> On Jan 7, 2019, at 6:37 AM, Jonathan Ballet <jbal...@edgelab.ch> wrote:
>
> Hi,
>
> 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:
>
> ```
> seed_provider:
> - class_name: org.apache.cassandra.locator.SimpleSeedProvider
> parameters:
> - 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?
>
Yes - generally you can make any node a seed if you want
> * Is it possible to "promote" a new seed node without having to restart all
> the nodes?
nodetool reloadseeds
There are a few weird edge cases where seeds are reloaded automatically and we
don’t document how or why (it’s a side effect of an error condition in hosts
going up/down, but it’s generally pretty minor unless your seed provider is
broken)
(Also true that you could write a seed provider that did this automatically)
> 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
>
>
> 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.
>
> * 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.
Decreases effectiveness of probabilistic gossiping with seed for convergence
> 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.)
>
No
>
> Thanks for helping me to understand better how seeds are working!
>
> Jonathan
>
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