Annotations strategy

[linki]

We'll support several annotations from the beginning, some of them are supposed to be deprecated soon, others will be added over time as new requirements come up.

@justinsb throw in an idea where we store JSON as the value of an annotation which can then be used to encode more complicated data in order to support more advanced but less common use cases.

We could design our annotations around a single annotation holding JSON. This would be the source of truth for our controller. Keys can be added over time to support new features (CNAME, TTL, provider specific stuff). Though, normally one would use the simpler annotations using plain text values (e.g. external-dns/hostname: foo) but internally external-dns would convert the simpler annotations to the source-of-truth-struct before processing.

This way, most users could just use the simple annotations that are exposed but more adavanced users could directly set the JSON based annotation to have access to everything that external-dns provides.

This can also help with solving backwards-incopatible changes and with rolling out new versions. New features could be made available solely via some keys in the JSON before being promoted to top-level annotations if they prove to be useful.

@justinsb @ideahitme @iterion Any thoughs?

[ideahitme]

I like the idea and totally makes sense to me. Plus making it easier to manage the logic :)

Additionally, we might think of storing the serialized json in the resource itself, similarly to what kubernetes does with kubernetes.io/created-by:

"kubernetes.io/created-by": "{\"kind\":\"SerializedReference\",\"apiVersion\":\"v1\",\"reference\":{\"kind\":\"ReplicaSet\",\"namespace\":\"kube-system\",\"name\":\"mate-617493630\",\"uid\":\"...\",\"apiVersion\":\"extensions\",\"resourceVersion\":\"72660398\"}}\n",

If we store the last updated version of the referring to the resourceVersion, this could greatly simplify the procedure. For example:

"external-dns.kubernetes.io/config": "{\"hostname\":\"foo.com\", \"resourceVersion\":\"123\"}"
"kubernetes.io/created-by": "{..., \"resourceVersion\":\"124\"}" 

Since 124 != 123 - trigger update

[iterion]

This would also be a good idea for when we implement TPRs (#21). We would already have the format we require set up and implementation should be really easy.

[linki]

I was wondering if we should rather use alpha annotations while external-dns is still unstable?

Looking at other projects I think it makes to target alpha.external-dns.kubernetes.io for now so that we allow us to change them at any time if we discover they aren't suitable. I know that we discussed the desired annotations for quite some time but you never know if they really works out.

[linki]

I moved the discussion about using alpha annotations to #42

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