[FEATURE] ISM action to auto scale down writers on index migration for Reader/Writer Separation #1531
Description
Is your feature request related to a problem?
In Reader/Writer Separation architecture, when indices migrate through ISM lifecycle states (e.g., Hot → Warm → Cold), writer shards (primary + write replicas) continue to consume resources even when no longer receiving write traffic.
For example, in a typical log management use case, Hot tier indices receive active writes for 7 days, then transition to Warm tier where writes become minimal or stop entirely. There's currently no automated way to optimize these resources while keeping indices searchable.
What solution would you like?
A new ISM action (e.g., optimize_for_search) that automatically scales down writer shards during state transitions.
- Leverage the existing
_scaleAPI (Search only replicas (scale to zero) with Reader/Writer Separation OpenSearch#17299) to setsearch_only: true - Scale down writer shards (primary + write replicas) while maintaining search replicas
- Include prerequisite validation (Remote Store enabled, Segment Replication enabled, etc.)
What alternatives have you considered?
- Manual API calls: Users could manually call the
_scaleAPI for each index, but this doesn't scale well for clusters managing thousands of indices with ISM policies - External automation: Building custom scripts/tools outside ISM, but this adds complexity and doesn't integrate with existing lifecycle management workflows
Do you have any additional context?
- I am interested in contributing this feature
- This addresses a follow-up item in the Reader/Writer Separation META issue: [META] Reader/Writer Separation OpenSearch#15306
- Related PR: Search only replicas (scale to zero) with Reader/Writer Separation OpenSearch#17299 (Search only replicas - scale to zero)