In the past, we have discussed various approaches to replicate sequences by decoding the sequence changes from WAL. However, we faced several challenges to achieve the same, some of which are due to the non-transactional nature of sequences. The major ones were: (a) correctness of the decoding part, some of the problems were discussed at [1][2][3] (b) handling of sequences especially adding certain sequences automatically (e.g. sequences backing SERIAL/BIGSERIAL columns) for built-in logical replication is not considered in the proposed work [1] (c) there were some performance concerns in not so frequent scenarios [4] (see performance issues), we can probably deal with this by making sequences optional for builtin logical replication
It could be possible that we can deal with these and any other issues with more work but as the use case for this feature is primarily major version upgrades it is not clear that we want to make such a big change to the code or are there better alternatives to achieve the same. This time at pgconf.dev (https://2024.pgconf.dev/), we discussed alternative approaches for this work which I would like to summarize. The various methods we discussed are as follows: 1. Provide a tool to copy all the sequences from publisher to subscriber. The major drawback is that users need to perform this as an additional step during the upgrade which would be inconvenient and probably not as useful as some built-in mechanism. 2. Provide a command say Alter Subscription ... Replicate Sequences (or something like that) which users can perform before shutdown of the publisher node during upgrade. This will allow copying all the sequences from the publisher node to the subscriber node directly. Similar to previous approach, this could also be inconvenient for users. 3. Replicate published sequences via walsender at the time of shutdown or incrementally while decoding checkpoint record. The two ways to achieve this are: (a) WAL log a special NOOP record just before shutting down checkpointer. Then allow the WALsender to read the sequence data and send it to the subscriber while decoding the new NOOP record. (b) Similar to the previous idea but instead of WAL logging a new record directly invokes a decoding callback after walsender receives a request to shutdown which will allow pgoutput to read and send required sequences. This approach has a drawback that we are adding more work at the time of shutdown but note that we already waits for all the WAL records to be decoded and sent before shutting down the walsender during shutdown of the node. Any other ideas? I have added the members I remember that were part of the discussion in the email. Please feel free to correct me if I have misunderstood or missed any point we talked about. Thoughts? [1] - https://www.postgresql.org/message-id/e4145f77-6f37-40e0-a770-aba359c50b93%40enterprisedb.com [2] - https://www.postgresql.org/message-id/CAA4eK1Lxt%2B5a9fA-B7FRzfd1vns%3DEwZTF5z9_xO9Ms4wsqD88Q%40mail.gmail.com [3] - https://www.postgresql.org/message-id/CAA4eK1KR4%3DyALKP0pOdVkqUwoUqD_v7oU3HzY-w0R_EBvgHL2w%40mail.gmail.com [4] - https://www.postgresql.org/message-id/12822961-b7de-9d59-dd27-2e3dc3980c7e%40enterprisedb.com -- With Regards, Amit Kapila.
