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From ibrahim El-sanosi <>
Subject Re: lightweight transactions with potential problem?
Date Wed, 26 Aug 2015 11:05:24 GMT
OKKKKKKKKK. I see what the purpose of acknowledgment round here. So
acknowledgment is optional here, depend on CL setting as we normally do in
So we can say that acknowledgment is not really related to Paxos phase, it
depends on CL in Cassandra?


On Wed, Aug 26, 2015 at 11:50 AM, Sylvain Lebresne <>

> On Wed, Aug 26, 2015 at 12:19 PM, ibrahim El-sanosi <
>> wrote:
>> Yes, Sylvain, your answer makes more sense. The phase is in Paxos
>> protocol sometimes called learning or decide phase, BUT this phase does not
>> have acknowledgment round, just learning or decide message from the
>> proposer to learners. So why we need acknowledgment round with commit phase
>> in lightweight transactions?
> It's not _needed_ as far as Paxos is concerned. But it's useful in the
> context of Cassandra. The commit phase is about actually persisting to
> replica the update decided by the Paxos algorithm and thus making that
> update visible to non paxos reads. Being able to apply normal consistencies
> to this phase is thus useful, since it allows user to get visibility
> guarantees even for non-paxos reads if they so wish, and that's exactly
> what we do and why we optionally wait on acknowledgments (and I say
> optionally because how many acks we wait on depends on the user provided
> consistency level and if that's CL.ANY then the whole Paxos operation
> actually return without waiting on any of those acks).

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