Re: Tolerable amount of CAS queries?

[Jeff Jirsa]

On 2017-07-21 06:41 (-0700), Jan Algermissen  
wrote: 
> 
> IOW, suppose I
> 
> - have a cluster spanning geographic regions
> - restrict the CAS queries to key spaces that are only replicated in a 
> single region and I use LOCAL_SERIAL CL
> 
> would 100 CAS queries per second that in the normal case do not conflict 
> (== work in different partition keys) be sort of 'ok'?
> 

While the only way to know for sure is to test it in your particular 
environment, I would expect it to be fine for most healthy clusters, as long as 
it's really not contended (different partition keys). 



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Tolerable amount of CAS queries?

[Jan Algermissen]

Hi,

I just read [1] which describes a lease implementation using CAS 
queries. It applies a TTL to the lease which needs to be refreshed 
periodically by the lease holder.


I use such a pattern myself since a couple of years, so no surprise 
there.


However, the article uses CAS queries not only for acquiring the lease, 
but also for TTL updates and active lease canceling. Given a default TTL 
of three minutes is described, the amount of CAS queries might be ok.


What if I go for much shorter TTLs, eg 5 seconds to minimise the time 
another peer takes over if the current lease owner crashes or is 
stopped? Using some safety margin for updating the TTL, we'd end up with 
a CAS query every 3 seconds or so. If we have a bunch of such leases, 
we'd likely see 10 or more such CAS queries a second.


I am looking for advice whether such a high number of CAS queries could 
be tolerable at all? I'd assume there is not much contention on the same 
lease, is the overhead of a CAS query basically that it leads to 4 or 
sometimes significantly more 'queries' in the C* cluster?


IOW, suppose I

- have a cluster spanning geographic regions
- restrict the CAS queries to key spaces that are only replicated in a 
single region and I use LOCAL_SERIAL CL


would 100 CAS queries per second that in the normal case do not conflict 
(== work in different partition keys) be sort of 'ok'?


Or should it rather be in the range of 10/s?

Jan


[1] https://www.datastax.com/dev/blog/consensus-on-cassandra

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