Add leaf node remotes config

[variadico]

Currently, there is no way to specify leaf node remotes. This adds config
support for leaf node remotes and also includes some example YAML to deploy on
Kubernetes.

Fixes #280

[wallyqs]

I think we should probably also support the array version:

URL         []string `json:"urls,omitempty"`

[variadico]

Is this possible NATS config?

leafnodes {
    remotes = [
        {
          url: ["nats-leaf://s3cr3t@foo", "nats-leaf://s3cr3t@bar"]
        },
    ]
}

[wallyqs]

yes also possible

[variadico]

Ah, okay. Sounds good. 👍

[wallyqs]

both these are valid:

leafnodes {
    remotes = [
        {
          url: "nats-leaf://s3cr3t@foo,nats-leaf://s3cr3t@bar"
        },
    ]
}

array version:

leafnodes {
    remotes = [
        {
          urls: ["nats-leaf://s3cr3t@foo", "nats-leaf://s3cr3t@bar"]
        },
    ]
}

[wallyqs]

url can be many comma separated, but can also be an array urls: ["nats://...", "nats://..."] can be common in case to explicit about the clusters

[wallyqs]

LGTM! Might be good to have support for array version of the remotes too, also we need to add the option for remote leafnode TLS certs and the volume in a similar way as it is done here with the credentials.

[variadico]

Okay, so looks like the controller code generator doesn't like if we use interface{} types in the YAML. This means we can't overload the type of url in YAML.

However, I exposed two keys in the YAML, so this is possible.

  leafnodeConfig:
    remotes:
    - url: nats://1.2.3.4:7422
      credentials: /etc/nats-creds/user.ncreds

# and

  leafnodeConfig:
    remotes:
    - urls: [nats://1.2.3.4:7422, nats://1.2.3.4:7423]
      credentials: /etc/nats-creds/user.ncreds

# and technically even this

  leafnodeConfig:
    remotes:
    - url: nats://1.2.3.4:7422
      urls: [nats://1.2.3.4:7422, nats://1.2.3.4:7423]
      credentials: /etc/nats-creds/user.ncreds

# in the above case, all of the URLs get combined into 1 array of URLs

All combinations of these URL keys get merged into this.

leafnodes {
    remotes = [
        {
          urls: [nats://1.2.3.4:7422, ...]
        },
    ]
}

Change: fafdb24

[wallyqs]

LGTM!

[Kampe]

Question for you all, when I start my nats servers I should see some connection logs about the leaf nodes connecting? Is there a way to verify the connections?

[variadico]

Yeah, you should see something like this.

nats-server -c /tmp/leaf.conf 
....
[3704] 2019/12/09 09:55:31.548308 [INF] Listening for client connections on 127.0.0.1:4111
...
[3704] 2019/12/09 09:55:31.549404 [INF] Connected leafnode to "localhost"

Source: https://docs.nats.io/nats-server/configuration/leafnodes