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https://issues.apache.org/jira/browse/KUDU-1188?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Mike Percy updated KUDU-1188:
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Parent: KUDU-430
> For snapshot read correctness, enforce simple form of leader leases
> -------------------------------------------------------------------
>
> Key: KUDU-1188
> URL: https://issues.apache.org/jira/browse/KUDU-1188
> Project: Kudu
> Issue Type: Sub-task
> Components: tserver
> Affects Versions: Public beta
> Reporter: David Alves
>
> Since raft doesn't allow holes in the log, a new leader is guaranteed to have
> all the writes that preceded its election and to have them in flight when
> elected (meaning mvcc will have those transactions in flight, meaning a
> snapshot read will wait for them to complete). So, for writes, leases aren't
> really necessary. This is contrary to paxos in spanner where there is no
> timestamp propagation and the log might have holes and leases are required to
> enforce write correctness.
> However some form of lease is necessary to enforce read consistency. In
> particular in the following case:
> Leader A, accepts a write at time 10 which commits and has no following
> writes, it then serves a snapshot read at 15, and crashed.
> Leader B is elected but has a slow clock which reads 11 when he's ready to
> serve writes. It then accepts a write at time 13.
> The snapshot read at 15 is now broken.
> A simple form to avoid this is to have each replica promise, on each ack,
> that if ever elected leader it won't accept writes or serve snapshot read
> until a certain period, say 2 secs has passed since that ack. On the leader
> side, the leader is only allowed to serve snapshot read up to 2 seconds since
> _a majority_ of replicas has ack'd. which in practice means 1 replica usually.
> With such a mechanism in place, if the lease is 5, then leader B wouldn't
> accept the write at time 13 and would instead wait until 15 had passed, not
> breaking the snapshot read.
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