Placement constraints

Placement constraints allow you to customize where a service is deployed in the DC/OS cluster. Placement constraints use the Marathon operators syntax. For example, [["hostname", "UNIQUE"]] ensures that at most one pod instance is deployed per agent.

A common task is to specify a list of whitelisted systems to deploy to. To achieve this, use the following syntax for the placement constraint:

"placement_constraints": "hostname:LIKE:192.168.2.193",

IMPORTANT: Be sure to include excess capacity in such a scenario so that if one of the whitelisted systems goes down, there is still enough capacity to repair your service.

Updating placement constraints

Clusters change, and so will your placement constraints. However, already-running service pods will not be affected by changes in placement constraints. This is because altering a placement constraint might invalidate the current placement of a running pod, and the pod will not be relocated automatically, as doing so is a destructive action. We recommend using the following procedure to update the placement constraints of a pod:

1. Update the placement constraint definition in the service.
2. For each affected pod, perform a pod replace procedure, one at a time. This will (destructively) move the pod to be in accordance with the new placement constraints.

Prometheus server placement constraints

To control where the Prometheus server is placed, please specify the placement constraints, as in the following:

{
  "prometheus": {
    "placement_constraints": "[[\"hostname\", \"IS\", \"10.0.0.73\"]]"
  }
}

Grafana server placement constraints

To control where the Grafana server is placed, please specify the placement constraints, as in the following:

{
  "grafana": {
    "placement_constraints": "[[\"hostname\", \"IS\", \"10.0.0.73\"]]"
  }
}

Scheduler placement constraints

To control where the service’s scheduler is placed, please specify the placement constraints, as in the following. Note that the option type is an array.

{
  "service": {
    "placement_constraints": [["hostname", "IS", "10.0.0.73"]]
  }
}