Re: Multi-Master Logical Replication

On Thu, Apr 28, 2022 at 5:20 AM Peter Smith <smithpb2250(at)gmail(dot)com> wrote:
>
> MULTI-MASTER LOGICAL REPLICATION
>
> 1.0 BACKGROUND
>
> Let’s assume that a user wishes to set up a multi-master environment
> so that a set of PostgreSQL instances (nodes) use logical replication
> to share tables with every other node in the set.
>
> We define this as a multi-master logical replication (MMLR) node-set.
>
> <please refer to the attached node-set diagram>
>
> 1.1 ADVANTAGES OF MMLR
>
> - Increases write scalability (e.g., all nodes can write arbitrary data).
> - Allows load balancing
> - Allows rolling updates of nodes (e.g., logical replication works
> between different major versions of PostgreSQL).
> - Improves the availability of the system (e.g., no single point of failure)
> - Improves performance (e.g., lower latencies for geographically local nodes)

Thanks for working on this proposal. I have a few high-level thoughts,
please bear with me if I repeat any of them:

1. Are you proposing to use logical replication subscribers to be in
sync quorum? In other words, in an N-masters node, M (M >= N)-node
configuration, will each master be part of the sync quorum in the
other master?
2. Is there any mention of reducing the latencies that logical
replication will have generally (initial table sync and
after-caught-up decoding and replication latencies)?
3. What if "some" postgres provider assures an SLA of very few seconds
for failovers in typical HA set up with primary and multiple sync and
async standbys? In this context, where does the multi-master
architecture sit in the broad range of postgres use-cases?
4. Can the design proposed here be implemented as an extension instead
of a core postgres solution?
5. Why should one use logical replication for multi master
replication? If logical replication is used, isn't it going to be
something like logically decode and replicate every WAL record from
one master to all other masters? Instead, can't it be achieved via
streaming/physical replication?

Regards,
Bharath Rupireddy.