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/**
 * A connector for group membership protocol. Supports two modes:
 * 1) "joining" (becoming the member, leaving the group, subscribing to change notifications)
 * 2) "observing" (fetching group state, subscribing to change notifications)
 *
 */
trait GroupMembershipClient {
  /**
   * Each instance of this class is tightly coupled with exactly one group,
   * once set (during initialization) cannot be changed
   * @return unique group identifier among all application groups
   */
  def group: String

  /**
   * Become a member of this group. Throw an exception in case of ID conflict
   * @param id unique member identifier among members of this group
   * @param data supplemental data to be stored along with member ID
   */
  def join(id: String, data: String): Unit

  /**
   * Stop membership in this group
   * @param id unique member identifier among members of this group
   */
  def leave(id: String): Unit

  /**
   * Fetch membership of this group
   * @return IDs of the members of this group
   */
  def membershipList(): Set[String]

  /**
   * Fetch detailed membership of this group
   * @return IDs and corresponding supplemental data of the members of this group
   */
  def membership(): Map[String, String]

  /**
   * Register permanent on group change listener.
   * There is no guarantee listener will be fired on ALL events (due to session reconnects etc)
   * @param listener see [[GroupChangeListener]]
   */
  def addListener(listener: GroupChangeListener)

  /**
   * Deregister on group change listener
   * @param listener see [[GroupChangeListener]]
   */
  def removeListener(listener: GroupChangeListener)

  /**
   * 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
}
 
/**
  * A callback fired on group change event
*/
trait GroupChangeListener {
    /**
     * Event fired when the group membership has changed (member(s) joined and/or left)
     * @param membership new membership of the group
     */
    def onGroupChange(membership: Set[String])
}