Re: Initial Schema Sync for Logical Replication

On Fri, Mar 24, 2023, at 8:57 AM, houzj(dot)fnst(at)fujitsu(dot)com wrote:
> 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.

[1] https://www.postgresql.org/message-id/fb7894e4-b44e-4ae3-a74d-7c5650f69f1a%40app.fastmail.com
[2] https://www.postgresql.org/message-id/78149fa6-4c77-4128-8518-197a631c29c3%40app.fastmail.com

--
Euler Taveira
EDB https://www.enterprisedb.com/