[Pull-based Ingestion] Add all-active ingestion mode documentation

[varunbharadwaj]

Description

Add documentation for all-active ingestion mode and related APIs in pull-based ingestion. This mode has been introduced as a document replication equivalent with additional fields added in the GetIngestionState API.

Issues Resolved

Closes #11121

Version

3.3 onwards

Checklist

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[kolchfa-aws]

Thank you, @varunbharadwaj! I provided suggestions based on my assumption that the ingestion mode depends entirely on the index’s replication type (segment vs. document replication). If that’s the case, what does the all_active parameter do? If an index is in segment replication, then you can't set all_active but if it's in document replication, what's the difference between all_active true and false?

[varunbharadwaj]

Thank you, @varunbharadwaj! I provided suggestions based on my assumption that the ingestion mode depends entirely on the index’s replication type (segment vs. document replication). If that’s the case, what does the all_active parameter do? If an index is in segment replication, then you can't set all_active but if it's in document replication, what's the difference between all_active true and false?

Thanks for the quick review @kolchfa-aws . The all-active mode is only supported when the replication type is DOCUMENT (the default replication type today). We do not want to support this mode in segment replication mode. However, the all-active mode does not really do any replication at all, and therefore a different concept from replication type.

In all-active mode, the replicas also process the indexing updates similar to document replication mode, and hence i was thinking it can be portrayed as a doc-rep equivalent in pull-based ingestion.

[varunbharadwaj]

If an index is in segment replication, then you can't set all_active but if it's in document replication, what's the difference between all_active true and false?

If all_active = true, then replication type must be document. If all_active = false, replication type must be segment.

[varunbharadwaj]

Updated the ingestion mode descriptions and added an image for each mode for clarity.

[natebower]

LGTM