Knative Eventing 中 Channel 如何注入默认 Provisioner

场景

通常的在创建Broker时，我们需要通过 spec.ChannelTemplate 指定使用某个具体的 Channel Provisioner。例如这样的Broker:

apiVersion: eventing.knative.dev/v1alpha1
kind: Broker
metadata:name: pubsub-channel
spec:channelTemplate:provisioner:apiVersion: eventing.knative.dev/v1alpha1kind: ClusterChannelProvisionername: gcp-pubsub

这里通过spec.ChannelTemplate 指定了名称为gcp-pubsub的provisioner。那么我们也遇到过这样的Broker：

apiVersion: eventing.knative.dev/v1alpha1
kind: Broker
metadata:name: default

并没有指定使用某个具体的 channel, 但创建完Broker之后会发现已经创建出来了Channel:

apiVersion: eventing.knative.dev/v1alpha1
kind: Channel
metadata:...name: default-broker-8ml79namespace: defaultownerReferences:- apiVersion: eventing.knative.dev/v1alpha1blockOwnerDeletion: truecontroller: truekind: Brokername: defaultuid: 2e4c3332-6755-11e9-a81f-00163f005e02
spec:provisioner:apiVersion: eventing.knative.dev/v1alpha1kind: ClusterChannelProvisionername: in-memory
...

分析

我们知道 Broker创建之后，会通过 reconcile controller 会创建相应的Channel, 也就是下面这段代码：

// newChannel creates a new Channel for Broker 'b'.
func newChannel(b *v1alpha1.Broker, l map[string]string) *v1alpha1.Channel {var spec v1alpha1.ChannelSpecif b.Spec.ChannelTemplate != nil {spec = *b.Spec.ChannelTemplate}return &v1alpha1.Channel{ObjectMeta: metav1.ObjectMeta{Namespace:    b.Namespace,GenerateName: fmt.Sprintf("%s-broker-", b.Name),Labels:       l,OwnerReferences: []metav1.OwnerReference{*metav1.NewControllerRef(b, schema.GroupVersionKind{Group:   v1alpha1.SchemeGroupVersion.Group,Version: v1alpha1.SchemeGroupVersion.Version,Kind:    "Broker",}),},},Spec: spec,}
}

分析上面这段代码，我们可以很清楚得出这样的结论：如果Broker中设置了Spec.ChannelTemplate, 那么Channel中会直接使用ChannelTemplate所对应的provisioner。
但如果没有设置的话，那么Channel中的spec应该设置为nil。但事实上设置了in-memory provisioner，那么这个是在哪里注入的呢？

注入机制

经过定位源代码，我们发现在channel_defaults.go中，发现下面这段代码：

func (c *Channel) SetDefaults(ctx context.Context) {if c != nil && c.Spec.Provisioner == nil {// The singleton may not have been set, if so ignore it and validation will reject the// Channel.if cd := ChannelDefaulterSingleton; cd != nil {prov, args := cd.GetDefault(c.DeepCopy())c.Spec.Provisioner = provc.Spec.Arguments = args}}c.Spec.SetDefaults(ctx)
}

分析一下，我们可以看到当c.Spec.Provisioner==nil时，会设置默认的Provisioner。
进一步分析ChannelDefaulterSingleton, 我们可以在webhook中赋予了实现设置：

...
// Watch the default-channel-webhook ConfigMap and dynamically update the default
// ClusterChannelProvisioner.
channelDefaulter := channeldefaulter.New(logger.Desugar())
eventingv1alpha1.ChannelDefaulterSingleton = channelDefaulter
configMapWatcher.Watch(channeldefaulter.ConfigMapName, channelDefaulter.UpdateConfigMap)
...

接着分析发现 ChannelDefaulter 实现了 GetDefault 方法：

// GetDefault determines the default provisioner and arguments for the provided channel.
func (cd *ChannelDefaulter) GetDefault(c *eventingv1alpha1.Channel) (*corev1.ObjectReference, *runtime.RawExtension) {// Because we are treating this as a singleton, be tolerant to it having not been setup at all.if cd == nil {return nil, nil}if c == nil {return nil, nil}config := cd.getConfig()if config == nil {return nil, nil}// TODO Don't use a single default, instead use the Channel's arguments to determine the type of// Channel to use (e.g. it can say whether it needs to be persistent, strictly ordered, etc.).dp := getDefaultProvisioner(config, c.Namespace)cd.logger.Info("Defaulting the ClusterChannelProvisioner", zap.Any("defaultClusterChannelProvisioner", dp))return dp, nil
}

并且这里是通过一个ConfigMap设置使用的默认provisioner, 这个ConfigMap名称为default-channel-webhook, 没错可以在 Knative Eventing 安装文件中发现这个资源：

apiVersion: v1
data:default-channel-config: |clusterdefault:apiversion: eventing.knative.dev/v1alpha1kind: ClusterChannelProvisionername: in-memorynamespacedefaults:some-namespace:apiversion: eventing.knative.dev/v1alpha1kind: ClusterChannelProvisionername: some-other-provisioner
kind: ConfigMap
metadata:name: default-channel-webhooknamespace: knative-eventing

那么分析到此，我们梳理一下整个注入的流程：