feat: BigTable online store

docs/reference/online-stores/bigtable.md

@@ -0,0 +1,56 @@
+# Bigtable online store
+
+## Description
+
+The [Bigtable](https://cloud.google.com/bigtable) online store provides support for
+materializing feature values into Cloud Bigtable. The data model used to store feature
+values in Bigtable is described in more detail
+[here](../../specs/online_store_format.md#google-bigtable-online-store-format).
+
+## Getting started
+
+In order to use this online store, you'll need to run `pip install 'feast[gcp]'`. You
+can then get started with the command `feast init REPO_NAME -t gcp`.
+
+## Example
+
+{% code title="feature_store.yaml" %}
+```yaml
+project: my_feature_repo
+registry: data/registry.db
+provider: gcp
+online_store:
+  type: bigtable
+  project_id: my_gcp_project
+  instance: my_bigtable_instance
+```
+{% endcode %}
+
+The full set of configuration options is available in
+[BigtableOnlineStoreConfig](https://rtd.feast.dev/en/latest/#feast.infra.online_stores.bigtable.BigtableOnlineStoreConfig).
+
+## Functionality Matrix
+
+The set of functionality supported by online stores is described in detail [here](overview.md#functionality).
+Below is a matrix indicating which functionality is supported by the Bigtable online store.
+
+|                                                           | Bigtable |
+|-----------------------------------------------------------|----------|
+| write feature values to the online store                  | yes      |
+| read feature values from the online store                 | yes      |
+| update infrastructure (e.g. tables) in the online store   | yes      |
+| teardown infrastructure (e.g. tables) in the online store | yes      |
+| generate a plan of infrastructure changes                 | no       |
+| support for on-demand transforms                          | yes      |
+| readable by Python SDK                                    | yes      |
+| readable by Java                                          | no       |
+| readable by Go                                            | no       |
+| support for entityless feature views                      | yes      |
+| support for concurrent writing to the same key            | no       |
+| support for ttl (time to live) at retrieval               | no       |
+| support for deleting expired data                         | no       |
+| collocated by feature view                                | yes      |
+| collocated by feature service                             | no       |
+| collocated by entity key                                  | yes      |
+
+To compare this set of functionality against other online stores, please see the full [functionality matrix](overview.md#functionality-matrix).

docs/specs/online_store_format.md

@@ -92,6 +92,29 @@ Other types of entity keys are not supported in this version of the specificatio
 
 ![Datastore Online Example](datastore_online_example.png)
 
+## Google Bigtable Online Store Format
+
+[Bigtable storage model](https://cloud.google.com/bigtable/docs/overview#storage-model)
+consists of massively scalable tables, with each row keyed by a "row key". The rows in a
+table are stored lexicographically sorted by this row key.
+
+We use the following structure to store feature data in Bigtable:
+
+* All feature data for an entity or a specific group of entities is stored in the same
+  table. The table name is derived by concatenating the lexicographically sorted names
+  of entities.
+* This implementation only uses one column family per table, named `features`.
+* Each row key is created by concatenating a hash derived from the specific entity keys
+  and the name of the feature view. Each row only stores feature values for a specific
+  feature view. This arrangement also means that feature values for a given group of
+  entities are colocated.
+* The columns used in each row are named after the features in the feature view.
+  Bigtable is perfectly content being sparsely populated.
+* By default, we store 1 historical value of each feature value. This can be configured
+  using the `max_versions` setting in `BigtableOnlineStoreConfig`. This implementation
+  of the online store does not have the ability to revert any given value to its old
+  self. To use the historical version, you'll have to use custom code.
+
 ## Cassandra/Astra DB Online Store Format
 
 ### Overview
@@ -250,4 +273,4 @@ message BoolList {
   repeated bool val = 1;
 }
 
-```
+```