Load Balancing

In a Kubernetes cluster, depending on the flow of traffic direction, there are two kinds of load balancing:

  • Load balancing for the traffic within a Kubernetes cluster
  • Load balancing for the traffic coming from outside the cluster

Load balancing for internal traffic

Load balancing within a Kubernetes cluster is accessed through a ClusterIP service type. ClusterIP presents a single IP address to the client and load balances the traffic going to this IP to the backend servers. The actual load balancing happens using iptables or IPVS configuration. The kube-proxy Kubernetes component programs these. The iptables mode of operation uses DNAT rules to distribute direct trafik to real servers, whereas IPVS leverages in-kernel transport-layer load-balancing. A comparision between these two methods can be found here. By default, kube-proxy runs in iptables mode. The kube-proxy configuration can be altered by updating the kube-proxy configmap in the kube-system namespace.

Load balancing for external traffic

External traffic destined for the Kubernetes service requires a service of type LoadBalancer, through which external clients connect to your internal service. Under the hood, it uses a load balancer provided by the underlying infrastructre to direct the traffic.

In the Cloud

In cloud deployments, the load balancer is provided by the cloud provider. For example, in AWS, the service controller communicates with the AWS API to provision an ELB service which targets the cluster nodes.

On-premises

For an on-premises deployment, Kommander ships with MetalLB, which provides load-balancing services.

NOTE: Making a configuration change in the ConfigMap for the metallb application might not result in the config change applying. This is intentional behavior. MetalLB refuses to adopt changes to the ConfigMap that breaks existing Services. You can force MetalLB to load those changes by deleting the metallb controller pod:

kubectl -n kommander delete pod -l app=metallb,component=controller

To use MetalLB:

  1. Identify and reserve a virtual IP (VIP) address range in your networking infrastructure.

  2. Configure your networking infrastructure so that the reserved IP addresses is reachable:

    • from all hosts specified in the inventory file.
    • from the computer used to deploy Kubernetes.

NOTE: Make sure the MetalLB subnet does not overlap with podSubnet and serviceSubnet.

Your configuration is complete if the reserved virtual IP addresses are in the same subnet as the rest of the cluster nodes. If it is in a different subnet you may need to configure appropriate routes to ensure connectivity with the virtual IP address. If the virtual IP addresses share an interface with the primary IP address of the interface, you must disable any IP or MAC spoofing from the infrastructure firewall.

MetalLB can be configured in two modes: Layer2 and BGP.

The following example illustrates how to enable MetalLB and configure it with the Layer2 mode:

---
apiVersion: v1
kind: ConfigMap
metadata:
  name: metallb-overrides
data:
  values.yaml: |
    configInline:
      address-pools:
      - name: default
        protocol: layer2
        addresses:
        - 10.0.50.25-10.0.50.50
---
apiVersion: apps.kommander.d2iq.io/v1alpha2
kind: AppDeployment
metadata:
  name: metallb
spec:
  appRef:
    name: metallb-0.12.2
    kind: ClusterApp
  configOverrides:
    name: metallb-overrides

The number of virtual IP addresses in the reserved range determines the maximum number of LoadBalancer service types you can create in the cluster.

MetalLB in bgp mode implements only a subset of the BGP protocol. In particular, it only advertises the virtual IP to peer BGP agent.

The following example illustrates the BGP configuration in the overrides ConfigMap:

apiVersion: v1
kind: ConfigMap
metadata:
  name: metallb-overrides
data:
  values.yaml: |
    configInline:
      peers:
      - my-asn: 64500
        peer-asn: 64500
        peer-address: 172.17.0.4
      address-pools:
      - name: my-ip-space
        protocol: bgp
        addresses:
        - 172.40.100.0/24

In the above configuration, peers defines the configuration of the BGP peer, such as peer ip address and autonomous system number (asn). The address-pools section is similar to layer2, except for the protocol.

MetalLB also supports advanced BGP configuration, which can be found here.