﻿---
title: Time-bound indices and dimension-based routing
description: Unlike regular data streams that only write to the most recent backing index, time series data streams (TSDS) use time-bound backing indices that accept...
url: https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/5510/manage-data/data-store/data-streams/time-bound-tsds
products:
  - Elasticsearch
applies_to:
  - Elastic Cloud Serverless: Generally available
  - Elastic Stack: Generally available
---

# Time-bound indices and dimension-based routing
Unlike regular data streams that only write to the most recent backing index, time series data streams (TSDS) use time-bound backing indices that accept documents based on their timestamp values. This page provides details and best practices to help you work with time-bound indices.

## How time-bound indices work

Each TSDS backing index has a time range for accepted `@timestamp` values, defined by two settings:
- [`index.time_series.start_time`](https://docs-v3-preview.elastic.dev/elastic/elasticsearch/tree/main/reference/elasticsearch/index-settings/time-series#index-time-series-start-time): The earliest accepted timestamp (inclusive)
- [`index.time_series.end_time`](https://docs-v3-preview.elastic.dev/elastic/elasticsearch/tree/main/reference/elasticsearch/index-settings/time-series#index-time-series-end-time): The latest accepted timestamp (exclusive)

When you add a document to a TSDS, Elasticsearch adds the document to the appropriate backing index based on its `@timestamp` value. This means a TSDS can write to multiple backing indices simultaneously, not just the most recent one.
![time bound indices](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/5510/manage-data/images/elasticsearch-reference-time-bound-indices.svg)

If no backing index can accept a document's `@timestamp` value, Elasticsearch rejects the document.
Elasticsearch automatically configures `index.time_series.start_time` and `index.time_series.end_time` settings as part of the index creation and rollover process.

### Accepted time range for adding data

A TSDS is designed to ingest current metrics data. When the TSDS is first created, the initial backing index has the following settings:
- An `index.time_series.start_time` value set to `now - index.look_back_time`
- An `index.time_series.end_time` value set to `now + index.look_ahead_time`

Only data that falls within this range is indexed.
To check the accepted time range for writing to a TSDS, use the [get data stream API](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-indices-get-data-stream):
```json
```

<tip>
  Writes within the accepted time range might still be rejected. The following actions can affect the writable time range, either because they make a backing index read-only or remove it:
  - [Delete](https://docs-v3-preview.elastic.dev/elastic/elasticsearch/tree/main/reference/elasticsearch/index-lifecycle-actions/ilm-delete)
  - [Downsample](https://docs-v3-preview.elastic.dev/elastic/elasticsearch/tree/main/reference/elasticsearch/index-lifecycle-actions/ilm-downsample)
  - [Force merge](https://docs-v3-preview.elastic.dev/elastic/elasticsearch/tree/main/reference/elasticsearch/index-lifecycle-actions/ilm-forcemerge)
  - [Read only](https://docs-v3-preview.elastic.dev/elastic/elasticsearch/tree/main/reference/elasticsearch/index-lifecycle-actions/ilm-readonly)
  - [Searchable snapshot](https://docs-v3-preview.elastic.dev/elastic/elasticsearch/tree/main/reference/elasticsearch/index-lifecycle-actions/ilm-searchable-snapshot)
  - [Shrink](https://docs-v3-preview.elastic.dev/elastic/elasticsearch/tree/main/reference/elasticsearch/index-lifecycle-actions/ilm-shrink), which might then revert the read-only status at the end of the action
  Index lifecycle management will **not** proceed with executing these actions until [`index.time_series.end_time`](https://docs-v3-preview.elastic.dev/elastic/elasticsearch/tree/main/reference/elasticsearch/index-settings/time-series#index-time-series-end-time) has passed.
</tip>


### Dimension-based routing

In addition to time-based routing, time series data streams use dimension-based routing to determine which shard to route data to. Documents with the same dimensions are routed to the same shards, using one of two strategies:
<definitions>
  <definition term="Index dimensions Elastic Stack: Generally available since 9.2 Elastic Cloud Serverless: Generally available">
    Routing based on the internally managed `index.dimensions` setting.
  </definition>
  <definition term="Routing path">
    Routing based on the [`index.routing_path`](https://docs-v3-preview.elastic.dev/elastic/elasticsearch/tree/main/reference/elasticsearch/index-settings/time-series#index-routing-path)  setting (as a fallback).
  </definition>
</definitions>

The `index.dimensions`-based strategy offers better ingest performance. It uses a list of dimension paths that is automatically updated (and is not user-configurable). This strategy is not available for time series data streams with dynamic templates that set `time_series_dimension: true`.
To disable routing based on `index.dimensions`, set [`index.index_dimensions_tsid_strategy_enabled`](https://docs-v3-preview.elastic.dev/elastic/elasticsearch/tree/main/reference/elasticsearch/index-settings/time-series#index-dimensions-tsid-strategy-enabled) to `false`,
or manually set the [`index.routing_path`](https://docs-v3-preview.elastic.dev/elastic/elasticsearch/tree/main/reference/elasticsearch/index-settings/time-series#index-routing-path) to the dimensions you want to use:
```json
"settings": {
  "index.mode": "time_series",
  "index.routing_path": ["host", "service"]
}
```

Documents with the same dimension values are routed to the same shard, improving compression and query performance for time series data.
The `index.routing_path` setting supports wildcards (for example, `dim.*`) and can dynamically match new fields.