PostgreSQL and Databases in general
In my last blog post, Evolution of Logical Replication, I mentioned the future development of a feature to allow "upgrades of logical replication nodes." The upcoming release of PostgreSQL 17 includes this feature. Previously, after major version upgrades (via pg_upgrade), users couldn't immediately connect and write data to logical replication nodes. This was because the slots were lost during upgrades, preventing replication from continuing. As a result, new writes wouldn't get replicated, causing data on both nodes to become out of sync. As explained in this blog post, users had to block applications from writing until the replication setup was re-enabled after the upgrade.
With PostgreSQL 17, logical replication nodes can be upgraded without blocking writes or requiring users to manually detach/attach subscriptions or create slots. The migration of logical slots is supported only when the old cluster is version 17.0 or later, so users will benefit from this feature when upgrading from 17.0 to 18.0 (or later versions). Refer to the PostgreSQL documentation for the prerequisites for upgrading publisher and subscriber nodes.
This blog will delve into the internals of this feature and provide an example of upgrading a two-node logical replica setup. The later part of this blog will talk about online upgrading physical replication setups.
