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The quorum package contains logic to reason about committed indexes as well as vote outcomes for both majority and joint quorums. The package is oblivious to the existence of learner replicas. The plan is to hook this up to etcd/raft in subsequent commits.
154 lines
3.0 KiB
Plaintext
154 lines
3.0 KiB
Plaintext
# The empty quorum commits "everything". This is useful for its use in joint
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# quorums.
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committed
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----
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<empty majority quorum>∞
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# A single voter quorum is not final when no index is known.
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committed cfg=(1) idx=(_)
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----
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idx
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? 0 (id=1)
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0
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# When an index is known, that's the committed index, and that's final.
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committed cfg=(1) idx=(12)
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----
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idx
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> 12 (id=1)
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12
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# With two nodes, start out similarly.
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committed cfg=(1, 2) idx=(_,_)
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----
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idx
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? 0 (id=1)
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? 0 (id=2)
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0
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# The first committed index becomes known (for n1). Nothing changes in the
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# output because idx=12 is not known to be on a quorum (which is both nodes).
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committed cfg=(1, 2) idx=(12,_)
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----
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idx
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x> 12 (id=1)
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? 0 (id=2)
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0
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# The second index comes in and finalize the decision. The result will be the
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# smaller of the two indexes.
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committed cfg=(1,2) idx=(12,5)
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----
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idx
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x> 12 (id=1)
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> 5 (id=2)
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5
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# No surprises for three nodes.
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committed cfg=(1,2,3) idx=(_,_,_)
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----
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idx
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? 0 (id=1)
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? 0 (id=2)
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? 0 (id=3)
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0
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committed cfg=(1,2,3) idx=(12,_,_)
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----
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idx
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xx> 12 (id=1)
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? 0 (id=2)
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? 0 (id=3)
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0
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# We see a committed index, but a higher committed index for the last pending
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# votes could change (increment) the outcome, so not final yet.
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committed cfg=(1,2,3) idx=(12,5,_)
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----
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idx
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xx> 12 (id=1)
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x> 5 (id=2)
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? 0 (id=3)
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5
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# a) the case in which it does:
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committed cfg=(1,2,3) idx=(12,5,6)
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----
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idx
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xx> 12 (id=1)
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> 5 (id=2)
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x> 6 (id=3)
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6
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# b) the case in which it does not:
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committed cfg=(1,2,3) idx=(12,5,4)
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----
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idx
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xx> 12 (id=1)
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x> 5 (id=2)
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> 4 (id=3)
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5
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# c) a different case in which the last index is pending but it has no chance of
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# swaying the outcome (because nobody in the current quorum agrees on anything
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# higher than the candidate):
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committed cfg=(1,2,3) idx=(5,5,_)
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----
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idx
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x> 5 (id=1)
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> 5 (id=2)
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? 0 (id=3)
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5
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# c) continued: Doesn't matter what shows up last. The result is final.
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committed cfg=(1,2,3) idx=(5,5,12)
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----
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idx
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> 5 (id=1)
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> 5 (id=2)
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xx> 12 (id=3)
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5
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# With all committed idx known, the result is final.
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committed cfg=(1, 2, 3) idx=(100, 101, 103)
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----
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idx
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> 100 (id=1)
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x> 101 (id=2)
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xx> 103 (id=3)
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101
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# Some more complicated examples. Similar to case c) above. The result is
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# already final because no index higher than 103 is one short of quorum.
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committed cfg=(1, 2, 3, 4, 5) idx=(101, 104, 103, 103,_)
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----
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idx
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x> 101 (id=1)
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xxxx> 104 (id=2)
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xx> 103 (id=3)
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> 103 (id=4)
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? 0 (id=5)
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103
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# A similar case which is not final because another vote for >= 103 would change
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# the outcome.
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committed cfg=(1, 2, 3, 4, 5) idx=(101, 102, 103, 103,_)
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----
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idx
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x> 101 (id=1)
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xx> 102 (id=2)
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xxx> 103 (id=3)
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> 103 (id=4)
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? 0 (id=5)
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102
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