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From Edward Capriolo <edlinuxg...@gmail.com>
Subject Re: Benefit of LOCAL_SERIAL consistency
Date Wed, 07 Dec 2016 15:01:45 GMT
On Wed, Dec 7, 2016 at 8:25 AM, DuyHai Doan <doanduyhai@gmail.com> wrote:

> The reason you don't want to use SERIAL in multi-DC clusters is the
> prohibitive cost of lightweight transaction (in term of latency),
> especially if your data centers are separated by continents. A ping from
> London to New York takes 52ms just by speed of light in optic cable. Since
> LightWeight Transaction involves 4 network round-trips, it means at least
> 200ms just for raw network transfer, not even taking into account the cost
> of processing the operation....
>
> You're right to raise a warning about mixing LOCAL_SERIAL with SERIAL.
> LOCAL_SERIAL guarantees you linearizability inside a DC, SERIAL guarantees
> you linearizability across multiple DC.
>
> If I have 3 DCs with RF = 3 each (total 9 replicas) and I did an INSERT IF
> NOT EXISTS with LOCAL_SERIAL in DC1, then it's possible that a subsequent
> INSERT IF NOT EXISTS on the same record succeeds when using SERIAL because
> SERIAL on 9 replicas = at least 5 replicas. Those 5 replicas which respond
> can come from DC2 and DC3 and thus did not apply yet the previous INSERT...
>
> On Wed, Dec 7, 2016 at 2:14 PM, Hiroyuki Yamada <mogwaing@gmail.com>
> wrote:
>
>> Hi,
>>
>> I have been using lightweight transactions for several months now and
>> wondering what is the benefit of having LOCAL_SERIAL serial consistency
>> level.
>>
>> With SERIAL, it achieves global linearlizability,
>> but with LOCAL_SERIAL, it only achieves DC-local linearlizability,
>> which is missing point of linearlizability, I think.
>>
>> So, for example,
>> once when SERIAL is used,
>> we can't use LOCAL_SERIAL to achieve local linearlizability
>> since data in local DC might not be updated yet to meet quorum.
>> And vice versa,
>> once when LOCAL_SERIAL is used,
>> we can't use SERIAL to achieve global linearlizability
>> since data is not globally updated yet to meet quorum .
>>
>> So, it would be great if we can use LOCAL_SERIAL if possible and
>> use SERIAL only if local DC is down or unavailable,
>> but based on the example above, I think it is not possible, is it ?
>> So, I am not sure about what is the good use case for LOCAL_SERIAL.
>>
>> The only case that I can think of is having a cluster in one DC for
>> online transactions and
>> having another cluster in another DC for analytics purpose.
>> In this case, I think there is no big point of using SERIAL since data
>> for analytics sometimes doesn't have to be very correct/fresh and
>> data can be asynchronously replicated to analytics node. (so using
>> LOCAL_SERIAL for one DC makes sense.)
>>
>> Could anyone give me some thoughts about it ?
>>
>> Thanks,
>> Hiro
>>
>
>
You're right to raise a warning about mixing LOCAL_SERIAL with SERIAL.
LOCAL_SERIAL guarantees you linearizability inside a DC, SERIAL guarantees
you linearizability across multiple DC.

I am not sure what of the state of this is anymore but I was under the
impression the linearizability of lwt was in question. I never head it
specifically addressed.

https://issues.apache.org/jira/browse/CASSANDRA-6106

Its hard to follow 6106 because most of the tasks are closed 'fix later'
 or closed 'not a problem' .

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