Updating the Rollover page

manage-data/lifecycle/index-lifecycle-management/rollover.md

@@ -9,50 +9,91 @@ products:
 
 # Rollover [index-rollover]
 
-When indexing time series data like logs or metrics, you can’t write to a single index indefinitely. To meet your indexing and search performance requirements and manage resource usage, you write to an index until some threshold is met and then create a new index and start writing to it instead. Using rolling indices enables you to:
+In {{es}}, the [rollover action](elasticsearch://reference/elasticsearch/index-lifecycle-actions/ilm-rollover.md) replaces your active write index with a new one whenever your index grows too large, too old, or stores too many documents.
+This is particularly useful for time-series data, such as logs or metrics, where index growth is continuous and performance can degrade over time.
 
-* Optimize the active index for high ingest rates on high-performance *hot* nodes.
-* Optimize for search performance on *warm* nodes.
-* Shift older, less frequently accessed data to less expensive *cold* nodes,
-* Delete data according to your retention policies by removing entire indices.
+Without rollover, a single index can grow until searches slow down, shard relocation takes hours, and nodes run low on disk space.
 
-We recommend using [data streams](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-indices-create-data-stream) to manage time series data. Data streams automatically track the write index while keeping configuration to a minimum.
+The rollover feature is an important part of how [index lifecycle](../index-lifecycle-management/index-lifecycle.md) and [data stream lifecycles](../data-stream.md) work to keep your indices fast and manageable. By switching the write target of an index, the rollover action provides the following benefits:
 
-Each data stream requires an [index template](../../data-store/templates.md) that contains:
+* **Automation** - works with ILM to remove manual index rotation tasks and allows for granular control over retention cycles
+* **Optimized performance** - keeps shard sizes within recommended limits (10-50 GB)
+* **Queries run faster** - improves search performance
 
-* A name or wildcard (`*`) pattern for the data stream.
-* The data stream’s timestamp field. This field must be mapped as a [`date`](elasticsearch://reference/elasticsearch/mapping-reference/date.md) or [`date_nanos`](elasticsearch://reference/elasticsearch/mapping-reference/date_nanos.md) field data type and must be included in every document indexed to the data stream.
-* The mappings and settings applied to each backing index when it’s created.
+Rollover can be triggered via the [API](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-indices-rollover) or ILM.
 
-Data streams are designed for append-only data, where the data stream name can be used as the operations (read, write, rollover, shrink etc.) target. If your use case requires data to be updated in place, you can instead manage your time series data using [index aliases](../../data-store/aliases.md). However, there are a few more configuration steps and concepts:
+## How rollover works in ILM
 
-* An *index template* that specifies the settings for each new index in the series. You optimize this configuration for ingestion, typically using as many shards as you have hot nodes.
-* An *index alias* that references the entire set of indices.
-* A single index designated as the *write index*. This is the active index that handles all write requests. On each rollover, the new index becomes the write index.
+You define a rollover action in the hot phase of an index lifecycle policy. It will run when any of the configured thresholds are met.
+You can configure the following rollover conditions:
 
-::::{note}
-When an index is rolled over, the previous index’s age is updated to reflect the rollover time. This date, rather than the index’s `creation_date`, is used in {{ilm}} `min_age` phase calculations. [Learn more](../../../troubleshoot/elasticsearch/index-lifecycle-management-errors.md#min-age-calculation).
+* **Size** - an index will rollover when it reaches a set size, for example 50 GB.
+* **Age** - an index will rollover when an index reaches a certain age, for example 7 days.
+* **Document count** - an index will rollover when a shard contains a certain number of documents, for example 2 million.
 
+::::{tip}
+Rolling over to a new index based on size, document count, or age is preferable to time-based rollovers. Rolling over at an arbitrary time often results in many small indices, which can have a negative impact on performance and resource usage.
 ::::
 
+After rollover occurs in the hot phase of an ILM policy, indices move to other index lifecycle phases like warm, cold, frozen, and delete. Rollover creates a new write index while the old one continues through the lifecycle phases.
 
+**Special rules:**
 
-## Automatic rollover [ilm-automatic-rollover]
+* Empty indices rollover is blocked even if they have an associated `max_age` that would otherwise result in a roll over occurring. A policy can override this behavior if you set `min_docs: 0` in the rollover conditions. This can also be disabled on a cluster-wide basis if you set `indices.lifecycle.rollover.only_if_has_documents` to `false`.
+* Forced rollover occurs if any shard reaches 200 million documents. Usually, a shard will reach 50 GB long before it reaches 200 million documents, but this isn’t the case for space efficient data sets.
 
-{{ilm-init}} and the data stream lifecycle enable you to automatically roll over to a new index based on conditions like the index size, document count, or age. When a rollover is triggered, a new index is created, the write alias is updated to point to the new index, and all subsequent updates are written to the new index.
+## Recommended approaches
 
-::::{tip}
-Rolling over to a new index based on size, document count, or age is preferable to time-based rollovers. Rolling over at an arbitrary time often results in many small indices, which can have a negative impact on performance and resource usage.
-::::
+Decide your approach to index rotation based on your use case and requirements.
 
+| Use case               | Recommended approach                                      | Benefits                              |
+| ---------------------- | --------------------------------------------------------- | ------------------------------------- |
+| Logs, metrics          | [Data streams](rollover.md#rollover-data-stream)          | Automatic rollover, minimal setup     |
+| Legacy indexing setup  | [Alias-based rollover](rollover.md#rollover-with-aliases) | Granular control over rollover timing |
+| Small, static datasets | No rollover                                               | Simpler management                    |
 
-::::{important}
-Empty indices will not be rolled over, even if they have an associated `max_age` that would otherwise result in a roll over occurring. A policy can override this behavior, and explicitly opt in to rolling over empty indices, by adding a `"min_docs": 0` condition. This can also be disabled on a cluster-wide basis by setting `indices.lifecycle.rollover.only_if_has_documents` to `false`.
-::::
+:::{tip}
+For new projects, use data streams. They're simple to manage and handle rollover automatically.
+:::
 
 
-::::{important}
-The rollover action implicitly always rolls over a data stream or alias if one or more shards contain 200000000 or more documents. Normally a shard will reach 50GB long before it reaches 200M documents, but this isn’t the case for space efficient data sets. Search performance will very likely suffer if a shard contains more than 200M documents. This is the reason of the builtin limit.
-::::
+### Rotating your indices with data streams [rollover-data-stream]
 
+We recommend using [data streams](../../data-store/data-streams.md) to manage time series data. Data streams automatically track the write index while keeping configuration to a minimum.
+When targeting a data stream, the new backing index becomes the data stream's writing index. The generation of new backing indices is incremented automatically when it reaches a specified age or size.
+
+Each data stream requires an [index template](../../data-store/templates.md) that contains the following:
+
+* A name or wildcard (`*`) pattern for the data stream.
+* The data stream’s timestamp field. This field must be mapped as a [`date`](elasticsearch://reference/elasticsearch/mapping-reference/date.md) or [`date_nanos`](elasticsearch://reference/elasticsearch/mapping-reference/date_nanos.md) field data type and must be included in every document indexed to the data stream.
+* The mappings and settings applied to each backing index when it’s created.
+
+For more information about this approach, refer to the [Manage time series data with data streams](../index-lifecycle-management/tutorial-automate-rollover.md#manage-time-series-data-with-data-streams) tutorial.
+
+**Data streams naming pattern**<br>
+{{es}} uses a structured naming convention for the backing indices of data streams, following this pattern:
+
+```console
+.ds-<DATA-STREAM-NAME>-<yyyy.MM.dd>-<GENERATION>
+```
+For more information about data stream naming patterns, refer to the [Generation](../../data-store/data-streams.md#data-streams-generation) section of the Data streams page.
+
+### Rotating your indices with aliases [rollover-with-aliases]
+
+Data streams are designed for append-only data, where the data stream name can be used as the operations (read, write, rollover, shrink etc.) target. If your use case requires data to be updated in place, you can instead manage your time series data using [index aliases](../../data-store/aliases.md). However, this approach requires additional configuration steps, including bootstrapping the initial index. For more details about this approach, refer to the [Manage time series data without data streams](../index-lifecycle-management/tutorial-automate-rollover.md#manage-time-series-data-without-data-streams) tutorial.
+
+:::{important}
+To roll over an index alias, the alias and its [write index](../../data-store/aliases.md#write-index) must meet the following conditions:
+
+* The index name must match the pattern `^.-\d+$*`, for example `my-index-000001`.
+* The `index.lifecycle.rollover_alias` must be configured as the alias to roll over.
+* The index must be the write index for the alias.
+:::
+
+::::{note}
+When an index is rolled over, the previous index’s age is updated to reflect the rollover time. This date, rather than the index’s `creation_date`, is used in {{ilm}} `min_age` phase calculations. [Learn more](../../../troubleshoot/elasticsearch/index-lifecycle-management-errors.md#min-age-calculation).
+
+::::
 
+**Alias-based naming pattern**<br>
+When configured correctly, the newly created write index will have a similar name to one that's been rolled over, however the six-digit, zero-padded suffix will be incremented. For example before rollover, the write index was called `my-index-000001` and after rollover, the newly created index becomes `my-index-000002` and also becomes the new write index. The alias typically shares the base name which in this example is `my-index`.