Hello, Riak is based on eventual consistency concept, but sometimes it's desirable to have absolute consistency. I though it would be possible to implement this on top of Riak with the help of pr and wr parameters of fetch and store, but when testing this I sometimes get worrying results.
I have at test setup with Riak nodes in three virtual machines running on my desktop machine. The test bucket has n_val=3 and allow_mult=true. On my desktop machine I constantly read from every node like this: watch curl -v "'http://192.168.100.21:8098/riak/test/foo?pr=quorum'" When I pause two of the virtual machines, the remaining one starts to fail with error message "PR-value unsatisfied: 1/2", just as expected. But occasionally it will returns 200 OK for about 10 seconds, before it starts returning "PR-value unsatisfied: 1/2". I can't predictably reproduce this situation, but it's seems to happen when I resume a machine that was paused when the other machines where running too. Pausing virtual machines to simulate downtime might be a bit unrealistic. Still it concerns me that Riak can return stale data. Here is an example: First I wrote a value when all nodes where up. Then I pause one of the nodes and wrote a new value. Then I paused the two remaining nodes and resume the first node. For a few seconds the first node then returned 200 OK with the old value when though I supplied pr=quorum. I have also been able to do successful writes with w, dw and pw=quorum with only one machine running. I thing the node also returned 200 OK when the object was read with pr=quorum right after. The tests where done with Riak 1.0.0-1 running on Ubuntu 10.10 64 bit. -- Thomas Bakketun, Senior software developer T. +47 21 53 69 40 Copyleft Solutions www.copyleftsolutions.com _______________________________________________ riak-users mailing list riak-users@lists.basho.com http://lists.basho.com/mailman/listinfo/riak-users_lists.basho.com
