Marathon provider no longer allows for tasks from different apps within the same backend

[TrevinTeacutter]

Do you want to request a feature or report a bug?

bug

What did you do?

We have the need for new apps to be included within the same backend so that they are added
to the wrr pool of an existing app in order for zero downtime deployments. This was originally
working with Traefik 1.4.0-rc1 and continued to work until 1.4.2. We didn't try to upgrade
until recently and we are pretty sure this template change caused the issue:

02035d4#diff-080894c4c7f765921235ddb2ceb82140

What did you expect to see?

We expect to see multiple tasks belonging to different marathon apps within the same backend.

What did you see instead?

An error due to a backend already being 'configured' within the template

Output of traefik version: (What version of Traefik are you using?)

1.4.1 (This is the last version that worked for this)
1.4.2 (First version that failed for the use-case)
1.4.6
1.5.0.rc5

What is your environment & configuration (arguments, toml, provider, platform, ...)?

################################################################
# Global configuration
################################################################

InsecureSkipVerify = true
defaultEntryPoints = ["http","https"]
email = "test@traefik.io"
storage = "traefik/acme/account"
entryPoint = "https"

[accessLog]

[entryPoints]
  [entryPoints.http]
  address = ":80"
  [entryPoints.https]
  address = ":443"
    [entryPoints.https.tls]
      [[entryPoints.https.tls.certificates]]
      CertFile = "/etc/ssl/traefik.crt"
      KeyFile = "/etc/ssl/traefik.key"

[retry]
attempts = -1

[forwardingTimeouts]

# dialTimeout is the amount of time to wait until a connection to a backend server can be established.
#
# Optional
# Default: "30s"
#
dialTimeout = "3s"

################################################################
# Web configuration backend
################################################################

[web]
address = ":9090"

################################################################
# File configuration backend
################################################################
trace = true

################################################################
# Mesos/Marathon configuration backend
################################################################

[marathon]
endpoint = "https://leader.mesos/marathon"
watch = true
domain = "marathon.localhost"
exposedByDefault = true
marathonLBCompatibility = true

[marathon.DCOSCredentials]
UID = "svc-traefik"
PrivateKey = "/etc/ssl/service-account.key"
LoginEndpoint = "https://leader.mesos/acs/api/v1/auth/login"

[marathon.TLS]
InsecureSkipVerify = true

If applicable, please paste the log output in debug mode (--debug switch)

time="2018-01-15T18:51:11Z" level=error msg="Failed to render Marathon configuration template: Near line 66 (last key parsed 'backends'): Key 'backends.foo' has already been defined." 

[ldez]

Related to #2333

[emilevauge]

@TrevinTeacutter Could you provide your Marathon application descriptor (json) that produces this error ?

[TrevinTeacutter]

{
  "id": "/foo-v000",
  "instances": 2,
  "cpus": 0.1,
  "mem": 32,
  "disk": 0,
  "gpus": 0,
  "constraints": [],
  "fetch": [],
  "storeUrls": [],
  "backoffSeconds": 1,
  "backoffFactor": 1.15,
  "maxLaunchDelaySeconds": 3600,
  "container": {
    "type": "DOCKER",
    "volumes": [],
    "docker": {
      "image": "docker.io/nginx",
      "network": "BRIDGE",
      "portMappings": [
        {
          "containerPort": 8080,
          "protocol": "tcp",
          "name": "main",
          "labels": {}
        }
      ],
      "privileged": false,
      "parameters": [],
      "forcePullImage": true
    }
  },
  "healthChecks": [],
  "readinessChecks": [],
  "dependencies": [],
  "upgradeStrategy": {
    "minimumHealthCapacity": 1,
    "maximumOverCapacity": 1
  },
  "secrets": {},
  "unreachableStrategy": {
    "inactiveAfterSeconds": 300,
    "expungeAfterSeconds": 600
  },
  "killSelection": "YOUNGEST_FIRST",
  "requirePorts": false,
  "labels": {
    "traefik.main.backend": "test.foo",
    "traefik.main.protocol": "http",
    "traefik.protocol": "http",
    "traefik.main.portIndex": "0",
    "traefik.enable": "true",
    "traefik.main.frontend.rule": "Host:test.foo.hax"
  },
  "env": {}
}

{
  "id": "/foo-v001",
  "instances": 2,
  "cpus": 0.1,
  "mem": 32,
  "disk": 0,
  "gpus": 0,
  "constraints": [],
  "fetch": [],
  "storeUrls": [],
  "backoffSeconds": 1,
  "backoffFactor": 1.15,
  "maxLaunchDelaySeconds": 3600,
  "container": {
    "type": "DOCKER",
    "volumes": [],
    "docker": {
      "image": "docker.io/nginx",
      "network": "BRIDGE",
      "portMappings": [
        {
          "containerPort": 8080,
          "protocol": "tcp",
          "name": "main",
          "labels": {}
        }
      ],
      "privileged": false,
      "parameters": [],
      "forcePullImage": true
    }
  },
  "healthChecks": [],
  "readinessChecks": [],
  "dependencies": [],
  "upgradeStrategy": {
    "minimumHealthCapacity": 1,
    "maximumOverCapacity": 1
  },
  "secrets": {},
  "unreachableStrategy": {
    "inactiveAfterSeconds": 300,
    "expungeAfterSeconds": 600
  },
  "killSelection": "YOUNGEST_FIRST",
  "requirePorts": false,
  "labels": {
    "traefik.main.backend": "test.foo",
    "traefik.main.protocol": "http",
    "traefik.protocol": "http",
    "traefik.main.portIndex": "0",
    "traefik.enable": "true",
    "traefik.main.frontend.rule": "Host:test.foo.hax"
  },
  "env": {}
}

We should expect to see this in the Web UI for the ones that aren't broken:

In the ones that are broken, state will never change because the error prevents any updates being made.

[MrMarkW]

Has any done any investigation of this issue?

[TrevinTeacutter]

We removed this line in the marathon template in our local fork of 1.5.2 Traefik and this reverted the behavior to that like 1.4.1 but to ensure the issue this was added for is fixed as well there would probably need to be smarter logic to group all apps together by backends defined (if any).

[traefiker]

Closed by #3109.