[Tarantool-patches] [PATCH v2 4/5] raft: introduce split vote detection
Serge Petrenko
sergepetrenko at tarantool.org
Tue Jan 25 13:17:52 MSK 2022
21.01.2022 02:02, Vladislav Shpilevoy пишет:
>>> +static void
>>> +raft_check_split_vote(struct raft *raft)
>>> +{
>>> + /* When leader is known, there is no election. Thus no vote to split. */
>>> + if (raft->leader != 0)
>>> + return;
>>> + /* Not a candidate = can't trigger term bump anyway. */
>>> + if (!raft->is_candidate)
>>> + return;
>>> + /*
>>> + * WAL write in progress means the state is changing. All is rechecked
>>> + * when it is done.
>>> + */
>>> + if (raft->is_write_in_progress)
>>> + return;
>>> + if (!raft_has_split_vote(raft))
>>> + return;
>>> + assert(raft_ev_is_active(&raft->timer));
>>> + /*
>>> + * Could be already detected before. The timeout would be updated by now
>>> + * then.
>>> + */
>>> + if (raft->timer.repeat < raft->election_timeout)
>>> + return;
>> I don't think you should decrease timer.repeat.
>> This 'vote speedup' is for a single term only.
>>
>> Besides the check below about delay >= remaining is enough
>> to test if split vote detection was already triggered.
> I update timer.repeat as a flag that the split vote was already seen
> during this term. If I drop this check, each next vote after the first
> detection of split vote will lead to potential timeout decrease depending
> on random. I wanted to decrease it only once - when split vote is seen
> first time. Even managed to add a test for it.
>
> The alternative was to store a flag 'has_split_vote' somewhere in struct
> raft, but I didn't want to add it.
Ok, I see now. Thanks for the explanation!
>
>>> +
>>> + assert(raft->state == RAFT_STATE_FOLLOWER ||
>>> + raft->state == RAFT_STATE_CANDIDATE);
>>> + struct ev_loop *loop = raft_loop();
>>> + struct ev_timer *timer = &raft->timer;
>>> + double delay = raft_new_random_election_shift(raft);
>>> + /*
>>> + * Could be too late to speed up anything - probably the term is almost
>>> + * over anyway.
>>> + */
>>> + double remaining = raft_ev_timer_remaining(loop, timer);
>>> + if (delay >= remaining)
>>> + delay = remaining;
>>> + say_info("RAFT: split vote is discovered - %s, new term in %lf sec",
>>> + raft_scores_str(raft), delay);
>>> + raft_ev_timer_stop(loop, timer);
>>> + raft_ev_timer_set(timer, delay, delay);
>>
>> ...
>>> diff --git a/test/unit/raft_test_utils.h b/test/unit/raft_test_utils.h
>>> index c68dc3b22..2138a829e 100644
>>> --- a/test/unit/raft_test_utils.h
>>> +++ b/test/unit/raft_test_utils.h
>>> @@ -32,6 +32,7 @@
>>> #include "fakesys/fakeev.h"
>>> #include "fiber.h"
>>> #include "raft/raft.h"
>>> +#include "raft/raft_ev.h"
>> Why do you need it here?
> The tests now use raft_ev_is_active() which is defined in this header. I
> decided to add it here instead of unit/raft.c because the idea of
> raft_test_utils.h is, among other things, to do all the boilerplate work
> such as necessary inclusions.
Ok.
>
> New version of the commit after the comment from v1 about new raft
> members:
>
> ====================
> raft: introduce split vote detection
Thanks for the fixes! LGTM.
--
Serge Petrenko
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