I think that Oleg may have misunderstood how replicas are selected. If you have 3 nodes in your cluster and a RF of 2, Cassandra first selects what two nodes, out of the 3 will get data, then, and only then does it write it out. The selection is based on the row key, the token of the node, and you choice of partitioner. This means that Cassandra does not need to store what node is responsible for a given row. That information can be recalculated whenever it is needed.
The error that you are getting is because you may have 2 nodes up, those are not the nodes that Cassandra will use to store data. ----- Original Message ----- From: "Nate McCall" <n...@datastax.com> To: hector-us...@googlegroups.com Cc: "Cassandra Users" <user@cassandra.apache.org> Sent: Friday, September 2, 2011 4:44:01 PM Subject: Re: HUnavailableException: : May not be enough replicas present to handle consistency level. In your options, you have configured 2 replicas for each data center: Options: [DC2:2, DC1:2] If one of those replicas is down, then LOCAL_QUORUM will fail as there is only one replica left 'locally.' On Fri, Sep 2, 2011 at 3:35 PM, Oleg Tsvinev <oleg.tsvi...@gmail.com> wrote: > from http://www.datastax.com/docs/0.8/consistency/index: > > <A “quorum” of replicas is essentially a majority of replicas, or RF / > 2 + 1 with any resulting fractions rounded down.> > > I have RF=2, so majority of replicas is 2/2+1=2 which I have after 3rd > node goes down? > > On Fri, Sep 2, 2011 at 1:22 PM, Nate McCall <n...@datastax.com> wrote: >> It looks like you only have 2 replicas configured in each data center? >> >> If so, LOCAL_QUORUM cannot be achieved with a host down same as with >> QUORUM on RF=2 in a single DC cluster. >> >> On Fri, Sep 2, 2011 at 1:40 PM, Oleg Tsvinev <oleg.tsvi...@gmail.com> wrote: >>> I believe I don't quite understand semantics of this exception: >>> >>> me.prettyprint.hector.api.exceptions.HUnavailableException: : May not >>> be enough replicas present to handle consistency level. >>> >>> Does it mean there *might be* enough? >>> Does it mean there *is not* enough? >>> >>> My case is as following - I have 3 nodes with keyspaces configured as >>> following: >>> >>> Replication Strategy: org.apache.cassandra.locator.NetworkTopologyStrategy >>> Durable Writes: true >>> Options: [DC2:2, DC1:2] >>> >>> Hector can only connect to nodes in DC1 and configured to neither see >>> nor connect to nodes in DC2. This is for replication by Cassandra >>> means, asynchronously between datacenters DC1 and DC2. Each of 6 total >>> nodes can see any of the remaining 5. >>> >>> and inserts with LOCAL_QUORUM CL work fine when all 3 nodes are up. >>> However, this morning one node went down and I started seeing the >>> HUnavailableException: : May not be enough replicas present to handle >>> consistency level. >>> >>> I believed if I have 3 nodes and one goes down, two remaining nodes >>> are sufficient for my configuration. >>> >>> Please help me to understand what's going on. >>> >> >
