[DISCUSS] add per node DNS selector

[spacewander]

When resolving domain, currently we take one of the DNS record randomly. When we want to do service discovery via DNS, sometimes we hope the domain resolving will return all the records. For example, with:

"nodes": {
    "test.consul.service": 1
}

We may want to get all IPs instead of a random one.

So my purpose is to provide a per-node dns selector to control the behavior.

For example,

"nodes": {
    {"host": "test.consul.service", "weight": 1, "selector": "random"}
}

is equal to previous configuration.

While

"nodes": {
    {"host": "test.consul.service", "weight": 1, "selector": "all"},
    {"host": "1.1.1.1", "weight": 1}
}

with test.consul.service be resolved as 1.1.1.2 and 1.1.1.3 will get this result:

"nodes": {
    {"host": "1.1.1.1", "weight": 2},
    {"host": "1.1.1.2", "weight": 1},
    {"host": "1.1.1.3", "weight": 1},
}

Note that all the IPs from test.consul.service share the same weight.
And the selector doesn't affect SRV record, which is a difference story.

[tokers]

Maybe add a new DNS based service discovery mechanism for the "selector: all" is better?

[sysulq]

Nice to see this enhancement, it's really useful when we do discovery through DNS way.

And host 1.1.1.1 should be recounted based on the number and weight of resovled IPs base on test.consul.service.

By the way, is there any plan to support SRV record?