@startuml
participant client
participant connect
participant logic

client -> connect: 离线消息同步
connect -> logic: 离线消息同步

logic -> logic: 如果seq!=0,同步序列号大于seq的消息，否则，同步用户未收到的消息

logic --> connect: 返回离线消息
connect --> client: 返回离线消息

client -> connect: 消息ack
connect -> logic: 消息ack
@enduml