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From aaron morton <aa...@thelastpickle.com>
Subject Re: Get CL ONE / NTS
Date Wed, 14 Sep 2011 22:27:20 GMT
> Are you advising CL.ONE does not worth the game when considering
> read performance ?
Consistency is not performance, it's a whole new thing to tune in your application. If you
have performance issues deal with those as performance issues, better code / data model /
hard ware. 

> By the way, I do not have consistency problem at all - data is only written
> once
Nobody expects a consistency problem. It's chief weapon is surprise. Surprise and fear. It's
two weapons are fear and surprise. And so forth http://www.youtube.com/watch?v=Ixgc_FGam3s

If you write at LOCAL QUORUM in DC 1 and DC 2 is down at the start of the request, a hint
will be stored in DC 1. Some time later when DC 2 comes back that hint will be sent to DC
2. If in the mean time you read from DC 2 at CL ONE you will not get that change. With Read
Repair enabled it will repair in the background and you may get a different response on the
next read (Am guessing here, cannot remember exactly how RR works cross DC) 

 Cheers



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

On 15/09/2011, at 10:07 AM, Pierre Chalamet wrote:

> 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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