Bootstrap Expect Behavior

Consul
1

I have been experimenting with Consul/Nomad as a potential solution to our product. Our solution ships out 4 hosts and I was curious if I startup Consul with systemd on all four with the bootstrap-expect=3 what will happen with the fourth host. My concern is that once all four hosts are discovered (mdns) that I would have a even number of servers. Does consul handle this? It would be great if the bootstrap-expect behavior is that we have only 3 servers and the 4th automatically is made a client. The documentation doesn’t specify this behavior

2

I have always used the bootstrap_expect on the servers, and also kept the count == no.of servers

I am not sure is leaving out the -server parameter will work.

I’ll let someone from HashiCorp confirm

BTW, servers are usually where the application workload should not run, so as not to overwhelm the Consul/Nomad servers.

3

“Expected” means that this is the minimum amount to build a cluster/ start leader election.
Additional nodes can join every time.
Consul can handle even numbers of nodes, even if it’s not recommended.

I’ll let someone from HashiCorp confirm.

4

My assumption from reading the documentation is that if we set the minimum number of servers to 3 is that consul will wait until that number is satisfied. What I am asking is the behavior of consul when the expect is set to 3 and a 4th server joins. That would result in 4 consul servers which is not ideal. I did not know if Hashicorp had code in to set that forth into a client to prevent a even number, that is the ask here

5

I don’t think so, because a cluster of five nodes is recommended, too. So the bootstrap_expect parameter would be also a maximum or the former client (4th node) will have to be promoted… And what’s the purpose of a client that should be a server instead?

But proof me wrong. :slight_smile:

6

If you are using bootstrap_expect=3 and are booting 4 servers, all of them are servers. Consul does not convert the 4th to a client. bootstrap_expect just makes the servers wait until there are 3 servers before it starts the leader election between these servers. You can always join more servers after that. Which is basically what happens when you are booting the 4th.

2 Likes