Read-your-own-writes consistency in Cassandra

Read-your-own-writes consistency is great improvement from the so called eventual consistency: if I change my profile picture I don't care if others see the change a minute later, but it looks weird if after a page reload I still see the old one.

Can this be achieved in Cassandra without having to do a full r开发者_开发技巧ead-check on more than one node?

Using ConsistencyLevel.QUORUM is fine while reading an unspecified data and n>1 nodes are actually being read. However when client reads from the same node as he writes in (and actually using the same connection) it can be wasteful - some databases will in this case always ensure that the previously written (my) data are returned, and not some older one. Using ConsistencyLevel.ONE does not ensure this and assuming it leads to race conditions. Some test showed this: http://cassandra-user-incubator-apache-org.3065146.n2.nabble.com/per-connection-quot-read-after-my-write-quot-consistency-td6018377.html

My hypothetical setup for this scenario is 2 nodes, replication factor 2, read level 1, write level 1. This leads to eventual consistency, but I want read-your-own-writes consistency on reads.

Using 3 nodes, RF=3, RL=quorum and WL=quorum in my opinion leads to wasteful read request if I being consistent only on "my" data is enough.

// seo: also known as: session consistency, read-after-my-write consistency

Good question.

We've had http://issues.apache.org/jira/browse/CASSANDRA-876 open for a while to add this, but nobody's bothered finishing it because

1. CL.ONE is just fine for a LOT of workloads without any extra gymnastics
2. Reads are so fast anyway that doing the extra one is not a big deal (and in fact Read Repair, which is on by default, means all the nodes get checked anyway, so the difference between CL.ONE and higher is really more about availability than performance)

That said, if you're motivated to help, ask on the ticket and I'll be happy to point you in the right direction.

I've been following Cassandra development for a little while and I haven't seen a feature like this mentioned.

That said, if you only have 2 nodes with a replication factor of 2, I would question whether Cassandra is the best solution. You are going to end up with the entire data set on each node, so a more traditional replicated SQL setup might be simpler and more widely tested. Cassandra is very promising but it is still only version 0.8.2 and problems are regularly reported on the mailing list.

The other way to solve the 'see my own updates' problem would be to cache the results somewhere closer to the client, whether in the web server, the application layer, or using something like memcached.