Add external-dns(v0.14.0) as a dependency

[mikenairn]

Adds external-dns (latest released version v0.14.0) as a go module dependency and updates the dnsrecord controller and dns provider logic to align with an external-dns approach to dns management.

Updates

dnsrecord changes:

• Replace v1alpha1.Endpoint with externaldns Endpoint
• Add GetRootDomain method to DNSRecord, returns the shortest domain that is shared across all spec.Endpoints dns names.

provider changes:

• Remove Ensure and Delete and instead include externaldnsprovider.Provider in provider.Provider interface.
• Add provider.Config struct to hold common provider configuration, currently it only handles filters. Update provider Factory constructors accept the config as input.
• Set DomainFilter, ZoneTypeFilter and ZoneIDFilter in provider implementations from provider.Config.

AWS provider changes:

• Implement AdjustEndpoints to change our generic provider specific fields for weight and geo into aws ones understood by external-dns.

Google provider changes:

• Copy external-dns google provider
• Adds implementations of funtions required to translate endpoints and resourceRecordSets into the different states.
  • endpointsToProviderFormat - converts a list of endpoints into a google specific format.
  • endpointsFromResourceRecordSets - converts a list of ResourceRecordSet into endpoints (google format).
  • resourceRecordSetFromEndpoint - converts an endpoint(google format) into a ResourceRecordSet.

controller changes:

• Update DNSRecord controller to use applyChanges which follows an external-dns process to determine a set of changes(Create/Update/Delete) that are required to be made against the provider.
• Use noop registry, no owner is currently set or checked for any records.
• Setup provider filters:
  • DomainFilter = root domain for endpoints determined by calling dnsRecord.GetRootDomain()
  • ZoneTypeFilter = "", public or private zones considered.
  • ZoneIDFilter = set to single hosted zone id (managedZone.Status.ID)
• Use sync policy, all changes will be applied (Create, Update and Deletes)
• Use plan with above and using the current zone(plan.Current) and spec endpoints(plan.Desired) as input.

additional changes:

downgrade gatewayapiv1 to v0.7.1. This is required since the external dns aws provider is including the external dns source package which has a dependency on Gateway API v0.7.1. Downgrading is less than ideal, but dns-operator shouldn't have a dependency on gateway api, and the health check code that is currently causing it will likely be removed soon anyway in favour of provider health checks. There is also an opportunity to change external-dns to prevent it including the source package since it's including here for no reason other than getting a version
https://github.com/kubernetes-sigs/external-dns/blob/master/provider/aws/session.go#L72

Validation

As part of the POC several example dnsrecords were created to run through the various different scenarios on Google and AWS. These were all specific to my own accounts so are not added here. I did add them to the poc branch though so can be used, with modification, for testing here.

A test script is also available in this branch to step through each of the scenarios manually.

A test suite based on these will be created in the very near future,

Note: The tests for "multicluster with no control plane" are not expected to work currently

closes #47

[mikenairn]

See description for the reason why this is downgraded

[mikenairn]

We might want to replace this by adding a new rootDomain field to the dnsrecord spec, but this will work for now.

[philbrookes]

Could you put the samples you showed us in the demo somewhere, so I can try this out locally?

[mikenairn]

Could you put the samples you showed us in the demo somewhere, so I can try this out locally?

Added a "Validation" section to the end of the description. Has links to the example DNSRecords i was using.

[philbrookes]

/lgtm