ReplicaSets

A ReplicaSetReplicaSetKeeps a stated number of identical pods alive, and nothing else. keeps a number of pods alive. That’s the whole object. There’s no second feature hiding behind it.

kind: ReplicaSet
metadata:
  name: web-6c8f4b
spec:
  replicas: 3
  template:
    spec:
      containers:
        - name: web
          image: shop/web:2.8

A number, and a pod templatePod templateThe description of a pod held inside another object, rather than a pod of its own., which is just a pod without a name, since it’s about to be three of them.

Its controller counts the pods that belong to it, compares that against replicas, and creates or deletes one to close the gap. Two pods when it wants three, so it makes one. Four when it wants three, so it just deletes one. Then it goes back to sleep.

You’ve done this job already, by hand, for about ten minutes in lesson 2.

It counts, it doesn’t track

Here’s the part that really trips people up. The controller just counts how many pods exist and compares that to a number. Which three they are never comes into it.

So the pods are interchangeable, and they’re interchangeable all the way down. Delete one and you get a fourth pod, with a new name and a new address. “Put that one back” isn’t an operation that exists anywhere. There’s nothing to put back. There’s a count that’s one short and a controller that fixes counts.

Which pods are its own, then? Each one it creates gets an owner referenceOwner referenceA note on an object saying which object it was created for. pointing at the set, and the count is a count of pods carrying that.

The thing it won’t do

So nobody writes one

You can apply the YAML at the top of this lesson and it’ll work just fine. Then the first time you ship a new version you’ll be deleting pods one at a time, watching the count, hoping nothing busy happens while you’re down to one.

That’s just the object being exactly one thing. Keeping a count steady is all a ReplicaSet is for, and something that kept the count steady and made decisions about versions would be two objects welded together.

So there are two objects, and this is the lower one. The other sits a layer up, and all it does is make these and decide what their counts should be.