On Mon, Jan 7, 2019 at 6:54 PM Magnus Hagander <mag...@hagander.net> wrote: > > On Mon, Jan 7, 2019 at 9:01 AM Masahiko Sawada <sawada.m...@gmail.com> wrote: >> >> Hi, >> >> Logical replication enables us to replicate data changes to different >> major version PostgreSQL as the doc says[1]. However the current >> logical replication can work fine only if replicating to a newer major >> version PostgreSQL such as from 10 to 11. Regarding using logical >> replication with older major version, say sending from 11 to 10, it >> will stop when a subscriber receives a truncate change because it's >> not supported at PostgreSQL 10. I think there are use cases where >> using logical replication with a subscriber of an older version >> PostgreSQL but I'm not sure we should support it. >> >> Of course in such case we can set the publication with publish = >> 'insert, update, delete' to not send truncate changes but it requres >> users to recognize the feature differences between major vesions and >> in the future it will get more complex. So I think it would be better >> to be configured autometically by PostgreSQL. >> >> To fix it we can make subscribers send its supporting message types to >> the publisher at a startup time so that the publisher doesn't send >> unsupported message types on the subscriber. Or as an another idea, we >> can make subscribers ignore unsupported logical replication message >> types instead of raising an error. Feedback is very welcome. >> >> [1] https://www.postgresql.org/docs/devel/logical-replication.html > > > How would that work in practice? > > If an 11 server is sent a message saying "client does not support truncate", > and immediately generates an error, then you can no longer replicate even if > you turn off truncate. And if it delays it until the actual replication of > the item, then you just get the error on the primary ìnstead of the standby? > > I assume you are not suggesting a publication with truncation enabled should > just ignore replicating truncation if the downstream server doesn't support > it? Because if that's the suggestion, then a strong -1 from me on that. >
I'm thinking that the we can make the pgoutput plugin recognize that the downstream server doesn't support it and not send it. For example, even if we create a publication with publish = 'truncate' we send nothing due to checking supported message types by pgoutput plugin if the downstream server is PostgreSQL server and its version is older than 10. Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center
