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From "Pierre Chalamet" <pie...@chalamet.net>
Subject RE: Get CL ONE / NTS
Date Wed, 14 Sep 2011 22:07:04 GMT
Thanks Aaron, didn't seen your answer before mine.

I do agree for 2/ I might have read error. Good suggestion to use
EACH_QUORUM  - it could be a good trade off to read at this level if ONE
fails.

Maybe using LOCAL_QUORUM might be a good answer and will avoid headache
after all. Are you advising CL.ONE does not worth the game when considering
read performance ?

By the way, I do not have consistency problem at all - data is only written
once (and if more it is always the same data) and read several times across
DC. I only have replication problems. That's why I'm more inclined to use
CL.ONE for read if possible.

Thanks,
- Pierre


-----Original Message-----
From: aaron morton [mailto:aaron@thelastpickle.com] 
Sent: Wednesday, September 14, 2011 11:48 PM
To: user@cassandra.apache.org; pierre@chalamet.net
Subject: Re: Get CL ONE / NTS

Your current approach to Consistency opens the door to some inconsistent
behavior. 

> 1/ Will I have an error because DC2 does not have any copy of the data ? 
If you read from DC2 at CL ONE and the data is not replicated it will not be
returned. 

> 2/ Will Cassandra try to get the data from DC1 if nothing is found in DC2
?
Not at CL ONE. If you used CL EACH QUORUM then the read will go to all the
DC's. If DC2 is behind DC1 then you will get the data form DC1. 

> 3/ In case of partial replication to DC2, will I see sometimes errors
about servers not holding the data in DC2 ?
Depending on the API call and the client, working at CL ONE, you will see
either errors or missing data. 

> 4/ Does Get CL ONE failed as soon as the fastest server to answer tell it
does not have the data or does it waits until all servers tell they do not
have the data ? 
 yes

Consider 

using LOCAL QUORUM for write and read, will make things a bit more
consistent but not add inter DC overhead into the request latency. Still
possible to not get data in DC2 if it is totally disconnected from the DC1 

write at LOCAL QUORUM and read at EACH QUORUM . Will so you can always read,
requests in DC2 will fail if DC1 is not reachable. 

Hope that helps. 

 
-----------------
Aaron Morton
Freelance Cassandra Developer
@aaronmorton
http://www.thelastpickle.com

On 15/09/2011, at 1:33 AM, Pierre Chalamet wrote:

> Hello,
> 
> I have 2 datacenters. Cassandra is configured as follow:
> - RackInferringSnitch
> - NetworkTopologyStrategy for CF
> - strategy_options: DC1:3 DC2:3
> 
> Data are written using CL LOCAL_QUORUM so data written from one datacenter
will eventually be replicated to the other datacenter. Data is always
written exactly once. 
> 
> On the other side, I'd like to improve the read path. I'm using actually
the CL ONE since data is only written once (ie: timestamp is more or less
meaningless in my case).
> 
> This is where I have some doubts: if data is written on DC1 and
tentatively read from DC2 while the data is still not replicated or
partially replicated (for whatever good reason since replication is async),
what is the behavior of Get with CL ONE / NTS ? 
> 1/ Will I have an error because DC2 does not have any copy of the data ? 
> 2/ Will Cassandra try to get the data from DC1 if nothing is found in DC2
?
> 3/ In case of partial replication to DC2, will I see sometimes errors
about servers not holding the data in DC2 ?
> 4/ Does Get CL ONE failed as soon as the fastest server to answer tell it
does not have the data or does it waits until all servers tell they do not
have the data ? 
> 
> Thanks a lot,
> - Pierre



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