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Composable SDK

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Kubernetes object specifications often require constant values for their fields. When deploying an entire application with many different resources, this limitation often results in the need for staged deployments, because some resources have to be deployed first in order to determine what data to provide for the specifications of dependent resources. This undermines the declarative nature of Kubernetes object specification and requires workflows, manual step-by-step instructions and/or brittle automated scripts for the deployment of applications as a whole.

The Composable SDK can be used to add cross-resource references to any existing CRD, so that values no longer need to be hardwired. This feature allows dynamic configuration of a resource, meaning that its fields can be resolved after it has been deployed.

See this tutorial, in which we add cross-references to the memcached-operator (provided as a sample for operator-sdk) using the Composable SDK.

Installation

To install, run:

go get github.com/composable-operator/composable/sdk

Types

The Composable SDK offers the following types to be used in a CRD definition:

type ObjectRef struct {
	GetValueFrom ComposableGetValueFrom `json:"getValueFrom"`
}

type ComposableGetValueFrom struct {
	Kind               string   `json:"kind"`
	APIVersion         string   `json:"apiVersion,omitempty"`
	Name               string   `json:"name,omitempty"`
	Labels             []string `json:"labels,omitempty"`
	Namespace          string   `json:"namespace,omitempty"`
	Path               string   `json:"path"`
	FormatTransformers []string `json:"format-transformers,omitempty"`
}

An ObjectRef can be used to specify the type of any field of a CRD definition, allowing the value to be determined dynamically. For a detailed explanation of how to specify an object reference according to this schema, see here.

The Composable SDK offers the following types to be used as part of a Reconciler in a controller:

type ResolveObject interface {
	ResolveObject(ctx context.Context, in, out interface{}) error
}

type KubernetesResourceResolver struct {
	Client          client.Client
	ResourcesClient discovery.ServerResourcesInterface
}

The interface ResolveObject provides a function to resolve object references (see below). The struct KubernetesResourceResolver implements it and can be used as part of a Reconciler struct in a CRD controller (see tutorial). It requires a Client and a ServerResourceInterface used to query Kubernetes about existing resources.

A ServerResourceInterface can be instantiated as follows:

discovery.NewDiscoveryClientForConfigOrDie(cfg)

where discovery is the package k8s.io/client-go/discovery, and cfg is a rest.Config.

Functions

The Composable SDK offers the following function for resolving the value of cross-resource references.

func (k KubernetesResourceResolver) ResolveObject(ctx context.Context, object, resolved interface{}) error {

The function ResolveObject takes a context, an object to resolve, and a blank object resolved that will contain the result of resolving cross-resource references. It assumes that the input object has a namespace, which is then used as the default namespace when references do not specify one. This function will cast the result to the type of the resolved object, provided that appropriate data transforms have been included in the reference definitions (see tutorial for an example).

The ResolveObject function uses caching for looking up objects in order to ensure that a consistent view of the data is obtained. If any data is not available at the time of the lookup, it returns an error. So this function either resolves the entire object or it doesn't -- there are no partial results.

The return value of ResolveObject is an error and the Composable SDK offers a series of functions to determine the nature of the error. This is used to decide whether the error needs to be returned by the Reconcile function or not.

func IsIllFormedRef(err error) bool 

func IsKindNotFound(err error) bool 

func IsObjectNotFound(err error) bool 

func IsValueNotFound(err error) bool 

func IsRefNotFound(err error) bool

Function IsIllFormedRef indicates that that a cross-resource reference is ill-formed (in which case retrying reconciliation would probably not help). Function IsKindNotFound indicates that the kind of the reference does not exist. IsObjectNotFound indicates that the object itself does not exist, and IsValueNotFound that the value within the object does not exist. Finally, IsRefNotFound is true if either IsKindNotFound, IsObjectNotFound, or IsValueNotFound are true.