[stable/redis] Correct master host env value

[WyriHaximus]

What this PR does / why we need it:

The REDIS_MASTER_HOST env variable currently passed to slaves currently points to a non-existing service namely redis-master-0.redis-headless.default.svc.local. Which is a mix of pod names and services that is never going to work. So this PR changes that to redis-master.default.svc.local targeting the redis master service.

Which issue this PR fixes

• fixes [stable/redis] Slave fails to connect to master with connection timeout #17245 (possibly, for sure the latest comment, but there is a lot going on in that issue)

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• DCO signed
• Chart Version bumped
• Title of the PR starts with chart name (e.g. [stable/mychartname])

[k8s-ci-robot]

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[k8s-ci-robot]

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[WyriHaximus]

/assign @desaintmartin

[juan131]

Hi @WyriHaximus

That's a resolvable hostname since we're using a "headless" service to add those entries in the DNS configuration. In other words, it creates a hostname that can be used to target a specific pod.

Here you have an example:

$ helm install stable/redis --name redis
$ kubectl get svc
NAME             TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)    AGE
...
redis-headless   ClusterIP   None            <none>        6379/TCP   3m36s
redis-master     ClusterIP   10.96.143.224   <none>        6379/TCP   3m36s
redis-slave      ClusterIP   10.96.113.236   <none>        6379/TCP   3m36s
$ kg endpoints
NAME             ENDPOINTS                                         AGE
...
redis-headless   172.17.0.5:6379,172.17.0.6:6379,172.17.0.7:6379   3m41s
redis-master     172.17.0.5:6379                                   3m41s
redis-slave      172.17.0.6:6379,172.17.0.7:6379                   3m41s

As you can see there are 3 services (one for master pods, one for slave pods, and there's a headless service).

We can check that we can resolve every hostname:

$ kubectl run --restart=Never -it busybox --image=busybox -- sh
/ # nslookup redis-master.default.svc.cluster.local
Server:		10.96.0.10
Address:	10.96.0.10:53

Name:	redis-master.default.svc.cluster.local
Address: 10.96.143.224
...
/ # nslookup redis-master-0.redis-headless.default.svc.cluster.local
Server:		10.96.0.10
Address:	10.96.0.10:53

Name:	redis-master-0.redis-headless.default.svc.cluster.local
Address: 172.17.0.5

As you can see, redis-master.default.svc.cluster.local resolves to the master service IP: 10.96.143.224; while redis-master-0.redis-headless.default.svc.cluster.local resolves to the master pod IP: 172.17.0.5

[juan131]

/hold

[WyriHaximus]

Hey @juan131,

Thank you for your detailed response. When I started doing test runs on how to set up my personal kubernetes cluster this was one of the things I ran into using minikube. At the time installing it with helm install redis stable/redis --values https://raw.githubusercontent.com/helm/charts/master/stable/redis/values-production.yaml would yield the first error of this comment: #17245 (comment)

Fast forward to yesterday I did some light reading on headless services and the idea excites me. But I assumed you could only do headless.NAMESPACE.svc.cluster.local not POD.headless.NAMESPACE.svc.cluster.local. Hence this PR.

Now today after you commented I started doing some experiments. That helm install works on minikube kudo's for that 👍 . But it doesn't on digitalocean's hosted kubernetes, which I'm using for my cluster. Did a test on a fresh cluster to be sure. But the nslookup redis-master-0.redis-headless.default.svc.cluster.local inside busybox doesn't return anything. So I'm going to look into why it's failing there specifically.

[WyriHaximus]

Did some more digging, it's for sure not the issue I thought that had to be fixed in this PR. The IP resolves but it can't connect for some reason. Will keep digging on why this is happening