Introduce IndexSettingDeprecatedInV8AndRemovedInV9 Setting property

[alexey-ivanov-es]

Index-scope settings deprecated in one major version cannot be removed in the next major version after the deprecation because they might still be used in indices created by the previous version. These settings should remain in the code but be prohibited when new indices are created.

To achieve this, such settings should be marked as IndexSettingDeprecatedInVXAndRemovedInVY and there is a special check to prohibit their usage in newer indices. In V9 Elasticsearch Elasticsearch will still support reading indices created by V7, so IndexSettingDeprecatedInV7AndRemovedInV8 will remain in the code.

Currently, there are plans to deprecate index-scope settings in 8.18 and remove them in 9.0. This change introduces the IndexSettingDeprecatedInV8AndRemovedInV9 constant to mark these settings.

The check to enforce the prohibition of settings removed in V9 will be introduced separately once we have IndexVersion after marking all relevant settings as removed.

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[elasticsearchmachine]

Hi @alexey-ivanov-es, I've created a changelog YAML for you.

[JVerwolf]

Does IndexSettingDeprecatedInV8AndRemovedInV9 also need to be added to the following?:

• public boolean isDeprecatedAndRemoved()
• org.elasticsearch.xpack.migrate.action.CreateIndexFromSourceTransportAction#filterSettings

[alexey-ivanov-es]

@JVerwolf
No:

The check to enforce the prohibition of settings removed in V9 will be introduced separately once we have IndexVersion after marking all relevant settings as removed.

Before we have IndexVersion for V9 release these settings should continue to work as deprecated, so we shouldn't throw exception if they are used and we shouldn't filter out them during reindexing

[JVerwolf]

LGTM

[cbuescher]

Quick question after reading this:

Currently, there are plans to deprecate index-scope settings in 8.18 and remove them in 9.0. This change introduces the IndexSettingDeprecatedInV8AndRemovedInV9 constant to mark these settings.

Does "remove" here mean using the setting in 9 will throw an error, but the settings constant remains for backwards compatibility with indices created in 7 or 8? I assume so but "removal" somehow sounds like removal of the settings themselves from the codebase. We cannot really do that now for settings that might still be used in version 7 indices.

[alexey-ivanov-es]

@cbuescher

Does "remove" here mean using the setting in 9 will throw an error, but the settings constant remains for backwards compatibility with indices created in 7 or 8?

Yes, that's correct - users won't be able to use the setting anymore, but it will remain in the codebase. As you rightly said, it's still needed for backward compatibility and this is exactly why we need IndexSettingDeprecatedInV8AndRemovedInV9.