@ElementCollection
  @CollectionTable(name = "Cluster_Servers", foreignKey = @ForeignKey(value = ConstraintMode.NO_CONSTRAINT))
  @MapKeyJoinColumn(name = "Server_id", foreignKey = @ForeignKey(value = ConstraintMode.NO_CONSTRAINT))
  @Column(name = "sequence")
  private Map<Server, Integer> servers = new HashMap<Server, Integer>();