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Loadbalancing Services

TL;DR

  1. I have multiple nodes (you'd benefit from MetalLB)
  2. I only need/want one node (just go with k3s svclb)

But why?

In Kubernetes, you don't access your containers / pods "directly", other than for debugging purposes. Rather, we have a construct called a "service", which is "in front of" one or more pods.

Consider that this is how containers talk to each other under Docker Swarm:

sequenceDiagram
    wordpress->>+mysql: Are you there?
    mysql->>+wordpress: Yes, ready to serve!

But this is how containers (pods) talk to each other under Kubernetes:

sequenceDiagram
    wordpress->>+mysql-service: Are you there?
    mysql-service->>+mysql-pods: Are you there?
    mysql-pods->>+wordpress: Yes, ready to serve!

Why do we do this?

  1. A service isn't pinned to a particular node, it's a virtual IP which lives in the cluster and doesn't change as pods/nodes come and go.
  2. Using a service "in front of" pods means that rolling updates / scaling of the pods can take place, but communication with the service is uninterrupted (assuming correct configuration).

Here's some more technical detail into how it works, but what you need to know is that when you want to interact with your containers in Kubernetes (either from other containers or from outside, as a human), you'll be talking to services.

Also, services are not exposed outside of the cluster by default. There are 3 levels of "exposure" for your Kubernetes services, briefly:

  1. ClusterIP (A service is only available to other services in the cluster - this is the default)
  2. NodePort (A mostly-random high-port on the node running the pod is forwarded to the pod)1
  3. LoadBalancer (Some external help is required to forward a particular IP into the cluster, terminating on the node running your pod)

For anything vaguely useful, only LoadBalancer is a viable option. Even though NodePort may allow you to access services directly, who wants to remember that they need to access Radarr on 192.168.1.44:34542 and Homer on 192.168.1.44:34532? Ugh.

Assuming you only had a single Kubernetes node (say, a small k3s deployment), you'd want 100% of all incoming traffic to be directed to that node, and so you wouldn't need a loadbalancer. You'd just point some DNS entries / firewall NATs at the IP of the cluster, and be done.

(This is the way k3s works by default, although it's still called a LoadBalancer)

Chef's notes 📓


  1. It is possible to be prescriptive about which port is used for a Nodeport-exposed service, and this is occasionally a valid deployment strategy, but you're usually limited to ports between 30000 and 32768. 

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    Last update: November 22, 2021
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