AOSC - Automatic Online Schema Change

AOSC moves an OpenSearch index to a pre-created target index while the source remains live. It backfills existing documents, replays source operation history, then briefly blocks source writes to finish catch-up and swap an alias.

Run a first migrationSee how it works

Online path

Backfill and replay run while the source index continues serving application traffic.

Explicit cutover

Source writes are blocked only for final replay, validation, and alias swap.

Operational visibility

Status APIs expose coordinator phase, shard phase, counters, and errors.

When AOSC fits

AOSC is for same-cluster index migrations where you need a new target index for mappings, settings, shard layout, or document shape, but the source index must keep receiving traffic during most of the move.

Cutover is not zero-interruption

Applications must retry writes rejected during the brief source write block. Successful cutovers have been observed in the 2s-30s range, but your window depends on index size, shard count, write load, and cluster-manager responsiveness.

Run it locallyBuild the plugin, start a test cluster, keep writes running, and watch cutover.Understand the flowFollow source writes, shard workers, catch-up, and alias movement step by step.Use the APIStart, monitor, cancel, list, and clean up migrations from REST endpoints.

Why AOSC?

Many OpenSearch index properties cannot be changed in place. Changing field types, analyzers, routing assumptions, or shard counts usually means creating a new index and copying data.

AOSC automates the parts that are risky to do by hand:

• Backfill source documents into the target.
• Replay source-shard operation history so writes during backfill are applied to the target.
• Track per-shard progress and expose status through REST APIs.
• Perform alias cutover after a source write block and final catch-up.

AOSC still requires planning. The target index must be created first, applications must use an alias for cutover, and applications must retry writes rejected during the cutover write block.