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custom resources: explain declarative API (#8543)

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Currently, the link just points to a description of
what Kubernetes objects are and does not
explicitly state what a declarative API is. It is
not immediately clear to the reader what the term
means.

This commit tries to explain it explicitly so that
the link adds to the meaning and it is clearer to
the reader.
This commit is contained in:
Nikhita Raghunath
2018-05-16 11:06:30 +05:30
committed by k8s-ci-robot
parent 6b1ac1285b
commit de9f222993
1 changed files with 6 additions and 1 deletions
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content/en/docs/concepts/api-extension/custom-resources.md
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@@ -28,7 +28,12 @@ Once a custom resource is installed, users can create and access its objects wit
### Custom controllers
On their own, custom resources simply let you store and retrieve structured data.
It is only when combined with a *controller* that they become a true [declarative API](/docs/concepts/overview/working-with-objects/kubernetes-objects/#understanding-kubernetes-objects). The controller interprets the structured data as a record of the user's desired state, and continually takes action to achieve and maintain that state.
It is only when combined with a *controller* that they become a true declarative API.
A [declarative API](/docs/concepts/overview/working-with-objects/kubernetes-objects/#understanding-kubernetes-objects)
allows you to _declare_ or specify the desired state of your resource and tries
to match the actual state to this desired state.
Here, the controller interprets the structured data as a record of the user's
desired state, and continually takes action to achieve and maintain this state.
A *custom controller* is a controller that users can deploy and update on a running cluster, independently of the cluster's own lifecycle. Custom controllers can work with any kind of resource, but they are especially effective when combined with custom resources. The [Operator](https://coreos.com/blog/introducing-operators.html) pattern is one example of such a combination. It allows developers to encode domain knowledge for specific applications into an extension of the Kubernetes API.