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From "Neil Conway" <>
Subject Re: Review Request 39325: Fixed race between coordinator election and recovery in replicated log.
Date Mon, 19 Oct 2015 23:29:15 GMT

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(Updated Oct. 19, 2015, 11:29 p.m.)

Review request for mesos, Jie Yu, Joris Van Remoortere, and Timothy Chen.


Address review comments.

Bugs: MESOS-3280

Repository: mesos


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

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 (updated)

  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 



"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.


Neil Conway

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