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Looks good to me except the last test. Looks quite ugly to me:( Still thinking 
about better solutions.

The code part looks good to me, we can commit that first (with one more unit 
test to test the coordinator election logic with IGNORE responses).

src/log/consensus.cpp (line 334)

    'type' is optional. Do you need to check `has_type()`?

src/log/consensus.cpp (line 356)

    This is a little hard to read. Can you introduce a small helper:
    bool isReject(const Response& response)
      if (response.has_type()) {
        // new replica
        return response.type() == PromiseResponse::REJECT;
      } else {
        // old replica
        return !response.okay();
    To be consistent, you might wanna introduce `isIgnore` as well.

src/log/coordinator.cpp (line 197)

    Please add a CHECK(response.has_type())?

src/messages/log.proto (line 142)

    s/old master/old coordinator/

src/messages/log.proto (line 163)

    Could you please put 'type' field right below 'okay' field?

src/messages/log.proto (line 213)

    Ditto. Please put it right below 'okay' field.

src/tests/log_tests.cpp (line 643)

    2 lines apart.

src/tests/log_tests.cpp (line 647)

    I think we put `:` in the next line.

src/tests/log_tests.cpp (line 703)

    No snake_case please. Also, we try not to use tailing underscore for member 
fields if possible.

src/tests/log_tests.cpp (line 733)

    Please add a brief descrption about what this test is doing (instead of 
just a JIRA number).

- Jie Yu

On Oct. 20, 2015, 8:45 p.m., Neil Conway wrote:
> -----------------------------------------------------------
> This is an automatically generated e-mail. To reply, visit:
> https://reviews.apache.org/r/39325/
> -----------------------------------------------------------
> (Updated Oct. 20, 2015, 8:45 p.m.)
> Review request for mesos, Jie Yu, Joris Van Remoortere, and Timothy Chen.
> Bugs: MESOS-3280
>     https://issues.apache.org/jira/browse/MESOS-3280
> Repository: mesos
> Description
> -------
> MESOS-3280. The basic problem is that replicas silently ignore inbound Promise
> and Write requests if they have not finished the recovery protocol yet 
> (because
> they can't safely vote on such requests). Hence, if we try to do a Paxos round
> while a quorum of nodes have not finished recovering, the Paxos round will 
> never
> complete. In particular, this might happen during coordinator election:
> coordinator election (which is implemented as performing a full Paxos round)
> starts as soon as the candidate coordinator replica has finished the recovery
> protocol. If several nodes start concurrently, a quorum of those nodes might
> still be executing the recovery protocol, and hence the coordinator will never
> be elected.
> To address this, add "ignored" responses to the Promise and Write 
> sub-protocols:
> if a proposer sees a quorum of "ignored" responses to a promise or write 
> request
> it has issued, it knows the request will never succeed.  When used for
> coordinator election, the current coding will retry immediately (without a
> backoff).
> Note that replicas will still silently drop promise/write requests if another
> kind of problem occurs (e.g., an I/O error prevents reading/writing log
> data). We might consider changing this, although it will require some thought:
> e.g., if a replica's disk is broken, sending an "ignored" message on every
> request might flood the network.
> * Test mock is incredibly ugly: it works, but we clearly need a better 
> approach
>   before committing this. I've been chatting with @tnachen to find a better
>   approach but haven't got anything that works yet.
> * Should we add a backoff when retrying after a failed coordinator election?
> * Should we also send back an "ignored" response if an I/O error occurs?
> Diffs
> -----
>   src/log/consensus.cpp 59f80d02d1d3c11683631f3fc5f6e923b5ebdf96 
>   src/log/coordinator.cpp 5500bca77f3020e0051010c5c178a20a3c7ad44a 
>   src/log/replica.hpp 33d3f1d9e89035936c67739898e73a06b391fcd0 
>   src/log/replica.cpp 75d39ff56822bf00fce9daf5c1e3befb75f2e039 
>   src/messages/log.proto d73b33f865963292af580945659ad0e800f2a204 
>   src/tests/log_tests.cpp f2dd47cfbe73fb18c360a637db009b7d391a782e 
> Diff: https://reviews.apache.org/r/39325/diff/
> Testing
> -------
> "make check" passes, including a new test that uses a newly constructed mock 
> to ensure we're testing the message schedule described above.
> I also wrote a script stops and starts mesos-master in a loop, removing the 
> replicated log each time. Without the patch, this occasionally fails with a 
> "registry fetch" timeout; with the patch, you can observe several scenarios 
> where coordinator election is reborted and retried because a quorum of 
> ignored responses is seen. Note that in some cases, we need to retry 
> coordinator election up to ~70 times (!), because we don't currently use a 
> backoff; that should probably be fixed, per comments above. But the important 
> point is that election eventually succeeds and we don't hang.
> Thanks,
> Neil Conway

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