[RFC] When to retire the serialization backward compat code for OpenSearch 2.11/2.12/2.13

[nibix]

In OpenSearch 2.11, a custom serialization mechanism for User objects and other serialized info was introduced. Due to unforeseen issues, it had to be rolled back in OpenSearch 2.14. See #4264 for the change that did the rollback. See #4670 for an example of the issues caused by 2.11/2.12/2.13 .

There is still compatibility code which allows mixed clusters with OpenSearch versions 2.11/2.12/2.13 to co-exist with nodes with newer nodes.

My question is: When can we remove this compatibility mode?

In other words: Which OpenSearch version shall be no longer mixable with 2.11/2.12/2.13? Is it OpenSearch 3?

[nibix]

Background: This is related to the "Immutable user object" effort #5168 and intended to make the logic and the code more simple.

[cwperks]

My question is: When can we remove this compatibility mode?

It can safely be removed in 3.0.0. The upgrade path for OpenSearch requires going to the last minor version of the previous major before going to the next major version for rolling upgrades.

Its not possible to mix 2.11/2.12/2.13 nodes with 3.0.0 nodes, my understanding is that the 3.0.0 would refuse to join the cluster.

To upgrade from 2.11 -> 3.0 with rolling upgrades would require 2.11 -> 2.19 -> 3.0.0

[nibix]

Confirmed by executing

        System.out.println("minimumCompatibilityVersion: " + Version.V_3_0_0.minimumCompatibilityVersion());

minimumCompatibilityVersion: 2.19.0