关于 Broker/Trigger 事件模型-阿里云开发者社区

本文选自《Knative 云原生应用开发指南》。

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Broker 和 Trigger

从 v0.5 开始，Knative Eventing 定义 Broker 和 Trigger 对象，从而能方便的对事件进行过滤（亦如通过 ingress 和 ingress controller 对网络流量的过滤一样）。

Broker 提供一个事件集，可以通过属性选择该事件集。它负责接收事件并将其转发给由一个或多个匹配 Trigger 定义的订阅者。
Trigger 描述基于事件属性的过滤器。同时可以根据需要创建多个 Trigger。

如图：

当前实现方式

Namespace

通过Namespace Reconciler (代码：eventing/pkg/reconciler/v1alpha1/namespace/namespace.go)创建 broker。Namespace Reconciler 会查询所有带knative-eventing-injection: enabled 标签的 namespace。如果存在这样标签的 namespace, 那么Namespace Reconciler将会进行如下处理操作：

创建 Broker 过滤器的 ServiceAccount：eventing-broker-filter
通过 RoleBinding 确保 ServiceAccount 的 RBAC 权限
创建名称为 default 的 Broker

// newBroker creates a placeholder default Broker object for Namespace 'ns'.
func newBroker(ns *corev1.Namespace) *v1alpha1.Broker {
    return &v1alpha1.Broker{
        ObjectMeta: metav1.ObjectMeta{
            Namespace: ns.Name,
            Name:      defaultBroker,
            Labels:    injectedLabels(),
        },
    }
}

Broker (事件代理)

通过Broker Reconciler进行处理 broker，对于每一个 broker, 会进行一下处理操作：

创建 'trigger'Channel。所有在 Broker 中的 event 事件都会发送到这个Channel，所有的 Trigger 会订阅这个Channel。
创建'filter'Deployment。这个 Deployment 会运行cmd/broker/filter。其目的是处理与此 Broker 相关的所有 Trigger 的数据平面。说白了其实就做了两件事情，从 Channel 中接收事件，然后转发给事件的订阅者。
创建'filter' Kubernetes Service。通过该 Service 提供'filter' Deployment的服务访问。
创建'ingress' Deployment。这个 Deployment 会运行 cmd/broker/ingress。其目的是检查进入 Broker的所有事件
创建'ingress' Kubernetes Service。通过该 Service提供'Ingress' Deployment的服务访问。
创建'ingress' Channel。这是一个 Trigger 应答的 Channel。目的是将 Trigger 中返回的事件通过 Ingress Deployment 回写到 Broker。理想情况下，其实不需要这个，可以直接将 Trigger 的响应发送给 k8s Service。但是作为订阅的场景，只允许我们向 Channel 发送响应信息，所以我们需要这个作为中介。
创建'ingress' Subscription。它通过'ingress' Channel来订阅'ingress' Service

代码如下：

func (r *reconciler) reconcile(ctx context.Context, b *v1alpha1.Broker) (reconcile.Result, error) {
    // 1. Trigger Channel is created for all events. Triggers will Subscribe to this Channel.
    // 2. Filter Deployment.
    // 3. Ingress Deployment.
    // 4. K8s Services that point at the Deployments.
    // 5. Ingress Channel is created to get events from Triggers back into this Broker via the
    //    Ingress Deployment.
    //   - Ideally this wouldn't exist and we would point the Trigger's reply directly to the K8s
    //     Service. However, Subscriptions only allow us to send replies to Channels, so we need
    //     this as an intermediary.
    // 6. Subscription from the Ingress Channel to the Ingress Service.

    if b.DeletionTimestamp != nil {
        // Everything is cleaned up by the garbage collector.
        return nil
    }

    if b.Spec.ChannelTemplate == nil {
        r.Logger.Error("Broker.Spec.ChannelTemplate is nil",
            zap.String("namespace", b.Namespace), zap.String("name", b.Name))
        return nil
    }

    gvr, _ := meta.UnsafeGuessKindToResource(b.Spec.ChannelTemplate.GetObjectKind().GroupVersionKind())
    channelResourceInterface := r.DynamicClientSet.Resource(gvr).Namespace(b.Namespace)
    if channelResourceInterface == nil {
        return fmt.Errorf("unable to create dynamic client for: %+v", b.Spec.ChannelTemplate)
    }

    track := r.channelableTracker.TrackInNamespace(b)

    triggerChannelName := resources.BrokerChannelName(b.Name, "trigger")
    triggerChannelObjRef := corev1.ObjectReference{
        Kind:       b.Spec.ChannelTemplate.Kind,
        APIVersion: b.Spec.ChannelTemplate.APIVersion,
        Name:       triggerChannelName,
        Namespace:  b.Namespace,
    }
    // Start tracking the trigger channel.
    if err := track(triggerChannelObjRef); err != nil {
        return fmt.Errorf("unable to track changes to the trigger Channel: %v", err)
    }

    logging.FromContext(ctx).Info("Reconciling the trigger channel")
    triggerChan, err := r.reconcileTriggerChannel(ctx, channelResourceInterface, triggerChannelObjRef, b)
    if err != nil {
        logging.FromContext(ctx).Error("Problem reconciling the trigger channel", zap.Error(err))
        b.Status.MarkTriggerChannelFailed("ChannelFailure", "%v", err)
        return err
    }
   ......

    filterDeployment, err := r.reconcileFilterDeployment(ctx, b)
    if err != nil {
        logging.FromContext(ctx).Error("Problem reconciling filter Deployment", zap.Error(err))
        b.Status.MarkFilterFailed("DeploymentFailure", "%v", err)
        return err
    }
    _, err = r.reconcileFilterService(ctx, b)
    if err != nil {
        logging.FromContext(ctx).Error("Problem reconciling filter Service", zap.Error(err))
        b.Status.MarkFilterFailed("ServiceFailure", "%v", err)
        return err
    }
    b.Status.PropagateFilterDeploymentAvailability(filterDeployment)

    ingressDeployment, err := r.reconcileIngressDeployment(ctx, b, triggerChan)
    if err != nil {
        logging.FromContext(ctx).Error("Problem reconciling ingress Deployment", zap.Error(err))
        b.Status.MarkIngressFailed("DeploymentFailure", "%v", err)
        return err
    }

    svc, err := r.reconcileIngressService(ctx, b)
    if err != nil {
        logging.FromContext(ctx).Error("Problem reconciling ingress Service", zap.Error(err))
        b.Status.MarkIngressFailed("ServiceFailure", "%v", err)
        return err
    }
    b.Status.PropagateIngressDeploymentAvailability(ingressDeployment)
    b.Status.SetAddress(&apis.URL{
        Scheme: "http",
        Host:   names.ServiceHostName(svc.Name, svc.Namespace),
    })

    ingressChannelName := resources.BrokerChannelName(b.Name, "ingress")
    ingressChannelObjRef := corev1.ObjectReference{
        Kind:       b.Spec.ChannelTemplate.Kind,
        APIVersion: b.Spec.ChannelTemplate.APIVersion,
        Name:       ingressChannelName,
        Namespace:  b.Namespace,
    }

    // Start tracking the ingress channel.
    if err = track(ingressChannelObjRef); err != nil {
        return fmt.Errorf("unable to track changes to the ingress Channel: %v", err)
    }

    ingressChan, err := r.reconcileIngressChannel(ctx, channelResourceInterface, ingressChannelObjRef, b)
    if err != nil {
        logging.FromContext(ctx).Error("Problem reconciling the ingress channel", zap.Error(err))
        b.Status.MarkIngressChannelFailed("ChannelFailure", "%v", err)
        return err
    }
    b.Status.IngressChannel = &ingressChannelObjRef
    b.Status.PropagateIngressChannelReadiness(&ingressChan.Status)

    ingressSub, err := r.reconcileIngressSubscription(ctx, b, ingressChan, svc)
    if err != nil {
        logging.FromContext(ctx).Error("Problem reconciling the ingress subscription", zap.Error(err))
        b.Status.MarkIngressSubscriptionFailed("SubscriptionFailure", "%v", err)
        return err
    }
    b.Status.PropagateIngressSubscriptionReadiness(&ingressSub.Status)

    return nil
}

Broker 示例：

apiVersion: eventing.knative.dev/v1alpha1
kind: Broker
metadata:
  name: default
spec:
  channelTemplateSpec:
    apiVersion: messaging.knative.dev/v1alpha1
    kind: InMemoryChannel

Trigger (触发器)

通过Trigger Reconciler进行处理 trigger，对于每一个 trigger, 会进行一下处理操作：

验证 Broker 是否存在
获取对应 Broker 的 Trigger Channel、 Ingress Channel 以及 Filter Service
确定订阅者的 URI
创建一个从 Broker 特定的 Channel 到这个 Trigger 的 kubernetes Service 的订阅。reply 被发送到

Broker 的 ingress Channel。
检查是否包含 knative.dev/dependency 的注释。

代码如下：

func (r *reconciler) reconcile(ctx context.Context, t *v1alpha1.Trigger) error {
    ......
    // 1. Verify the Broker exists.
    // 2. Get the Broker's:
    //   - Trigger Channel
    //   - Ingress Channel
    //   - Filter Service
    // 3. Find the Subscriber's URI.
    // 4. Creates a Subscription from the Broker's Trigger Channel to this Trigger via the Broker's
    //    Filter Service with a specific path, and reply set to the Broker's Ingress Channel.
    // 5. Find whether there is annotation with key "knative.dev/dependency".
    // If not, mark Dependency to be succeeded, else figure out whether the dependency is ready and mark Dependency correspondingly

    if t.DeletionTimestamp != nil {
        // Everything is cleaned up by the garbage collector.
        return nil
    }
    // Tell tracker to reconcile this Trigger whenever the Broker changes.
    brokerObjRef := corev1.ObjectReference{
        Kind:       brokerGVK.Kind,
        APIVersion: brokerGVK.GroupVersion().String(),
        Name:       t.Spec.Broker,
        Namespace:  t.Namespace,
    }

    if err := r.tracker.Track(brokerObjRef, t); err != nil {
        logging.FromContext(ctx).Error("Unable to track changes to Broker", zap.Error(err))
        return err
    }

    b, err := r.brokerLister.Brokers(t.Namespace).Get(t.Spec.Broker)
    if err != nil {
        logging.FromContext(ctx).Error("Unable to get the Broker", zap.Error(err))
        if apierrs.IsNotFound(err) {
            t.Status.MarkBrokerFailed("DoesNotExist", "Broker does not exist")
            _, needDefaultBroker := t.GetAnnotations()[v1alpha1.InjectionAnnotation]
            if t.Spec.Broker == "default" && needDefaultBroker {
                if e := r.labelNamespace(ctx, t); e != nil {
                    logging.FromContext(ctx).Error("Unable to label the namespace", zap.Error(e))
                }
            }
        } else {
            t.Status.MarkBrokerFailed("BrokerGetFailed", "Failed to get broker")
        }
        return err
    }
    t.Status.PropagateBrokerStatus(&b.Status)

    brokerTrigger := b.Status.TriggerChannel
    if brokerTrigger == nil {
        logging.FromContext(ctx).Error("Broker TriggerChannel not populated")
        r.Recorder.Eventf(t, corev1.EventTypeWarning, triggerChannelFailed, "Broker's Trigger channel not found")
        return errors.New("failed to find Broker's Trigger channel")
    }

    brokerIngress := b.Status.IngressChannel
    if brokerIngress == nil {
        logging.FromContext(ctx).Error("Broker IngressChannel not populated")
        r.Recorder.Eventf(t, corev1.EventTypeWarning, ingressChannelFailed, "Broker's Ingress channel not found")
        return errors.New("failed to find Broker's Ingress channel")
    }

    // Get Broker filter service.
    filterSvc, err := r.getBrokerFilterService(ctx, b)
    ......

    subscriberURI, err := r.uriResolver.URIFromDestination(*t.Spec.Subscriber, t)
    if err != nil {
        logging.FromContext(ctx).Error("Unable to get the Subscriber's URI", zap.Error(err))
        return err
    }
    t.Status.SubscriberURI = subscriberURI

    sub, err := r.subscribeToBrokerChannel(ctx, t, brokerTrigger, brokerIngress, filterSvc)
    if err != nil {
        logging.FromContext(ctx).Error("Unable to Subscribe", zap.Error(err))
        t.Status.MarkNotSubscribed("NotSubscribed", "%v", err)
        return err
    }
    t.Status.PropagateSubscriptionStatus(&sub.Status)

    if err := r.checkDependencyAnnotation(ctx, t); err != nil {
        return err
    }

    return nil
}

Trigger 示例：

apiVersion: eventing.knative.dev/v1alpha1
kind: Trigger
metadata:
  name: my-service-trigger
spec:
  broker: default
  filter:
    attributes:
      type: dev.knative.foo.bar
      myextension: my-extension-value
  subscriber:
    ref:
      apiVersion: serving.knative.dev/v1
      kind: Service
      name: my-service

总结

Broker/Trigger 模型出现的意义不仅在于其提供了消息过滤机制，更充分解耦了消息通道的实现，目前除了系统自身支持的基于内存的消息通道 InMemoryChannel 之外，还支持 Kafka、NATS Streaming 等消息服务。
此外结合 CloudEvent 进行事件统一标准传输，无论对于客户端接入事件源，还是消费端提供的消费事件服务，都能极大的提升了应用的跨平台可移植性。

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关于 Broker/Trigger 事件模型

Broker 和 Trigger

当前实现方式

Namespace

Broker (事件代理)

Trigger (触发器)

总结

云原生

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