Kubernetes: Services and Ingress

Why a pod has no stable address, the Service and its three types, selector and endpoints, port and targetPort, a Service's DNS name and the Ingress that routes HTTP traffic by host and path.

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Service: what is it?
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An object that gives a group of pods a stable address and DNS name, and spreads traffic across those its selector retains.
Pods are born and die with new addresses; the Service stays, and clients always aim at the same name.
A client pod calls http://web. On which container port does the traffic arrive?

```yaml
apiVersion: v1
kind: Service
metadata:
name: web
spec:
selector:
app: web
ports:
- port: 80
targetPort: 8080
`
- 8080
- 80
- 30080
- 443
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8080
The URL without a port aims at the Service's port, 80. The Service then redirects to the pods' targetPort: 8080. There is no nodePort here, so nothing on 30080.
A Service named db in the namespace data has the full DNS name …; a pod in the same namespace reaches it simply as db.
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A Service named db in the namespace data has the full DNS name db.data.svc.cluster.local; a pod in the same namespace reaches it simply as db.
The cluster DNS completes the name with the calling pod's namespace; from another namespace, write db.data.
Endpoints: what is it?
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The list of addresses of the ready pods a Service selects; it is kept up to date automatically.
If this list is empty (<none>), the Service has nobody to send traffic to: wrong selector, or no ready pod.

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