[Add] ConsulCatalog provider: Using K/V for properties (weight, load balancer, circuit breaker)

[samber]

See #253

[emilevauge]

Hi @samber, thanks a lot for your contribution :)

There is an issue with consul, there is no label :'( :
#146
hashicorp/consul#997

Your idea is great but I thought in the first place that I would use Consul tags to achieve that.
Something like:

{
  "service": {
    "name": "redis",
    "tags": ["traefik.backend=foo", "traefik.weight=10"],
    "address": "127.0.0.1",
    "port": 8000,
    "enableTagOverride": false,
    "checks": [
      {
        "script": "/usr/local/bin/check_redis.py",
        "interval": "10s"
      }
    ]
  }
}

WDYT?
/cc @vdemeester

[keis]

I think using tags would be better considering how you are most likely populating the consul catalog by some automatic scraping of another directory. e.g mesos-consul. Having another source of truth for some of the configuration does not sound like a good idea.

[samber]

We are building a Consul+Docker stack with Registrator to populate the Consul Catalog.

Using tags is a hack. Unfortunately, as you may know, storing kv-attributes in the consul catalog is discussed for 2y in the Consul ML and on Github. It's still not part of the road map.

It is a trade-off. For Registrator, a PR is going to be merge to use KV as attribute storage. gliderlabs/registrator#261

[emilevauge]

Using tags is a hack.

Could you elaborate on this? Because we would have to parse tags?

It seems much simpler to me to run a container using:

docker run -d --name redis.0 -p 10000:6379 \
 -e "SERVICE_NAME=db" \
 -e "SERVICE_TAGS=traefik.backend=foo,traefik.weight=10" \
 -e "SERVICE_REGION=us2" redis

which in Consul results in:

{
   "ID":"e2c1fdea9dc8:redis.0:6379",
   "Service":"db",
   "Tags":[
      "traefik.backend=foo",
      "traefik.weight=10"
   ],
   "Address":"172.17.0.4",
   "Port":6379
}

I don't say your solution is bad, but I just want to expose my point of view :)
As we use labels in Docker and Marathon backends, I think it would be more homogeneous to use tags in Consul Catalog (even if it's not ideal).
Besides, having services declared in consul catalog and their config somewhere else could lead to inconsistencies as we would have to update catalog and KV store (and there is no way to do that atomically :'( ).
For now, the PR gliderlabs/registrator#261 is not ready to be merged (ie panics on KV puts) and there has been no change since Oct 5, 2015.

[aaronchar]

For what it's worth I like the tags idea, seems easier to a mange.

[samber]

Ok, let's do that.
I'll update my branch and submit a new PR, probably next week.

Syntax of a tag: traefik.(loadbalancer | weight | circuitbreaker)=%value