Riak replication and quorum

Ben Tilly btilly at gmail.com
Fri May 13 12:00:56 EDT 2011


Here is my understanding.  Corrections welcome.

You're missing that Riak is happy to be "eventually consistent".  Drop
out 2 of your nodes, and it rebalances who is responsible for what,
then under the hood migrates and replicates its data more leisurely.

Data is still being written to 4 different hash buckets, but those
buckets are now distributed across 2 machines.

When the data is done migrating, you should not have lost any data.
But it might be temporarily available.

On Fri, May 13, 2011 at 8:13 AM, Peter Fales
<Peter.Fales at alcatel-lucent.com> wrote:
>
> I'm a Riak newbie, trying to get some familiarity with the system by
> runing some tests on Amazon EC2.   I'm seeing some behavior that I don't
> understand...
>
> I've set up a test where I create a 4-node cluster using 4 EC2 machines.
> I've created a bucket with n_val=4, r=quorum, and w=quorum.   For
> n_val=4, the quorum should be 3, so I thought I would have to have at
> least 3 nodes in service for my read and write operations to succeed.
> During my test, I start sending read/write requests to two of the nodes
> (and I see the CPU load go up on all four nodes, so I know they are
> talking to each other).  Then I reboot the other two nodes.  At that
> point, I was expecting the reads and writes to start failing, but in
> fact I usually don't see any problems.  (sometimes the query that is
> in progress at the time may fail or timeout, but if I establish a new
> connection to the server, and start sending read/write requests again,
> those requests will go through, even with only two of the 4 nodes in service)
>
> I suspect I'm just missing something obvious, but I don't understand how
> I can run with just two nodes.  What am I missing?
>
> --
> Peter Fales
> Alcatel-Lucent
> Member of Technical Staff
> 1960 Lucent Lane
> Room: 9H-505
> Naperville, IL 60566-7033
> Email: Peter.Fales at alcatel-lucent.com
> Phone: 630 979 8031
>
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