ILM defines four index lifecycle phases:
- Hot: The index is actively being updated and queried.
- Warm: The index is no longer being updated but is still being queried.
- Cold: The index is no longer being updated and is seldom queried. The information still needs to be searchable, but it’s okay if those queries are slower.
- Delete: The index is no longer needed and can safely be removed.
An index’s lifecycle policy specifies which phases are applicable, what actions are performed in each phase, and when it transitions between phases.
You can manually apply a lifecycle policy when you create an index. For time series indices, you need to associate the lifecycle policy with the index template used to create new indices in the series. When an index rolls over, a manually-applied policy isn’t automatically applied to the new index.
ILM moves indices through the lifecycle according to their age. To control the timing of these transitions, you set a minimum age for each phase. For an index to move to the next phase, all actions in the current phase must be complete and the index must be older than the minimum age of the next phase.
The minimum age defaults to zero, which causes ILM to move indices to the next phase as soon as all actions in the current phase complete.
If an index has unallocated shards and the cluster health status is yellow, the index can still transition to the next phase according to its index lifecycle management policy. However, because Elasticsearch can only perform certain clean up tasks on a green cluster, there might be unexpected side effects.
To avoid increased disk usage and reliability issues, address any cluster health problems in a timely fashion.
ILM controls the order in which the actions in a phase are executed and what steps are executed to perform the necessary index operations for each action.
When an index enters a phase, ILM caches the phase definition in the index metadata. This ensures that policy updates don’t put the index into a state where it can never exit the phase. If changes can be safely applied, ILM updates the cached phase definition. If they cannot, phase execution continues using the cached definition.
ILM runs periodically, checks to see if an index meets policy criteria,
and executes whatever steps are needed.
To avoid race conditions, ILM might need to run more than once to execute all of the steps
required to complete an action.
For example, if ILM determines that an index has met the rollover criteria,
it begins executing the steps required to complete the rollover action.
If it reaches a point where it is not safe to advance to the next step, execution stops.
The next time ILM runs, ILM picks up execution where it left off.
This means that even if
indices.lifecycle.poll_interval is set to 10 minutes and an index meets
the rollover criteria, it could be 20 minutes before the rollover is complete.
ILM supports the following actions in each phase.