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From Jeff Jirsa <jji...@gmail.com>
Subject Re: How seed nodes are working and how to upgrade/replace them?
Date Mon, 07 Jan 2019 16:23:15 GMT



> On Jan 7, 2019, at 6:37 AM, Jonathan Ballet <jballet@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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