Using Traefik with Consul

Consul Key/Value Store and Service Catalog

Consul has two functions built in to it: a Key/Value store and a Service Catalog
The Key/Value store is useful for defining variables to be shared in the cluster.
The Service Catalog is useful for integration with tools like Traefik as a service discovery tool to dynamically define hosts.

Service Catalog

The Consul Service Catalog is used as a service discovery tool.
The Service Catalog works byt defining a service by name, port, and healthcheck.

curl \
  -v \
  -XPUT \
  -d '{"service": {
        "name": "http-service",
        "tags": ["http-service"],
        "port": 80
      }
    }' \
  http://consul:8500/v1/agent/service/register

Once the service has been registered, consul will search the host where the consul agent is running for the port and healthcheck.
The service will be listed in the service catalog: consul catalog services

consul
http-service
nomad
nomad-client

Curl can be used to list the services

curl http://localhost:8500/v1/catalog/service/http-service
curl 'http://localhost:8500/v1/health/service/http-service?passing'

The power of the Service Catalog is that it includes a built-in DNS server for all the registered services

dig @127.0.0.1 -p 8600 web.service.consul
dig @127.0.0.1 -p 8600 web.service.consul SRV

Key/Value Store

curl http://0.0.0.0/v1/kv


consul kv put hello "world"
curl http://0.0.0.0:8500/v1/kv/hello
[
  {
    "LockIndex": 0,
    "Key": "hello",
    "Flags": 0,
    "Value": "ZGFwcA==",
    "CreateIndex": 75,
    "ModifyIndex": 75
  }
]


VALUE=$(curl http://0.0.0.0:8500/v1/kv/hello 2>/dev/null | jq '.[0].Value' | sed 's/"//g' | base64 -D)

echo "[$VALUE]"

world

Bootstrapping the Contents of a new Consul cluster

echo '{
  "web-service": {
    "name": "web",
    "tags": ["apache2"],
    "port": 80
  },
  "tomcat-service": {
    "name": "tomcat",
    "tags": ["tomcat"],
    "port": 8080
  },
  "webapp-service": {
    "name":  "webapp",
    "port":  5000,
    "tags":  ["flask", "backend"],
    "check": {
      "script":   "curl http://localhost:5000/healthcheck > /dev/null 2>&1",
      "interval": "10s"
    }
  }

}' | tee /etc/consul.d/web.json | jq '.'
consul agent -dev -config-dir=/etc/consul.d

Storing Traefik configuration in Consul

Traefik defines an entrypoint, a frontend, and a backend.
The entrypoint is a listener, think about it as an http or https point where traffic enters Traefik.
The entrypoint connects to a frontend. A frontend is a rule for connecting the incoming pipe to the backend outgoing pipe.
Consul can be used to store the configuration for Traefik so that a static on-disk config file is not needed.

Define Entrypoints

consul kv put traefik/entryPoints/http/address  ":80"
consul kv put traefik/entryPoints/http/compress true

Define Frontends

The frontend is a series of rules which determine how incoming traffic will flow from an entrypoint to the backend.
The rules used by Traefik are called matchers
Naturally a hostname request is the most common matching rule Host:dapp.hubner.org
Rules can be based on the HTTP request method: Method:GET
Regular expressions are supported in the rule so long as a named variable is used and the regex is contained in curly brackets: Method:{m:.*}
Multiple rules can be combined by separating them with a semicolon (;) : Method:GET;Method:POST

consul kv put traefik/frontends/dapp/backend dapp
consul kv put traefik/frontends/dapp/entrypoints http
consul kv put traefik/frontends/dapp/routes/dapp0/rule "Host:dapp.hubner.org"

Define Backends

consul kv put traefik/backends/dapp/servers/server0/url    "http://0.0.0.0:80"
consul kv put traefik/backends/dapp/servers/server0/weight 1

Configure docker container to register itself with the Consul Service Catalog on startup

IP=`ip addr | grep -E 'eth0.*state UP' -A2 | tail -n 1 | awk '{print $2}' | cut -f1 -d '/'`
SERVICENAME="dapp-service"
SERVICEPORT="5000"

read -r -d '' MSG << EOM
{
  "Name":    "$SERVICENAME",
  "address": "$IP",
  "port":    $SERVICEPORT,
  "Check": {
     "http": "http://$IP:$SERVICEPORT",
     "interval": "5s"
  }
}
EOM
 
curl -v -XPUT -d "$MSG" http://consul:8500/v1/agent/service/register && /app/main "$@"