First, I think the current publisher doesn't know the version number of
client(subscriber) so we need to check the feasibility of same. Also, having
client's version number checks doesn't seem to be a good idea.
walrcv_server_version().
Besides, I thought about the problems that will happen if we try to support
replicating New PG to older PG. The following examples assume that we support the
DDL replication in the mentioned PG.
1) Assume we want to replicate from a newer PG to a older PG where partition
table has not been introduced. I think even if the publisher is aware of
that, it doesn't have a good way to transform the partition related command,
maybe one could say we can transform that to inherit table, but I feel that
introduces too much complexity.
2) Another example is generated column. To replicate the newer PG which has
this feature to a older PG without this. I am concerned that is there a way
to transform this without causing inconsistent behavior.
Even if we decide to simply skip sending such unsupported commands or skip
applying them, then it's likely that the following dml replication will cause
data inconsistency.
As I mentioned in a previous email [1], the publisher can contain code to
decide if it can proceed or not, in case you are doing a downgrade. I said
downgrade but it can also happen if we decide to deprecate a syntax. For
example, when WITH OIDS was deprecated, pg_dump treats it as an acceptable
removal. The transformation can be (dis)allowed by the protocol version or
another constant [2].
So, it seems we cannot completely support this use case, there would be some
limitations. Personally, I am not sure if it's worth introducing complexity to
support it partially.
Limitations are fine; they have different versions. I wouldn't like to forbid
downgrade just because I don't want to maintain compatibility with previous
versions. IMO it is important to be able to downgrade in case of any
incompatibility with an application. You might argue that this isn't possible
due to time or patch size and that there is a workaround for it but I wouldn't
want to close the door for downgrade in the future.