Hi all, PostgreSQL 9.6 introduced the priority-based multiple synchronous replication and PostgreSQL 10 introduced the quorum-based one. Initially I was thinking to use both synchronous replication ways in combination but it's not supported yet for now. It's useful for some use cases where for example we have three replicas: nodeA, nodeB and nodeC. Two of them (nodeA and nodeC) are for read replica and another one (nodeB) is for the backup. In this case we would want to replicate the data synchronously to nodeB while replicating data to the nodeA and nodeC using quorum-based synchronous replication. To cover such a use case we need a feature allowing us to use both in combination. IIUC other use cases are also mentioned on earlier discussion.
To implement there are two ideas. 1. Use two synchronous replication ways in combination. For above example, we can set s_s_names = 'First 2 (nodeB, Any1(nodeA, nodeC))'. This approach allows us to set a nested set of nodes in s_s_names. We can consider supporting the more nested solution but one nested level would be enough for most cases. Also, it would be useful if one synchronization method can have multiple another synchronization method. For example, I can imagine a use case where each two backup data centers have two replicas and we want to send the data synchronously either one replica on each data center. We can set s_s_name = 'First 2( Any 1(nodeA, nodeB), Any 1(nodeC, nodeD))'. It might be over engineering but it would be worth to consider it. 2. Extend quorum-based synchronous replication so that we can specify the node that we want to replicate the data synchronously. For above example, if we can set s_s_names = 'Any 2 (nodeA, *nodeB, nodeC)' then the master server wait for nodeB and either nodeA or nodeC. That is, a special mark '*' means that the marked nodes is preferentially selected as synchronous server. This approach is more simpler than #1 approach but we cannot nest more than one and a synchronization method cannot have an another method.. Feedback and comment are very welcome. Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers