[FEATURE] Support for no_alias and min_state_age in ISM Transitions

[Tarun-kishore]

Is your feature request related to a problem?
Yes. OpenSearch ISM currently lacks conditions to:

• Determine if an index is still pointed by an alias.
• Control transitions based on the amount of time an index has spent in a particular ISM state.

This limits the ability to express real-world policies like:

• Transitioning to an archive state only after the index is unaliased. This is especially helpful when users query indices via aliases only.
• Delaying deletion for a fixed retention window (e.g., 7 days in archive before transitioning to delete).

Why min_state_age is required even though min_index_age is supported

min_index_age in ISM measures time since the index was created, which can lead to premature transitions if the index enters a new state much later in its lifecycle.

For example, if an index is created and stays in the hot state for some time — say 5 days — before transitioning to archive due to a dynamic condition like no_alias: true or min_doc_count, a transition based on min_index_age: 7d would cause the index to move to delete just 2 days after entering archive. This breaks the expectation that the index should remain in the archive state for a full 7 days. Since min_index_age measures time since index creation and is not tied to how long the index has been in its current state, it cannot enforce state-specific retention periods. In contrast, min_state_age starts counting from the moment the index enters a given state, making it possible to implement accurate, state-specific delays — like “stay in archive for 7 days before deletion” — regardless of when or why the transition to archive happened.

By contrast, min_state_age starts counting from when the index enters the current state, allowing policies to enforce precise retention windows or delay transitions. This enables reliable patterns like “stay in archive for 7 days” or “delay deletion for 24 hours after entering a cold state.”

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What solution would you like?
Introduce two new transition conditions in ISM:

1. no_alias (boolean)
   Enables ISM to transition based on whether an index has any alias.
   Example use cases:

• no_alias: true: transition only after index is removed from alias (e.g., post-rollover).
• no_alias: false: transition while alias is still present (e.g., ingest-to-archive pipelines).

2. min_state_age (string duration, e.g., 7d, 5m)

• Enables transitions only after an index has spent a minimum time in its current ISM state.
• Useful for policies requiring retention or cooling periods.

These conditions should be:

• Supported in ISM policies as standard transition conditions.
• Visible in ISM explain output.

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What alternatives have you considered?

• Manually removing aliases before transitions — error-prone and hard to automate.
• Estimating retention using min_index_age, which lacks per-state accuracy.
• Using external scripts to orchestrate retention windows.

These alternatives are more fragile and require custom operational overhead.

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Do you have any additional context?
These features were developed and tested locally. They are intended to make ISM more declarative and better suited to real-world search index lifecycle needs (e.g. retention cleanup, time-delayed archiving).

Here’s an example policy this would support:

{
  "policy": {
    "default_state": "hot",
    "states": [
      {
        "name": "hot",
        "actions": [],
        "transitions": [
          {
            "state_name": "archive",
            "conditions": {
              "no_alias": true
            }
          }
        ]
      },
      {
        "name": "archive",
        "actions": [],
        "transitions": [
          {
            "state_name": "delete",
            "conditions": {
              "min_state_age": "7d"
            }
          }
        ]
      },
      {
        "name": "delete",
        "actions": [{ "delete": {} }],
        "transitions": []
      }
    ]
  }
}