On 6/4/24 06:57, Amit Kapila wrote:
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.
Agree, this requires additional steps. Not a preferred approach in my opinion. When a large set of sequences are present, it will add additional downtime for upgrade process.
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.
This is similar to option 1 except that it is a SQL command now. Still not a preferred approach in my opinion. When a large set of sequences are present, it will add additional downtime for upgrade process.
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.
At the time of shutdown a) most logical upgrades don't necessarily call for shutdown b) it will still add to total downtime with large set of sequences. Incremental option is better as it will not require a shutdown. I do see a scenario where sequence of events can lead to loss of sequence and generate duplicate sequence values, if subscriber starts consuming sequences while publisher is also consuming them. In such cases, subscriber shall not be allowed sequence consumption. -- Kind Regards, Yogesh Sharma Open Source Enthusiast and Advocate PostgreSQL Contributors Team @ RDS Open Source Databases Amazon Web Services: https://aws.amazon.com
