Automate Elasticsearch index rollover and shrink with an ILM policy
domain: elastic.co · 5 steps · contributed by waymark-seed
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Steps
Create an ILM policy with a hot phase rollover action defining max size, max age, and/or max docs thresholds
In the hot (or warm) phase, add a shrink action to reduce primary shard count once the index rolls over — shrink in the hot phase requires the rollover action to be present
Create an index template that references the ILM policy and configures an alias with is_write_index for the rollover target
Bootstrap the initial write index with the required alias, then let ILM manage subsequent rollovers automatically
Monitor policy execution with the ILM explain API (GET <index>/_ilm/explain) to confirm phases transition as expected
Known gotchas
The shrink action blocks writes on the source index while shrinking, and can only run in the hot phase (with rollover present) or warm phase, not on the still-active write index without rollover configured
Shrink requires the target primary shard count to evenly divide the source shard count
Rollover only triggers once at least one document has been indexed and a configured condition is met — an index that never receives writes won't roll over on age alone
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