Decoding Failover

Venkata,

No problem - thanks for replying. I'll try to clarify.

Say I have databases A and B, where A is primary and B is a replica. I have
an application that executes 3 transactions against A, resulting in WAL log
sequence numbers 1, 2, and 3. The WAL is successfully replicated to B.

I have a logical deciding application which is pulling the logical stream,
and reads LSN 1 and 2 before the primary (A) fails. I decide to fail over
to B, which has all data on it and things work great.

Now the logical decoding client connects to B (the new primary). The
replication slot doesn't exist. So, it creates it and starts streaming.
This is where the problem lies - as it would begin streaming from LSN 4
(anything after what has already been committed), because I have no way
(that I can find) of restoring my "progress" through the WAL on a the
replicas.

As a result, my application never sees the event at LSN 3. In fact, I'm not
even sure how I could manually do this.

Hopefully that makes more sense.

Best,
Colin
On Mon, Aug 8, 2016 at 10:47 PM Venkata Balaji N <nag1010(at)gmail(dot)com> wrote:

> On Sun, Aug 7, 2016 at 9:29 PM, Colin Morelli <colin(dot)morelli(at)gmail(dot)com>
> wrote:
>
>> Venkata,
>>
>> Thanks for the reply. Unfortunately something like PgPool still won't
>> create the replication slots on all hosts, and record the LSN in a way that
>> is reusable on the secondary.
>>
>
> Yes, thats correct, pgPool does not have anything to do with replication
> slots. That is something which you need to manually configure.
>
>
>> This sort of puts logical decoding at odds with HA, correct? In the case
>> of master failover, there's no way to: a) know where in the stream you read
>> to, or b) convert that to something that can be understood by a replica
>> that's about to be promoted.
>>
>
> Can you please clarify, what did you exactly mean here ? are you referring
> to cascading standbys or something ? Failover is something which
> application has to do, to reconnect to the promoted standby. As far as
> logical decoding is concerned, you need to have a replication slot
> configured for the new master to ensure all the WALs which are not
> replicated to standby are retained. After promotion, the standby database
> becomes a standalone instance and the replication must be re-enabled. You
> can automate the standby promotion using tools like pgpool-II or pacemaker.
>
> I am trying to differentiate standby-promotion and failover mechanism here.
>
> Are there any scheduled improvements here either for 9.6 or 9.7?
>>
>
> I am not 100% sure, if there is anything developed from an failover
> mechanism perspective.
>
> Regards,
> Venkata B N
>
> Fujitsu Australia
>