In the original design of system indices (#50251), every system index would require application specific APIs for access via the REST layer. This works well for features that make use of system indices that live within the Elasticsearch repository but once other pieces of the stack and other features started looking at system indices then there are aspects that cause friction and pain points such as code living in multiple repositories and maintenance/ownership burden. In order to address this, system indices will be split into multiple types. What we've been calling system indices previously, will be known as internal system indices; these indices would belong to code that primarily lives and/or runs within Elasticsearch. Two new types of system indices will be introduced managed external system indices and unmanaged external system indices.

Both types of external system indices will make use of existing APIs to access system indices with a special header and value pair. The primary difference will be the management of the index; managed external system indices are those that Elasticsearch manages the creation, mappings, and settings of; unmanaged external system indices will be completely managed by a component that exists outside of Elasticsearch and performs the management using existing REST APIs.

More details about specific items to be implemented and changed will be added here.