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From Andreas Rudolph <andreas.rudo...@spontech-spine.com>
Subject Re: How to control location of data?
Date Wed, 11 Jan 2012 15:11:32 GMT
Hi!

> ...
> Again, it's probably a bad idea. 
I agree on that, now.

Thank you.

> 
> Cheers
> 
> -----------------
> Aaron Morton
> Freelance Developer
> @aaronmorton
> http://www.thelastpickle.com
> 
> On 11/01/2012, at 4:56 AM, Roland Gude wrote:
> 
>>  
>> Each node in the cluster is assigned a token (can be done automatically – but usually
should not)
>> The token of a node is the start token of the partition it is responsible for (and
the token of the next node is the end token of the current tokens partition)
>>  
>> Assume you have the following nodes/tokens (which are usually numbers but for the
example I will use letters)
>>  
>> N1/A
>> N2/D
>> N3/M
>> N4/X
>>  
>> This means that N1 is responsible (primary) for [A-D)
>>        N2 for [D-M)
>>        N3 for [M-X)
>> And N4 for [X-A)
>>  
>> If you have a replication factor of 1 data will go on the nodes like this:
>>  
>> B -> N1
>> E->N2
>> X->N4
>>  
>> And so on
>> If you have a higher replication factor, the placement strategy decides which node
will take replicas of which partition (becoming secondary node for that partition)
>> Simple strategy will just put the replica on the next node in the ring
>> So same example as above but RF of 2 and simple strategy:
>>  
>> B-> N1 and N2
>> E -> N2 and N3
>> X -> N4 and N1
>>  
>> Other strategies can factor in things like “put  data in another datacenter”
or “put data in another rack” or such things.
>>  
>> Even though the terms primary and secondary imply some means of quality or consistency,
this is not the case. If a node is responsible for a piece of data, it will store it.
>>  
>>  
>> But placement of the replicas is usually only relevant for availability reasons (i.e.
disaster recovery etc.)
>> Actual location should mean nothing to most applications as you can ask any node
for the data you want and it will provide it to you (fetching it from the responsible nodes).
>> This should be sufficient in almost all cases.
>>  
>> So in the above example again, you can ask N3 “what data is available” and it
will tell you: B, E and X, or you could ask it “give me X” and it will fetch it from N4
or N1 or both of them depending on consistency configuration and return the data to you.
>>  
>>  
>> So actually if you use Cassandra – for the application the actual storage location
of the data should not matter. It will be available anywhere in the cluster if it is stored
on any reachable node.
>>  
>> Von: Andreas Rudolph [mailto:andreas.rudolph@spontech-spine.com] 
>> Gesendet: Dienstag, 10. Januar 2012 15:06
>> An: user@cassandra.apache.org
>> Betreff: Re: AW: How to control location of data?
>>  
>> Hi!
>>  
>> Thank you for your last reply. I'm still wondering if I got you right...
>>  
>> ... 
>> A partitioner decides into which partition a piece of data belongs
>> Does your statement imply that the partitioner does not take any decisions at all
on the (physical) storage location? Or put another way: What do you mean with "partition"?
>>  
>> To quote http://wiki.apache.org/cassandra/ArchitectureInternals: "... AbstractReplicationStrategy
controls what nodes get secondary, tertiary, etc. replicas of each key range. Primary replica
is always determined by the token ring (...)"
>> 
>> 
>> ... 
>> You can select different placement strategies and partitioners for different keyspaces,
thereby choosing known data to be stored on known hosts.
>> This is however discouraged for various reasons – i.e.  you need a lot of knowledge
about your data to keep the cluster balanced. What is your usecase for this requirement? there
is probably a more suitable solution.
>>  
>> What we want is to partition the cluster with respect to key spaces.
>> That is we want to establish an association between nodes and key spaces so that
a node of the cluster holds data from a key space if and only if that node is a *member* of
that key space.
>>  
>> To our knowledge Cassandra has no built-in way to specify such a membership-relation.
Therefore we thought of implementing our own replica placement strategy until we started to
assume that the partitioner had to be replaced, too, to accomplish the task.
>>  
>> Do you have any ideas?
>>  
>> 
>> 
>> Von: Andreas Rudolph [mailto:Andreas.Rudolph@Spontech-Spine.com] 
>> Gesendet: Dienstag, 10. Januar 2012 09:53
>> An: user@cassandra.apache.org
>> Betreff: How to control location of data?
>>  
>> Hi!
>>  
>> We're evaluating Cassandra for our storage needs. One of the key benefits we see
is the online replication of the data, that is an easy way to share data across nodes. But
we have the need to precisely control on what node group specific parts of a key space (columns/column
families) are stored on. Now we're having trouble understanding the documentation. Could anyone
help us with to find some answers to our questions?
>> 
>> ·  What does the term "replica" mean: If a key is stored on exactly three nodes
in a cluster, is it correct then to say that there are three replicas of that key or are there
just two replicas (copies) and one original?
>> ·  What is the relation between the Cassandra concepts "Partitioner" and "Replica
Placement Strategy"? According to documentation found on DataStax web site and architecture
internals from the Cassandra Wiki the first storage location of a key (and its associated
data) is determined by the "Partitioner" whereas additional storage locations are defined
by "Replica Placement Strategy". I'm wondering if I could completely redefine the way how
nodes are selected to store a key by just implementing my own subclass of AbstractReplicationStrategy
and configuring that subclass into the key space.
>> ·  How can I suppress that the "Partitioner" is consulted at all to determine what
node stores a key first?
>> ·  Is a key space always distributed across the whole cluster? Is it possible to
configure Cassandra in such a way that more or less freely chosen parts of a key space (columns)
are stored on arbitrarily chosen nodes?
>>  
>> Any tips would be very appreciated :-)
>> 
> 



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