If you're ingesting continuously generated time-series data such as logs, events, and metrics into OpenSearch, you're likely in a scenario where the number of documents grows rapidly and you don't need to update older documents.
A typical workflow to manage time-series data involves multiple steps, such as creating a rollover index alias, defining a write index, and defining common mappings and settings for the backing indices.
Data streams simplify this process and enforce a setup that best suits time-series data, such as being designed primarily for append-only data and ensuring that each document has a timestamp field.
A data stream is internally composed of multiple backing indices. Search requests are routed to all the backing indices, while indexing requests are routed to the latest write index. [ISM]({{site.url}}{{site.baseurl}}/im-plugin/ism/index/) policies let you automatically handle index rollovers or deletions.
To create a data stream, you first need to create an index template that configures a set of indices as a data stream. The `data_stream` object indicates that it’s a data stream and not a regular index template. The index pattern matches with the name of the data stream:
```json
PUT _index_template/logs-template
{
"index_patterns": [
"my-data-stream",
"logs-*"
],
"data_stream": {},
"priority": 100
}
```
In this case, each ingested document must have an `@timestamp` field.
You also have the ability to define your own custom timestamp field as a property in the `data_stream` object. You can also add index mappings and other settings here, just as you would for a regular index template.
```json
PUT _index_template/logs-template-nginx
{
"index_patterns": "logs-nginx",
"data_stream": {
"timestamp_field": {
"name": "request_time"
}
},
"priority": 200,
"template": {
"settings": {
"number_of_shards": 1,
"number_of_replicas": 0
}
}
}
```
In this case, `logs-nginx` index matches both the `logs-template` and `logs-template-nginx` templates. When you have a tie, OpenSearch selects the matching index template with the higher priority value.
### Step 2: Create a data stream
After you create an index template, you can create a data stream.
You can use the data stream API to explicitly create a data stream. The data stream API initializes the first backing index:
```json
PUT _data_stream/logs-redis
PUT _data_stream/logs-nginx
```
You can also directly start ingesting data without creating a data stream.
Because we have a matching index template with a data_stream object, OpenSearch automatically creates the data stream:
You can see the name of the timestamp field, the list of the backing indices, and the template that's used to create the data stream. You can also see the health of the data stream, which represents the lowest status of all its backing indices.
To see more insights about the data stream, use the `_stats` endpoint:
To ingest data into a data stream, you can use the regular indexing APIs. Make sure every document that you index has a timestamp field. If you try to ingest a document that doesn't have a timestamp field, you get an error.
You can also set up an [Index State Management (ISM) policy]({{site.url}}{{site.baseurl}}/im-plugin/ism/policies/) to automate the rollover process for the data stream.
The ISM policy is applied to the backing indices at the time of their creation. When you associate a policy to a data stream, it only affects the future backing indices of that data stream.
### Step 6: Manage data streams in OpenSearch Dashboards
To manage data streams from OpenSearch Dashboards, open **OpenSearch Dashboards**, choose **Index Management**, select **Indices** or **Policy managed indices**.
You see a toggle switch for data streams that you can use to show or hide indices belonging to a data stream.
You can also use [asynchronous search]({{site.url}}{{site.baseurl}}/search-plugins/async/index/), [SQL]({{site.url}}{{site.baseurl}}/search-plugins/sql/index/), and [PPL]({{site.url}}{{site.baseurl}}/search-plugins/sql/ppl/index/) to query your data stream directly. You can also use the security plugin to define granular permissions for the data stream name.
