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/**
 * A registry for async data change notifications
 */
trait ListenerRegistry {
  /**
   * Register permanent callback for data change event
   * @param key the listenable data identifier
   * @param eventListener see [[ValueChangeListener]]
   */
  def addValueChangeListener(key: String, eventListener: ValueChangeListener): Unit

  /**
   * Deregister permanent callback for data change event
   * @param key the listenable data identifier
   * @param eventListener see [[EventListener]]
   * @tparam T type of the data ID
   */
  def removeValueChangeListener(key: String, eventListener: ValueChangeListener): Unit
 
  /**
   * Register permanent callback for key-set change event
   * @param namespace the listenable key-set identifier (e.g. parent path in Zookeeper, table name in Database etc)
   * @param eventListener see [[ValueChangeListener]]
   */
  def addKeySetChangeListener(namespace: String, eventListener: KeySetChangeListener): Unit

  /**
   * Deregister permanent callback for key-set change event
   * @param namespace the listenable key-set identifier (e.g. parent path in Zookeeper, table name in Database etc)
   * @param eventListener see [[ValueChangeListener]]
   */
  def removeKeySetChangeListener(namespace: String, eventListener: KeySetChangeListener): Unit

  /**
   * Setup everything needed for concrete implementation
   * @param context TBD. Should be abstract enough to be used by different implementations and
   *                at the same time specific because will be uniformly called from the Kafka code,
   *                regardless of the implementation
   */
  def init(context: Any): Unit

  /**
   * Release all acquired resources
   */
  def close(): Unit
}

/**
 * Callback on value change event
 */
trait ValueChangeListener {
  def valueChanged(newValue: Option[String])
}

/**
 * Callback on key-set change event
 */
trait KeySetChangeListener {
  def keySetChanged(newKeySet: Set[String])
}