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From aaron morton <aa...@thelastpickle.com>
Subject Re: advice for EC2 deployment
Date Tue, 26 Apr 2011 21:01:01 GMT
One difference between Cassandra and MySQL replication may be when the network IO happens.
Was the MySQL replication synchronous on transaction commit ?  I was only aware that it had
async replication, which means the client is not exposed to the network latency. In cassandra
the network latency is exposed to the client as it needs to wait for the CL number of nodes
to respond. 

If you use the PropertyFilePartitioner with the NetworkTopology you can manually assign machines
to racks / dc's based on IP. 
See conf/cassandra-topology.property file there is also an Ec2Snitch which (from the code)

/**
 * A snitch that assumes an EC2 region is a DC and an EC2 availability_zone
 *  is a rack. This information is available in the config for the node.

Recent discussion on DC aware CL levels 
http://www.mail-archive.com/user@cassandra.apache.org/msg11414.html

Hope that helps.
Aaron
 

On 27 Apr 2011, at 01:18, William Oberman wrote:

> Thanks Aaron!
> 
> Unless no one on this list uses EC2, there were a few minor troubles end of last week
through the weekend which taught me a lot about obscure failure modes in various applications
I use :-)  My original post was trying to be more redundant than fast, which has been by overall
goal from even before moving to Cassandra (my downtime from the EC2 madness was minimal, and
due to only having one single point of failure == the amazon load balancer).  My secondary
goal was  trying to make moving to a second region easier, but is that is causing problems
I can drop the idea.
> 
> I might be downplaying the cost of inter-AZ communication, but I've lived with that for
quite some time, for example my current setup of MySQL in Master-Master replication is split
over zones, and my webservers live in yet different zones.  Maybe Cassandra is "chattier"
than I'm used to?  (again, I'm fairly new to cassandra)
> 
> Based on that article, the discussion, and the recent EC2 issues, it sounds like it would
be better to start with:
> -6 nodes split in two AZs 3/3
> -Configure replication to do 2 in one AZ and one in the other (NetworkTopology treats
AZs as racks, so does RF=3,us-east=3 make this happen naturally?)
> -What does LOCAL_QUORUM do in this case?  Is there a "rack quorum"?  Or does the natural
latencies of AZs make LOCAL_QUORUM behave like a rack quorum?
> 
> will
> 
> On Tue, Apr 26, 2011 at 1:14 AM, aaron morton <aaron@thelastpickle.com> wrote:
> For background see this article:
> http://www.datastax.com/dev/blog/deploying-cassandra-across-multiple-data-centers
> 
> And this recent discussion 
> http://www.mail-archive.com/user@cassandra.apache.org/msg12502.html
> 
> Issues that may be a concern:
> - lots of cross AZ latency in us-east, e.g. LOCAL_QUORUM ops must wait cross AZ . Also
consider it during maintenance tasks, how much of a pain is it going to be to have latency
between every node.   
> - IMHO not having sufficient (by that I mean 3) replicas in a cassandra DC to handle
a single node failure when working at Quorum reduces the utility of the DC. e.g. with a local
RF of 2 in the west, the quorum is 2, and if you lose one node from the replica set you will
not be able to use local QUORUM for keys in that range. Or consider a failure mode where the
west is disconnected from the east.
> 
> Could you start simple with 3 replicas in one AZ in us-east and 3 replicas in an AZ+Region
?  Then work through some failure scenarios.  
> 
> Hope that helps. 
> Aaron
>   
> 
> On 22 Apr 2011, at 03:28, William Oberman wrote:
> 
>> Hi,
>> 
>> My service is not yet ready to be fully multi-DC, due to how some of my legacy MySQL
stuff works.  But, I wanted to get cassandra going ASAP and work towards multi-DC.  I have
two main cassandra use cases: one where I can handle eventual consistency (and all of the
writes/reads are currently ONE), and one where I can't (writes/reads are currently QUORUM).
 My test cluster is currently 4 smalls all in us-east with RF=3 (more to prove I can clustering,
than to have an exact production replica).  All of my unit tests, and "load tests" (again,
not to prove true max load, but to more to tease out concurrency issues) are passing now.
>> 
>> For production, I was thinking of doing:
>> -4 cassandra larges in us-east (where I am now), once in each AZ
>> -1 cassandra large in us-west (where I have nothing)
>> For now, my data can fit into a single large's 2 disk ephemeral using RAID0, and
I was then thinking of doing a RF=3 with us-east=2 and us-west=1.  If I do eventual consistency
at ONE, and consistency at LOCAL_QUORUM, I was hoping:
>> -eventual consistency ops would be really fast
>> -consistent ops would be pretty fast (what does LOCAL_QUORUM do in this case?  return
after 1 or 2 us-east nodes ack?)
>> -us-west would contain a complete copy of my data, so it's a good eventually consistent
"close to real time" backup  (assuming it can keep up over long periods of time, but I think
it should)
>> -eventually, when I'm ready to roll out in us-west I'll be able to change the replication
settings and that server in us-west could help seed new cassandra instances faster than the
ones in us-east
>> 
>> Or am I missing something really fundamental about how cassandra works making this
a terrible plan?  I should have plenty of time to get my multi-DC working before the instance
in us-west fills up (but even then, I should be able to add instances over there to stall
fairly trivially, right?).
>> 
>> Thanks!
>> 
>> will
> 
> 
> 
> 
> -- 
> Will Oberman
> Civic Science, Inc.
> 3030 Penn Avenue., First Floor
> Pittsburgh, PA 15201
> (M) 412-480-7835
> (E) oberman@civicscience.com


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