Data streamsedit

A data stream is a convenient, scalable way to ingest, search, and manage continuously generated time series data.

Time series data, such as logs, tends to grow over time. While storing an entire time series in a single Elasticsearch index is simpler, it is often more efficient and cost-effective to store large volumes of data across multiple, time-based indices. Multiple indices let you move indices containing older, less frequently queried data to less expensive hardware and delete indices when they’re no longer needed, reducing overhead and storage costs.

A data stream is designed to give you the best of both worlds:

  • The simplicity of a single named resource you can use for requests
  • The storage, scalability, and cost-saving benefits of multiple indices

You can submit indexing and search requests directly to a data stream. The stream automatically routes the requests to a collection of hidden backing indices that store the stream’s data.

You can use index lifecycle management (ILM) to automate the management of these backing indices. ILM lets you automatically spin up new backing indices, allocate indices to different hardware, delete old indices, and take other automatic actions based on age or size criteria you set. Use data streams and ILM to seamlessly scale your data storage based on your budget, performance, resiliency, and retention needs.

When to use data streamsedit

We recommend using data streams if you:

  • Use Elasticsearch to ingest, search, and manage large volumes of time series data
  • Want to scale and reduce costs by using ILM to automate the management of your indices
  • Index large volumes of time series data in Elasticsearch but rarely delete or update individual documents

Backing indicesedit

A data stream consists of one or more backing indices. Backing indices are hidden, auto-generated indices used to store a stream’s documents.

data streams diagram

To create backing indices, each data stream requires a matching index template. This template acts as a blueprint for the stream’s backing indices. It specifies:

  • One or more wildcard (*) patterns that match the name of the stream.
  • The mappings and settings for the stream’s backing indices.
  • That the template is used exclusively for data streams.

Every document indexed to a data stream must have a @timestamp field. This field can be mapped as a date or date_nanos field data type by the stream’s index template. If the template does not specify a mapping, the @timestamp field is mapped as a date field with default options.

The same index template can be used to create multiple data streams.

Generationedit

Each data stream tracks its generation: a six-digit, zero-padded integer that acts as a cumulative count of the data stream’s backing indices. This count includes any deleted indices for the stream. The generation is incremented whenever a new backing index is added to the stream.

When a backing index is created, the index is named using the following convention:

.ds-<data-stream>-<generation>

For example, the web-server-logs data stream has a generation of 34. The most recently created backing index for this data stream is named .ds-web-server-logs-000034.

Because the generation increments with each new backing index, backing indices with a higher generation contain more recent data. Backing indices with a lower generation contain older data.

A backing index’s name can change after its creation due to a shrink, restore, or other operations. However, renaming a backing index does not detach it from a data stream.

Read requestsedit

When a read request is sent to a data stream, it routes the request to all its backing indices. For example, a search request sent to a data stream would query all its backing indices.

data streams search request

Write indexedit

The most recently created backing index is the data stream’s only write index. The data stream routes all indexing requests for new documents to this index.

data streams index request

You cannot add new documents to a stream’s other backing indices, even by sending requests directly to the index.

Because it’s the only index capable of ingesting new documents, you cannot perform operations on a write index that might hinder indexing. These prohibited operations include:

Rolloveredit

When a data stream is created, one backing index is automatically created. Because this single index is also the most recently created backing index, it acts as the stream’s write index.

A rollover creates a new backing index for a data stream. This new backing index becomes the stream’s write index, replacing the current one, and increments the stream’s generation.

In most cases, we recommend using index lifecycle management (ILM) to automate rollovers for data streams. This lets you automatically roll over the current write index when it meets specified criteria, such as a maximum age or size.

However, you can also use the rollover API to manually perform a rollover. See Manually roll over a data stream.

Append-onlyedit

For most time series use cases, existing data is rarely, if ever, updated. Because of this, data streams are designed to be append-only.

You can send indexing requests for new documents directly to a data stream. However, you cannot send the update or deletion requests for existing documents directly to a data stream.

Instead, you can use the update by query and delete by query APIs to update or delete existing documents in a data stream. See Update documents in a data stream by query and Delete documents in a data stream by query.

If needed, you can update or delete a document by submitting requests to the backing index containing the document. See Update or delete documents in a backing index.

If you frequently update or delete existing documents, we recommend using an index alias and index template instead of a data stream. You can still use ILM to manage indices for the alias.