Allow sidecars to specify probe port

[emaildanwilson]

Currently the API for container livenessProbe and readinessProbe doesn't allow port to be specified since containerPort is already provided. In a multi-container use case, containerPort can only be specified on one container which provides a clear way to identify the container and port which receives traffic. As a side effect, it's currently not possible to configure a sidecar container with liveness/readiness probes using either httpGet or tcpSocket.

A non-ideal workaround for readinessProbe exists to design the primary container's health check code to also check the health of any dependent sidecars. This isn't a typical consideration of the service developer and would not work in many cases for third party containers.

Since k8s restarts the specific container that fails a livenessProbe and NOT the entire pod, this same workaround is not effective for livenessProbe and instead would result in unnecessary restarts of the primary container.

[emaildanwilson]

I can pick this up and work on a PR. before I do though...

Is there agreement that:

1. This should be fixed
2. The approach is to update the field mask to allow livenessProbe and readinessProbe to be defined on sidecar containers only.

cc @markusthoemmes

[mattmoor]

The workaround of proxying through the single exposed container doesn't work for exec probes, which are the defacto way of health checking GRPC services.

The point around the liveness probe restarting the container vs. the pod is another great point / problem with this suggested workaround.

My personal inclination would be to see us allow {liveness,readiness}Probe on sidecars.

cc @dprotaso @markusthoemmes @savitaashture
cc @dgerd (for historical purposes, and because we miss you 😉 )

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[markusthoemmes]

Ack, allowing the probes seems okay for sidecar containers.

[mattmoor]

/lifecycle frozen

[evankanderson]

/area
/kind api-change

/good-first-issue
Since it sounds like there's agreement on the general API shape. If not, remove /remove good-first-issue.

/triage acecpted

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Details

In response to this:

/area
/kind api-change

/good-first-issue
Since it sounds like there's agreement on the general API shape. If not, remove /remove good-first-issue.

/triage acecpted

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[knative-prow-robot]

@evankanderson: The label(s) area/api-change, triage/acecpted cannot be applied, because the repository doesn't have them.

Details

In response to this:

/area
/kind api-change

/good-first-issue
Since it sounds like there's agreement on the general API shape. If not, remove /remove good-first-issue.

/triage acecpted

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[evankanderson]

/kind api-change
/triage accepted