State server event arrivial order

State service has been implemented before. Detailed information is here: http://zhenhua-guo.blogspot.com/2007/12/state-service-implementation-zhenhua.html. However, there are several unsolved problems. Recently, I fixed one of them.
A typical procedure is:
(1) End user sends a workflow to agent server.
(2) Agent server transforms the original workflow into an internal format.
(3) Agent server sends the workflow to status server.
(4) Agent server submits the workflow to executor.
(5) Executor sends event messages to status server to report the progress of execution of the workflow.
The following senario is possible:
    When status server receives event messages from executor, it has not received the workflow from agent server. In other words, although workflow is sent by agent server before event messages are sent by executor, the arrivial order at the status server is not guaranteed.
In this case, the event messages will be lost and we have no chance to restore it.
[Solution]
Message buffer.
If the senario above happens, the received messages are buffered/saved temporarily at status server. Then, when a workflow is received by status server, all buffered/saved messages corresponding to that workflow will be applied to it.
[For example]
(1) Executor sends following message to status server.
    user "tony"                                    //user who owns the workflow
    workflow "weatherforecast"           //workflow id which uniquely identifies the workflow
    subjob: "solve equations"                //sub job
    status: started                                //status
(2) Status server receives that message.
    However, by checking its repository, status server finds that the corresponding workflow has not been received yet.
    So, that message is buffered.
(3) Status server receives the following workflow message from agent server.
    user "tony"
    workflow "weatherforecast"
    workflow content "<project>....</project>"
(4) Apply the message to the workflow and status of the workflow is changed to indicate that some element starts to execute.