LoadBalancer
In the last section, we managed to expose our podinfo service to the outside world by using a NodePort service. However, this approach has a few drawbacks:
- We need to know the IP address of one of the nodes in the cluster to access the service.
- We need to know the port on which the service is exposed on the node.
This makes working with NodePorts cumbersome for most use cases - if a node is in maintenance, clients need to look up the IP address of another node. If we want to expose multiple services, we need to keep track of the ports on which they are exposed on the nodes.
This is why LoadBalancer services exist. They are a special type of service that is exposed externally via a cloud provider's load balancer. This load balancer will route traffic to the nodes in the cluster, which in turn will route the traffic to the service.
Provisioning and configuration of said external load balancer is done by the cloud controller manager upon creation of the service.
The figure above shows the traffic flow when accessing a LoadBalancer service. The traffic is routed through the external load balancer to one of the nodes in the cluster. The node then routes the traffic to the service, and the service routes the traffic to one of the pods in the service.
Summing up, LoadBalancer extend the capabilities of NodePort services by providing a single point of entry to the cluster - the traffic flow stays the same, compared to a NodePort service. This makes them ideal for exposing services to the outside world.
Where's the lab?
As mentioned above, Kubernetes relies on the hosting cloud and the cloud controller manager to provision and configure the external load balancer. This means that we can't test LoadBalancer services in our lab environment without additional configuration that is beyond the scope of this course.
If you're interested in this topic, I recommend looking at the MetalLB project. It provides a software load balancer implementation that can be used in lab environments or on bare metal clusters.
Another option as of late is the Cilium CNI which supports the same mechanisms as MetalLB to provide load balancing functionality.
Created: September 18, 2023