# Adding a new online store

## Overview

Feast makes adding support for a new online store (database) easy. Developers can simply implement the [OnlineStore](https://github.com/feast-dev/feast/blob/v0.22-branch/sdk/python/feast/infra/online_stores/online_store.py#L26) interface to add support for a new store (other than the existing stores like Redis, DynamoDB, SQLite, and Datastore).

In this guide, we will show you how to integrate with MySQL as an online store. While we will be implementing a specific store, this guide should be representative for adding support for any new online store.

The full working code for this guide can be found at [feast-dev/feast-custom-online-store-demo](https://github.com/feast-dev/feast-custom-online-store-demo).

The process of using a custom online store consists of 3 steps:

1. Defining the `OnlineStore` class.
2. Defining the `OnlineStoreConfig` class.
3. Referencing the `OnlineStore` in a feature repo's `feature_store.yaml` file.
4. Testing the `OnlineStore` class.

## 1. Defining an OnlineStore class

{% hint style="info" %}
OnlineStore class names must end with the OnlineStore suffix!
{% endhint %}

The OnlineStore class broadly contains two sets of methods

* One set deals with managing infrastructure that the online store needed for operations
* One set deals with writing data into the store, and reading data from the store.

### 1.1 Infrastructure Methods

There are two methods that deal with managing infrastructure for online stores, `update` and `teardown`

* `update` is invoked when users run `feast apply` as a CLI command, or the `FeatureStore.apply()` sdk method.

The `update` method should be used to perform any operations necessary before data can be written to or read from the store. The `update` method can be used to create MySQL tables in preparation for reads and writes to new feature views.

* `teardown` is invoked when users run `feast teardown` or `FeatureStore.teardown()`.

The `teardown` method should be used to perform any clean-up operations. `teardown` can be used to drop MySQL indices and tables corresponding to the feature views being deleted.

{% code title="feast\_custom\_online\_store/mysql.py" %}

```python
def update(
    self,
    config: RepoConfig,
    tables_to_delete: Sequence[Union[FeatureTable, FeatureView]],
    tables_to_keep: Sequence[Union[FeatureTable, FeatureView]],
    entities_to_delete: Sequence[Entity],
    entities_to_keep: Sequence[Entity],
    partial: bool,
):
    """
    An example of creating manging the tables needed for a mysql-backed online store.
    """
    conn = self._get_conn(config)
    cur = conn.cursor(buffered=True)

    project = config.project

    for table in tables_to_keep:
        cur.execute(
            f"CREATE TABLE IF NOT EXISTS {_table_id(project, table)} (entity_key VARCHAR(512), feature_name VARCHAR(256), value BLOB, event_ts timestamp, created_ts timestamp,  PRIMARY KEY(entity_key, feature_name))"
        )
        cur.execute(
            f"CREATE INDEX {_table_id(project, table)}_ek ON {_table_id(project, table)} (entity_key);"
        )

    for table in tables_to_delete:
        cur.execute(
            f"DROP INDEX {_table_id(project, table)}_ek ON {_table_id(project, table)};"
        )
        cur.execute(f"DROP TABLE IF EXISTS {_table_id(project, table)}")


def teardown(
    self,
    config: RepoConfig,
    tables: Sequence[Union[FeatureTable, FeatureView]],
    entities: Sequence[Entity],
):
    """
    
    """
    conn = self._get_conn(config)
    cur = conn.cursor(buffered=True)
    project = config.project

    for table in tables:
        cur.execute(
            f"DROP INDEX {_table_id(project, table)}_ek ON {_table_id(project, table)};"
        )
        cur.execute(f"DROP TABLE IF EXISTS {_table_id(project, table)}")
```

{% endcode %}

### 1.2 Read/Write Methods

There are two methods that deal with writing data to and from the online stores.`online_write_batch` and `online_read`.

* `online_write_batch` is invoked when running materialization (using the `feast materialize` or `feast materialize-incremental` commands, or the corresponding `FeatureStore.materialize()` method.
* `online_read` is invoked when reading values from the online store using the `FeatureStore.get_online_features()` method.

{% code title="feast\_custom\_online\_store/mysql.py" %}

```python
def online_write_batch(
    self,
    config: RepoConfig,
    table: Union[FeatureTable, FeatureView],
    data: List[
        Tuple[EntityKeyProto, Dict[str, ValueProto], datetime, Optional[datetime]]
    ],
    progress: Optional[Callable[[int], Any]],
) -> None:
    conn = self._get_conn(config)
    cur = conn.cursor(buffered=True)

    project = config.project

    for entity_key, values, timestamp, created_ts in data:
        entity_key_bin = serialize_entity_key(entity_key).hex()
        timestamp = _to_naive_utc(timestamp)
        if created_ts is not None:
            created_ts = _to_naive_utc(created_ts)

        for feature_name, val in values.items():
            self.write_to_table(created_ts, cur, entity_key_bin, feature_name, project, table, timestamp, val)
        self._conn.commit()
        if progress:
            progress(1)

def online_read(
    self,
    config: RepoConfig,
    table: Union[FeatureTable, FeatureView],
    entity_keys: List[EntityKeyProto],
    requested_features: Optional[List[str]] = None,
) -> List[Tuple[Optional[datetime], Optional[Dict[str, ValueProto]]]]:
    conn = self._get_conn(config)
    cur = conn.cursor(buffered=True)

    result: List[Tuple[Optional[datetime], Optional[Dict[str, ValueProto]]]] = []

    project = config.project
    for entity_key in entity_keys:
        entity_key_bin = serialize_entity_key(entity_key).hex()
        print(f"entity_key_bin: {entity_key_bin}")

        cur.execute(
            f"SELECT feature_name, value, event_ts FROM {_table_id(project, table)} WHERE entity_key = %s",
            (entity_key_bin,),
        )

        res = {}
        res_ts = None
        for feature_name, val_bin, ts in cur.fetchall():
            val = ValueProto()
            val.ParseFromString(val_bin)
            res[feature_name] = val
            res_ts = ts

        if not res:
            result.append((None, None))
        else:
            result.append((res_ts, res))
    return result
```

{% endcode %}

## 2. Defining an OnlineStoreConfig class

Additional configuration may be needed to allow the OnlineStore to talk to the backing store. For example, MySQL may need configuration information like the host at which the MySQL instance is running, credentials for connecting to the database, etc.

To facilitate configuration, all OnlineStore implementations are **required** to also define a corresponding OnlineStoreConfig class in the same file. This OnlineStoreConfig class should inherit from the `FeastConfigBaseModel` class, which is defined [here](https://github.com/feast-dev/feast/blob/v0.22-branch/sdk/python/feast/repo_config.py#L44).

The `FeastConfigBaseModel` is a [pydantic](https://pydantic-docs.helpmanual.io) class, which parses yaml configuration into python objects. Pydantic also allows the model classes to define validators for the config classes, to make sure that the config classes are correctly defined.

This config class **must** container a `type` field, which contains the fully qualified class name of its corresponding OnlineStore class.

Additionally, the name of the config class must be the same as the OnlineStore class, with the `Config` suffix.

An example of the config class for MySQL :

{% code title="feast\_custom\_online\_store/mysql.py" %}

```python
class MySQLOnlineStoreConfig(FeastConfigBaseModel):
    type: Literal["feast_custom_online_store.mysql.MySQLOnlineStore"] = "feast_custom_online_store.mysql.MySQLOnlineStore"

    host: Optional[StrictStr] = None
    user: Optional[StrictStr] = None
    password: Optional[StrictStr] = None
    database: Optional[StrictStr] = None
```

{% endcode %}

This configuration can be specified in the `feature_store.yaml` as follows:

{% code title="feature\_repo/feature\_store.yaml" %}

```yaml
online_store:
    type: feast_custom_online_store.mysql.MySQLOnlineStore
    user: foo
    password: bar
```

{% endcode %}

This configuration information is available to the methods of the OnlineStore, via the`config: RepoConfig` parameter which is passed into all the methods of the OnlineStore interface, specifically at the `config.online_store` field of the `config` parameter.

{% code title="feast\_custom\_online\_store/mysql.py" %}

```python
def online_write_batch(
        self,
        config: RepoConfig,
        table: Union[FeatureTable, FeatureView],
        data: List[
            Tuple[EntityKeyProto, Dict[str, ValueProto], datetime, Optional[datetime]]
        ],
        progress: Optional[Callable[[int], Any]],
) -> None:

    online_store_config = config.online_store
    assert isinstance(online_store_config, MySQLOnlineStoreConfig)

    connection = mysql.connector.connect(
        host=online_store_config.host or "127.0.0.1",
        user=online_store_config.user or "root",
        password=online_store_config.password,
        database=online_store_config.database or "feast",
        autocommit=True
    )
```

{% endcode %}

## 3. Using the custom online store

After implementing both these classes, the custom online store can be used by referencing it in a feature repo's `feature_store.yaml` file, specifically in the `online_store` field. The value specified should be the fully qualified class name of the OnlineStore.

As long as your OnlineStore class is available in your Python environment, it will be imported by Feast dynamically at runtime.

To use our MySQL online store, we can use the following `feature_store.yaml`:

{% code title="feature\_repo/feature\_store.yaml" %}

```yaml
project: test_custom
registry: data/registry.db
provider: local
online_store: 
    type: feast_custom_online_store.mysql.MySQLOnlineStore
    user: foo
    password: bar
```

{% endcode %}

If additional configuration for the online store is \*\*not \*\*required, then we can omit the other fields and only specify the `type` of the online store class as the value for the `online_store`.

{% code title="feature\_repo/feature\_store.yaml" %}

```yaml
project: test_custom
registry: data/registry.db
provider: local
online_store: feast_custom_online_store.mysql.MySQLOnlineStore
```

{% endcode %}

## 4. Testing the OnlineStore class

Even if you have created the `OnlineStore` class in a separate repo, you can still test your implementation against the Feast test suite, as long as you have Feast as a submodule in your repo. In the Feast submodule, we can run all the unit tests with:

```
make test
```

The universal tests, which are integration tests specifically intended to test offline and online stores, can be run with:

```
make test-python-universal
```

The unit tests should succeed, but the universal tests will likely fail. The tests are parametrized based on the `FULL_REPO_CONFIGS` variable defined in `sdk/python/tests/integration/feature_repos/repo_configuration.py`. To overwrite these configurations, you can simply create your own file that contains a `FULL_REPO_CONFIGS`, and point Feast to that file by setting the environment variable `FULL_REPO_CONFIGS_MODULE` to point to that file. In this repo, the file that overwrites `FULL_REPO_CONFIGS` is `feast_custom_online_store/feast_tests.py`, so you would run

```
export FULL_REPO_CONFIGS_MODULE='feast_custom_online_store.feast_tests'
make test-python-universal
```

to test the MySQL online store against the Feast universal tests. You should notice that some of the tests actually fail; this indicates that there is a mistake in the implementation of this online store!