[FSTORE-1147] Online feature store notification system (#343)

Co-authored-by: Fabio Buso <fabio@hopsworks.ai>

Author: bubriks

docs/assets/images/guides/feature_group/create_online_enabled_feature_group.png

@@ -0,0 +1,100 @@
+---
+description: Documentation on Change Data Capture for feature groups in Hopsworks.
+---
+
+# Change Data Capture for feature groups
+
+## Introduction
+
+Changes to online-enabled feature groups can be captured by listening to events on specified topics.
+This optimizes the user experience by allowing users to proactively make predictions as soon as there is an update on the features.
+
+In this guide you will learn how to enable Change Data Capture (CDC) for online feature groups within Hopsworks, showing examples in HSFS APIs as well as the user interface.
+
+## Prerequisites
+
+Before you begin this guide we suggest you read the [Feature Group](../../../concepts/fs/feature_group/fg_overview.md) concept page to understand what a feature group is and how it fits in the ML pipeline.
+Subsequently [create a Kafka topic](../../projects/kafka/create_topic.md), this topic will be used for storing Change Data Capture events.
+
+## Using HSFS APIs
+
+### Create a Feature Group with Change Data Capture
+
+To enable Change Data Capture for an online-enabled feature group using the HSFS APIs you need to [create a feature group](./create.md) and set the `notification_topic_name` properties value to the previously created topic.
+
+=== "Python"
+
+    ```python
+    fg = fs.create_feature_group(
+      name="feature_group_name",
+      version=feature_group_version,
+      primary_key=feature_group_primary_keys,
+      online_enabled=True,
+      notification_topic_name="notification_topic_name")
+    ```
+
+### Update Feature Group Change Data Capture topic
+
+The notification topic name can be changed after the creation of the feature group.
+By setting the `notification_topic_name` value to `None` or empty string notification will be disabled.
+With the default configuration, it can take up to 30 minutes for these changes to take place since the onlinefs service internally caches feature groups.
+
+=== "Python"
+
+    ```python
+    fg.update_notification_topic_name(
+      notification_topic_name="new_notification_topic_name")
+    ```
+
+## Using UI
+
+### Create a Feature Group with Change Data Capture
+
+During the creation of the feature group enable online feature serving.
+When enabled you will be able to set the `CDC topic name` property.
+
+<p align="center">
+  <figure>
+    <img src="../../../../assets/images/guides/feature_group/create_online_enabled_feature_group.png" alt="Create online enabled feature group">
+  </figure>
+</p>
+
+### Update Feeature Group with Change Data Capture topic
+
+The notification topic name can be changed after creation by editing the feature group.
+By setting the `CDC topic name` value to empty the notifications will be disabled.
+With the default configuration, it can take up to 30 minutes for these changes to take place since the onlinefs service internally caches feature groups.
+
+<p align="center">
+  <figure>
+    <img src="../../../../assets/images/guides/feature_group/edit_online_enabled_feature_group.png" alt="Edit online enabled feature group">
+  </figure>
+</p>
+
+## Example of Change Data Capture event
+
+Once properly set up the online feature store service will produce events to the provided topic when data ingestion is completed for records.
+
+Here is an example output:
+
+```
+{
+  "projectId":119,  # project used for data ingestion
+  "featureStoreId":67,  # feature store where changes took place
+  "featureGroupId":14,  # feature group changed
+  "entry":{ # values of the affected feature group entry
+    "id":"15",
+    "text":"test"
+  },
+  "featureViews":[  # list of feature views affected
+    {
+      "id":9,  # id of the feature view
+      "name":"test",  # name of the feature view
+      "version":1,  # version of the feature view
+      "featurestoreId":67  # feature store where feature view resides
+    }
+  ]
+}
+```
+
+The list of `featureViews` in the event could be outdated for up to 10 minutes, due to internal logging in onlinefs service.

docs/assets/images/guides/feature_group/edit_online_enabled_feature_group.png

@@ -84,6 +84,7 @@ nav:
               - Feature Monitoring:
                   - Getting started: user_guides/fs/feature_group/feature_monitoring.md
                   - Advanced guide: user_guides/fs/feature_monitoring/feature_monitoring_advanced.md
+              - Notification: user_guides/fs/feature_group/notification.md
           - Feature View:
               - user_guides/fs/feature_view/index.md
               - Overview: user_guides/fs/feature_view/overview.md