Create the DataStreamNotification entity.

modules/core/data_stream/data_stream_notification/config/schema/data_stream_notification.schema.yml

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+# Schema for the configuration files of the Data stream notifications module.
+
+data_stream_notification.data_stream_notification.*:
+  type: config_entity
+  label: 'Data stream notification'
+  mapping:
+    id:
+      type: string
+      label: 'ID'
+    label:
+      type: label
+      label: 'Label'
+    data_stream:
+      type: integer
+      nullable: true
+      label: 'Data stream'
+    condition_operator:
+      type: string
+      label: 'Condition operator'
+    condition:
+      type: sequence
+      label: 'Conditions'
+      sequence:
+        type: data_stream_notification.condition.[type]
+    delivery:
+      type: sequence
+      label: 'Delivery'
+      sequence:
+        type: data_stream_notification.delivery.[type]

modules/core/data_stream/data_stream_notification/src/DataStreamNotificationPluginCollection.php

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+<?php
+
+namespace Drupal\data_stream_notification;
+
+use Drupal\Core\Plugin\DefaultLazyPluginCollection;
+
+/**
+ * A plugin collection for notification condition and delivery plugins.
+ *
+ * Overrides the DefaultLazyPluginCollection to use "type" as the plugin key.
+ */
+class DataStreamNotificationPluginCollection extends DefaultLazyPluginCollection {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $pluginKey = 'type';
+
+}

modules/core/data_stream/data_stream_notification/src/Entity/DataStreamNotification.php

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+<?php
+
+namespace Drupal\data_stream_notification\Entity;
+
+use Drupal\Core\Config\Entity\ConfigEntityBase;
+use Drupal\data_stream_notification\DataStreamNotificationPluginCollection;
+
+/**
+ * Defines the DataStreamNotification entity.
+ *
+ * @ConfigEntityType(
+ *   id = "data_stream_notification",
+ *   label = @Translation("Data stream notification"),
+ *   label_collection = @Translation("Data stream notifications"),
+ *   label_singular = @Translation("data stream notification"),
+ *   label_plural = @Translation("data stream notifications"),
+ *   label_count = @PluralTranslation(
+ *     singular = "@count data stream notification",
+ *     plural = "@count data stream notifications",
+ *   ),
+ *   handlers = {
+ *     "access" = "\Drupal\entity\EntityAccessControlHandler",
+ *     "permission_provider" = "\Drupal\entity\EntityPermissionProvider",
+ *   },
+ *   admin_permission = "administer data stream notification",
+ *   entity_keys = {
+ *     "id" = "id",
+ *     "label" = "label",
+ *     "status" = "status",
+ *   },
+ *   config_export = {
+ *     "id",
+ *     "label",
+ *     "data_stream",
+ *     "condition_operator",
+ *     "condition",
+ *     "delivery",
+ *   }
+ * )
+ *
+ * @ingroup farm
+ */
+class DataStreamNotification extends ConfigEntityBase implements DataStreamNotificationInterface {
+
+  /**
+   * The notification ID.
+   *
+   * @var string
+   */
+  protected string $id;
+
+  /**
+   * The notification label.
+   *
+   * @var string
+   */
+  protected string $label;
+
+  /**
+   * The data stream ID.
+   *
+   * @var int
+   */
+  protected int $data_stream;
+
+  /**
+   * The condition operator.
+   *
+   * @var string
+   */
+  protected string $condition_operator;
+
+  /**
+   * Stores all conditions of this notification.
+   *
+   * @var array
+   */
+  protected array $condition = [];
+
+  /**
+   * The condition plugin collection.
+   *
+   * @var \Drupal\data_stream_notification\DataStreamNotificationPluginCollection
+   */
+  protected $conditionCollection;
+
+  /**
+   * Stores all deliveries of this notification.
+   *
+   * @var array
+   */
+  protected array $delivery = [];
+
+  /**
+   * The delivery plugin collection.
+   *
+   * @var \Drupal\data_stream_notification\DataStreamNotificationPluginCollection
+   */
+  protected $deliveryCollection;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getPluginCollections() {
+    foreach (['condition', 'delivery'] as $type) {
+      $name = $type . 'Collection';
+      if (empty($this->$name)) {
+        $this->$name = new DataStreamNotificationPluginCollection(\Drupal::service("plugin.manager.data_stream_notification_$type"), $this->$type);
+      }
+    }
+    return [
+      'condition' => $this->conditionCollection,
+      'delivery' => $this->deliveryCollection,
+    ];
+  }
+
+}

modules/core/data_stream/data_stream_notification/src/Entity/DataStreamNotificationInterface.php

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+<?php
+
+namespace Drupal\data_stream_notification\Entity;
+
+use Drupal\Core\Config\Entity\ConfigEntityInterface;
+use Drupal\Core\Entity\EntityWithPluginCollectionInterface;
+
+/**
+ * Provides an interface for defining data stream entities.
+ */
+interface DataStreamNotificationInterface extends ConfigEntityInterface, EntityWithPluginCollectionInterface {
+
+}