How to promote agent to server?

Consul
1

Hi

We have 5 Consul servers (1.0.6) and a bunch of agents in our setup. We have lost one of the server nodes and would like to “promote” one of the agents to be a server. Is this as simple as editing /etc/consul.d/config.json, setting server=true and restarting Consul? Or do we have to wipe the data-dir first?

Thanks
David

2

Hi @david.tinker,

Setting server=true should be enough to make a Client agent into a Server, but there are few things to note before making this change.

  1. Server agents have few extra ports that should be open between nodes for them to function properly. So network configuration has to be taken care of before making the switch.

    • 8300/tcp: Server RPC (All clients and servers should be able to talk on this port)
    • 8302/tcp&udp: Serf WAN Port (All servers should be able to talk on this port)

  2. If you have enabled TLS and if you have verify_server_hostname set to true, then you will have to make sure that the service certificate has server.<datacenter_name>.<domain> in the SAN. (ref: Configuration | Consul by HashiCorp)

  3. Apart from the above two, you may want to consider moving the services registered if any on this node to different servers to make this a Consul server only node (from a performance perspective).

Having said the above, if you can issue a consul leave, wipe the data-dir, set server=true, and then start the agent then you (also satisfying the above requirements) can effectively think of it as if you are adding a brand new node.

These are the important points that came to my mind, I will let others add if there is anything that I have missed.

1 Like
3

Thanks. I ended up stopping consul, changing the config, wiping the data dir and starting it up. Worked seamlessly.

I didn’t think of doing a leave … maybe it did that itself? It was a graceful stop.

4

Yeah, by default when running as Client the default terminate behaviour is to leave the cluster (ref: Configuration | Consul by HashiCorp).