> You are right about the automatic fallback to ONE. Its quite possible, if 2 > nodes die for some reason I will have the same problem. So probably the right > thing to do would be to read/write at ONE only when we lose a DC by changing > some manual configuration. Since we shouldn't be losing DCs that often, this > should be an acceptable change. So my follow up questions would be -
Seems reasonable to have a human do it, since it seems that you really want QUORUM - so presumably there is some kind of negative impact and you don't want that sporadically happening every time there is a hiccup. But of course I don't know the context. > When would be the right time to start reading/writing at QUORUM again? I'd say usually as soon as possible, but it will depend on details of your situation. For example, if you have 2 DC:s with 5 nodes in one and 1 node in another, and there is a partition - the DC with just one node will start seeing "older" data (from the point of view of writes done in the 1-node DC) if you start asking for quorum since a lot of the time a quorum will be 4 nodes in the "other" DC. So if there is interest in preferring the "local" dc's copy of the data after an emergency fallback to CL.ONE, it may be detrimental to go QUORUM too early. But this will depend on what your application is actually doing and what is important to you. > Should we be marking the 2 nodes in the lost DC as down? > Should we be doing some administrative work on Cassandra before we start > reading/writing at QUORUM again? Are you talking about permanently losing a DC then, rather than just a transient partition? For non-permanent situations it seems counter-productive to mark other DC's nodes as down. Oh and btw, keep in mind you can choose to use LOCAL_QUORUM to get intra-site consistency (rather than ONE). As for administrative work: I can't answer in general since we're talking about very special circumstances, but at least it's valid to say that whenever you have some kind of issue that has caused inconsistency, running 'nodetool repair' (perhaps earlier than the standard weekly/whatever repair) is the most efficient way to achieve consistency again. -- / Peter Schuller
