# Overview

## Functionality

Here are the methods exposed by the `OnlineStore` interface, along with the core functionality supported by the method:

* `online_write_batch`: write feature values to the online store
* `online_read`: read feature values from the online store
* `update`: update infrastructure (e.g. tables) in the online store
* `teardown`: teardown infrastructure (e.g. tables) in the online store
* `plan`: generate a plan of infrastructure changes based on feature repo changes

There is also additional functionality not properly captured by these interface methods:

* support for on-demand transforms
* readable by Python SDK
* readable by Java
* readable by Go
* support for entityless feature views
* support for concurrent writing to the same key
* support for ttl (time to live) at retrieval
* support for deleting expired data

Finally, there are multiple data models for storing the features in the online store. For example, features could be:

* collocated by feature view
* collocated by feature service
* collocated by entity key

See this [issue](https://github.com/feast-dev/feast/issues/2254) for a discussion around the tradeoffs of each of these data models.

## Functionality Matrix

There are currently five core online store implementations: `SqliteOnlineStore`, `RedisOnlineStore`, `DynamoDBOnlineStore`, `SnowflakeOnlineStore`, and `DatastoreOnlineStore`. There are several additional implementations contributed by the Feast community (`PostgreSQLOnlineStore`, `HbaseOnlineStore` and `CassandraOnlineStore`), which are not guaranteed to be stable or to match the functionality of the core implementations. Details for each specific online store, such as how to configure it in a `feature_store.yaml`, can be found [here](https://docs.feast.dev/reference/online-stores).

Below is a matrix indicating which online stores support what functionality.

|                                                           | Sqlite | Redis | DynamoDB | Snowflake | Datastore | Postgres | Hbase | \[[Cassandra](https://cassandra.apache.org/_/index.html) / [Astra DB](https://www.datastax.com/products/datastax-astra?utm_source=feast)] | Milvus |
| --------------------------------------------------------- | ------ | ----- | -------- | --------- | --------- | -------- | ----- | ----------------------------------------------------------------------------------------------------------------------------------------- | ------ |
| write feature values to the online store                  | yes    | yes   | yes      | yes       | yes       | yes      | yes   | yes                                                                                                                                       | yes    |
| read feature values from the online store                 | yes    | yes   | yes      | yes       | yes       | yes      | yes   | yes                                                                                                                                       | yes    |
| update infrastructure (e.g. tables) in the online store   | yes    | yes   | yes      | yes       | yes       | yes      | yes   | yes                                                                                                                                       | yes    |
| teardown infrastructure (e.g. tables) in the online store | yes    | yes   | yes      | yes       | yes       | yes      | yes   | yes                                                                                                                                       | yes    |
| generate a plan of infrastructure changes                 | yes    | no    | no       | no        | no        | no       | no    | yes                                                                                                                                       | no     |
| support for on-demand transforms                          | yes    | yes   | yes      | yes       | yes       | yes      | yes   | yes                                                                                                                                       | yes    |
| readable by Python SDK                                    | yes    | yes   | yes      | yes       | yes       | yes      | yes   | yes                                                                                                                                       | yes    |
| readable by Java                                          | no     | yes   | no       | no        | no        | no       | no    | no                                                                                                                                        | no     |
| readable by Go                                            | yes    | yes   | no       | no        | no        | no       | no    | no                                                                                                                                        | no     |
| support for entityless feature views                      | yes    | yes   | yes      | yes       | yes       | yes      | yes   | yes                                                                                                                                       | yes    |
| support for concurrent writing to the same key            | no     | yes   | no       | no        | no        | no       | no    | no                                                                                                                                        | yes    |
| support for ttl (time to live) at retrieval               | no     | yes   | no       | no        | no        | no       | no    | no                                                                                                                                        | no     |
| support for deleting expired data                         | no     | yes   | no       | no        | no        | no       | no    | no                                                                                                                                        | no     |
| collocated by feature view                                | yes    | no    | yes      | yes       | yes       | yes      | yes   | yes                                                                                                                                       | no     |
| collocated by feature service                             | no     | no    | no       | no        | no        | no       | no    | no                                                                                                                                        | no     |
| collocated by entity key                                  | no     | yes   | no       | no        | no        | no       | no    | no                                                                                                                                        | yes    |


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.feast.dev/reference/online-stores/overview.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.