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From Eric Evans <eev...@acunu.com>
Subject Re: Update: Cassandra Virtual Nodes
Date Wed, 20 Jun 2012 21:25:47 GMT
So to followup on this, my take on the first discussion
(http://www.mail-archive.com/dev@cassandra.apache.org/msg03837.html)
was that there was some concern about how invasive this change would
be.  This was then followed by discussion about the trade-offs between
trying to push such a big change in all at once, versus an incremental
approach, and the associated risks if for some reason we weren't able
to follow through.

The point we're at now is our first shot at trying to thread this
needle.  As Sam says, it's at a point where it should be minimally
useful, and should be backward compatible when run with 1 token per
node (the default).  The patches I am referring to again are linked
from:

https://issues.apache.org/jira/browse/CASSANDRA-4121
https://issues.apache.org/jira/browse/CASSANDRA-4122
https://issues.apache.org/jira/browse/CASSANDRA-4125
https://issues.apache.org/jira/browse/CASSANDRA-4127

What we most need now is feedback, and consensus from the project.
Consensus on the proposed changes, the implementation, the scope of
the first iteration, and subsequent steps.

Thanks,

On Fri, Jun 1, 2012 at 11:50 AM, Sam Overton <sam@acunu.com> wrote:
> Work on our proposed design is being tracked on JIRA:
> https://issues.apache.org/jira/browse/CASSANDRA-4119
>
> We feel that our current implementation has sufficient functionality
> to be useful in its own right, but is still a fairly self-contained
> set of changes that can be incrementally improved - something which
> was expressed as desirable in the last discussion. This is a good
> point for a first round of review and to reach out to anyone
> interested in testing or contributing.
>
> == Obtaining the source ==
>
> The easiest way to test these changes is to clone our github repo and
> switch to the topic branch representing the top patch:
>
>> git clone git://github.com/acunu/cassandra.git
>> cd cassandra
>> git checkout --track remotes/origin/p/4127/01_migration_path
>
> Now just build with ant. To create a cluster with virtual-nodes
> support just un-comment/edit the following parameter in
> cassandra.yaml:
>> num_tokens: 256
>
> Make sure you have left initial_token blank. The node should
> automatically assign itself this many tokens. You can view the token
> assignment as usual with "nodetool ring" but this becomes fairly
> useless with any large number of hosts/tokens, which is why we have
> added "nodetool clusterinfo" which shows ring ownership without
> cluttering the output with the token assignment.
>
> If you want to test with a specific token assignment, initial_token
> now supports a comma-separated list of tokens which will override the
> num_tokens setting. This is just for convenience of testing - users
> should no longer need to manually specify tokens as balancing is
> automatic.
>
> == Patches ==
>
> The tickets which are in scope for this round of review are:
> https://issues.apache.org/jira/browse/CASSANDRA-4121
> https://issues.apache.org/jira/browse/CASSANDRA-4122
> https://issues.apache.org/jira/browse/CASSANDRA-4125
> https://issues.apache.org/jira/browse/CASSANDRA-4127
>
> Each of these tickets has links to the corresponding patches. The
> order they should be applied is just increasing numerical order of
> ticket number.
>
> These patches are based off another patch (currently pending review)
> for CASSANDRA-3881, which was an existing issue blocking virtual
> nodes.
> https://issues.apache.org/jira/browse/CASSANDRA-3881
>
> Links to the individual patches can also be found all in one place on
> the github wiki:
> https://github.com/acunu/cassandra/wiki/CASSANDRA-4119
>
> The current patches in the series are as follows:
>
> * p/4121/01_support_multiple_tokens_per_host
> support multiple tokens per host in TokenMetadata (CASSANDRA-4121)
> * p/4122/01_bootstrap_decommission
> support bootstrapping a node into the ring with multiple tokens and
> minor changes for decommission (CASSANDRA-4122)
> * p/4122/02_remove_tokens
> minor changes to support remove-token with multiple tokens (CASSANDRA-4122)
> * p/4125/01_admin_tools
> nodetool support (addition of nodetool clusterinfo and changes to
> nodetool ring) (CASSANDRA-4125)
> * p/4127/01_migration_path
> support for migration from 1-token to multi-token per node (CASSANDRA-4127)
>
> If you wish to contribute or work with a clone from github then it
> would be advisable to familiarise yourself with TopGit, the tool we
> have been using for branch-based patch queue management. We've written
> up a tutorial here:
> https://github.com/acunu/cassandra/wiki/TopGit-Tutorial
>
> == What's left? ==
>
> We haven't included any patches for tickets CASSANDRA-4123 and
> CASSANDRA-4124 which relate to the replication strategy and repair.
> Currently replication and repair "just work" with the current patches
> without any additional changes required.
>
> CASSANDRA-4126 relates to testing. We're running virtual nodes builds
> through our own test suites but we will also be writing new tests in
> addition.
>
>
> I look forward to your questions and comments!



-- 
Eric Evans
Acunu | http://www.acunu.com | @acunu

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