kube-arangodb/docs/design/constraints.md

layout	title	parent
page	Constraints	Design overview

Constraints

The ArangoDB operator tries to honor various constraints to support high availability of the ArangoDB cluster.

Run agents and dbservers on separate machines

It is essential for resilience and high availability that no two agents are running on the same node and no two dbservers are running running on the same node.

To ensure this, the agent and dbserver Pods are configured with pod-anti-affinity.

kind: Pod
spec:
  affinity:
    podAntiAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
      - weight: 100
        podAffinityTerm:
          labelSelector:
            matchExpressions:
            - key: app
              operator: In
              values:
              - arangodb
            - key: arangodb_cluster_name
              operator: In
              values:
              - <cluster-name>
            - key: role
              operator: In
              values:
              - agent (or dbserver)

The settings used for pod affinity are based on the spec.environment setting.

For a development environment we use preferredDuringSchedulingIgnoredDuringExecution so deployments are still possible on tiny clusters like minikube.

For production environments we enforce (anti-)affinity using requiredDuringSchedulingIgnoredDuringExecution.

Run coordinators on separate machines

It is preferred to run coordinators of separate machines.

To achieve this, the coordinator Pods are configured with pod-anti-affinity using the preferredDuringSchedulingIgnoredDuringExecution setting.

Run syncworkers on same machine as dbserver

It is preferred to run syncworkers on the same machine as dbservers.

To achieve this, the syncworker Pods are configured with pod-affinity using the preferredDuringSchedulingIgnoredDuringExecution setting.