(Part 3)Fix diffs in compute forwarding rule: remove networkTier field from global forwarding rule

[gemmahou]

Change description

Based on #2723. To review the change, check the last commit in this PR.

TF does not support networkTier field for global forwarding rule, It will always use GCP's default value "PREMIUM". Any value set by the user will be ignored and not sent to GCP.

Not sure if that's an ideal behavior, but let's try to match TF controller at this point.

Tests you have done

• Run make ready-pr to ensure this PR is ready for review.
• Perform necessary E2E testing for changed resources.

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[yuwenma]

I don't quite get this (but I like this comment). Could you describe what the Compute GCP service does for this field? I guess I'm confused because from your description it looks like TF sets the default "PREMIUM", in which case this code shall use "PREMIUM" as well to keep the behavior, but it sets 'nil'? Or, is the "PREMIUM" set by the GCP service if nil is passed in? If it is the latter, does the GCP return any errors/warnings?

Regarding the previous TF behavior, I guess it could relate to the spec "auto-correct" in state-into-spec:merge mode.

[gemmahou]

The latter one.
TF for global forwarding rule does not have "networkTier" field(see their documentation), that means whatever value provided by user, will be ignored in the request. If the field is unspecified, GCP will default it to "PREMIUM", no errors/warnings. I hope below example can explain that clearly:

Configuration yaml:

metadata:
 name: globalforwardingrule
spec:
 networkTier: "anything"
 (other fields)

request body with TF controller:

POST ...globalforwardingrule...
{
(other fields)
}

200 ok
{
operationType: insert,
progress: 0,
status: RUNNING,
}

response body with TF controller:

GET ...gloablforwardingrule...
200 ok
{
networkTier: "PREMIUM"
(other fields)
}

Quote the field description: For regional ForwardingRule, the valid values are 'PREMIUM' and 'STANDARD'. For GlobalForwardingRule, the valid value is 'PREMIUM'. If this field is not specified, it is assumed to be 'PREMIUM'. I think that explains why TF does not support this field for global forwarding rule and uses GCP default value.

[yuwenma]

Thanks for the explanation. I think

1. we want to include the networkTier field in status.observedState.
2. I think we don't need this line forwardingRule.NetworkTier = nil, with the previous TF-based state-into-spec: merge default, this guarantees backward compatibility for existing forwarding rules.

[gemmahou]

As discussed, we do not want to drop "networkTier" field in direct controller. The diff is acceptable.