I've done some gossip simulations in the past and found virtually no difference in the time it takes for messages to propagate in almost any sized cluster.  IIRC it always converges by 17 iterations.  Thus, I completely agree with Jeff's comment here.  If you aren't pushing 800-1000 nodes, it's not even worth bothering with.  Just be sure you have seeds in each DC.  

Something to be aware of - there's only a chance to gossip with a seed.  That chance goes down as cluster size increases, meaning seeds have less and less of an impact as the cluster grows.  Once you get to 100+ nodes, a given node is very rarely talking to a seed.

Just make sure when you start a node it's not in its own seed list and you're good.

On Tue, Jan 8, 2019 at 9:39 AM Jeff Jirsa <jjirsa@gmail.com> wrote:

On Tue, Jan 8, 2019 at 8:19 AM Jonathan Ballet <jballet@edgelab.ch> wrote:
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 <jjirsa@gmail.com> wrote:
> On Jan 7, 2019, at 6:37 AM, Jonathan Ballet <jballet@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.

Rolling upgrade shouldn't need to re-bootstrap. Only replacing a host should need a new bootstrap. That should be a new host in your list, so it seems like this should be fairly rare? 
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.)

New nodes, nodes going up/down, and schema propagation. 
Thanks for your answers!


Jon Haddad
twitter: rustyrazorblade