I'm trying to understand how seed nodes are working, when and how do they play a part in a Cassandra cluster, and how they should be managed and propagated to other nodes.

I have a cluster of 6 Cassandra nodes (let's call them #1 to #6), on which node #1 and #2 are seeds. All the configuration files of all the Cassandra nodes are currently configured with:

- class_name: org.apache.cassandra.locator.SimpleSeedProvider
  - seeds: 'IP #1,IP #2'

We are using a service discovery tool (Consul) which automatically registers new Cassandra nodes with its dedicated health-check and are able to generate new configuration based on the content of the service discovery status (with Consul-Template).

I'm working on how we could improve the upgrades of our servers and how to replace them completely (new instance with a new IP address).
What I would like to do is to replace the machines holding our current seeds (#1 and #2 at the moment) in a rolling upgrade fashion, on a regular basis:

* Is it possible to "promote" any non-seed node as a seed node?

* Is it possible to "promote" a new seed node without having to restart all the nodes?
  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).

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?

* The documentation says multiple times that not all nodes should be seed nodes, but I didn't really find any place about the consequences it has to have "too many" seed nodes. Also, relatively to the questions I asked above, is there any downsides of having changing seed nodes in a cluster? (with the exact same, at some point I define #1 and #2 to be seeds, then later #4 and #5, etc.)

Thanks for helping me to understand better how seeds are working!