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https://issues.apache.org/jira/browse/MESOS-3870?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14998720#comment-14998720
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Neil Conway commented on MESOS-3870:
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A process can't be enqueued onto the runq twice. This is prevented because a
process is only added to the runq when it receives an event and the process is
in state "BLOCKED"; once a process is on the runq, its state is changed to
"READY", so it won't be readded again in the future.
(https://github.com/apache/mesos/blob/master/3rdparty/libprocess/src/process.cpp#L2998-L3017)
> Prevent out-of-order libprocess message delivery
> ------------------------------------------------
>
> Key: MESOS-3870
> URL: https://issues.apache.org/jira/browse/MESOS-3870
> Project: Mesos
> Issue Type: Bug
> Components: libprocess
> Reporter: Neil Conway
> Priority: Minor
> Labels: mesosphere
>
> I was under the impression that {{send()}} provided in-order, unreliable
> message delivery. So if P1 sends <M1,M2> to P2, P2 might see <>, <M1>, <M2>,
> or <M1,M2> — but not <M2,M1>.
> I suspect much of the code makes a similar assumption. However, it appears
> that this behavior is not guaranteed. slave.cpp:2217 has the following
> comment:
> {noformat}
> // TODO(jieyu): Here we assume that CheckpointResourcesMessages are
> // ordered (i.e., slave receives them in the same order master sends
> // them). This should be true in most of the cases because TCP
> // enforces in order delivery per connection. However, the ordering
> // is technically not guaranteed because master creates multiple
> // connections to the slave in some cases (e.g., persistent socket
> // to slave breaks and master uses ephemeral socket). This could
> // potentially be solved by using a version number and rejecting
> // stale messages according to the version number.
> {noformat}
> We can improve this situation by _either_: (1) fixing libprocess to guarantee
> ordered message delivery, e.g., by adding a sequence number, or (2)
> clarifying that ordered message delivery is not guaranteed, and ideally
> providing a tool to force messages to be delivered out-of-order.
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