Hi Jeff, thanks for answering to most of my points! >From the reloadseeds' ticket, I followed to https://issues.apache.org/jira/browse/CASSANDRA-3829 which was very instructive, although a bit old.
On Mon, 7 Jan 2019 at 17:23, Jeff Jirsa <jji...@gmail.com> wrote: > > On Jan 7, 2019, at 6:37 AM, Jonathan Ballet <jbal...@edgelab.ch> wrote: > > > [...] > > > In essence, in my example that would be: > > > > - decide that #2 and #3 will be the new seed nodes > > - update all the configuration files of all the nodes to write the IP > addresses of #2 and #3 > > - DON'T restart any node - the new seed configuration will be picked > up only if the Cassandra process restarts > > > > * If I can manage to sort my Cassandra nodes by their age, could it be a > strategy to have the seeds set to the 2 oldest nodes in the cluster? (This > implies these nodes would change as the cluster's nodes get > upgraded/replaced). > > You could do this, seems like a lot of headache for little benefit. Could > be done with simple seed provider and config management > (puppet/chef/ansible) laying down new yaml or with your own seed provider > So, just to make it clear: sorting by age isn't a goal in itself, it was just an example on how I could get a stable list. Right now, we have a dedicated group of seed nodes + a dedicated group for non-seeds: doing rolling-upgrade of the nodes from the second list is relatively painless (although slow) whereas we are facing the issues discussed in CASSANDRA-3829 for the first group which are non-seeds nodes are not bootstrapping automatically and we need to operate them in a more careful way. What I'm really looking for is a way to simplify adding and removing nodes into our (small) cluster: I can easily provide a small list of nodes from our cluster with our config management tool so that new nodes are discovering the rest of the cluster, but the documentation seems to imply that seed nodes also have other functions and I'm not sure what problems we could face trying to simplify this approach. Ideally, what I would like to have would be: * Considering a stable cluster (no new nodes, no nodes leaving), the N seeds should be always the same N nodes * Adding new nodes should not change that list * Stopping/removing one of these N nodes should "promote" another (non-seed) node as a seed - that would not restart the already running Cassandra nodes but would update their configuration files. - if a node restart for whatever reason it would pick up this new configuration So: no node would start its life as a seed, only a few already existing node would have this status. We would not have to deal with the "a seed node doesn't bootstrap" problem and it would make our operation process simpler. > > I also have some more general questions about seed nodes and how they > work: > > > > * I understand that seed nodes are used when a node starts and needs to > discover the rest of the cluster's nodes. Once the node has joined and the > cluster is stable, are seed nodes still playing a role in day to day > operations? > > They’re used probabilistically in gossip to encourage convergence. Mostly > useful in large clusters. > How "large" are we speaking here? How many nodes would it start to be considered "large"? Also, about the convergence: is this related to how fast/often the cluster topology is changing? (new nodes, leaving nodes, underlying IP addresses changing, etc.) Thanks for your answers! Jonathan
