Knative Eventing

Knative Eventing is a system that is designed to address a common need for cloud native development and provides composable primitives to enable late-binding event sources and event consumers.

Design overview

Knative Eventing is designed around the following goals:

  1. Knative Eventing services are loosely coupled. These services can be developed and deployed independently on, and across a variety of platforms (for example Kubernetes, VMs, SaaS or FaaS).
  2. Event producers and event sources are independent. Any producer (or source), can generate events before there are active event consumers that are listening. Any event consumer can express interest in an event or class of events, before there are producers that are creating those events.
  3. Other services can be connected to the Eventing system. These services can perform the following functions:
    • Create new applications without modifying the event producer or event consumer.
    • Select and target specific subsets of the events from their producers.
  4. Ensure cross-service interoperability. Knative Eventing is consistent with the CloudEvents specification that is developed by the CNCF Serverless WG.

Event consumers

To enable delivery to multiple types of Services, Knative Eventing defines two generic interfaces that can be implemented by multiple Kubernetes resources:

  1. Addressable objects are able to receive and acknowledge an event delivered over HTTP to an address defined in their status.address.hostname field. As a special case, the core Kubernetes Service object also fulfils the Addressable interface.
  2. Callable objects are able to receive an event delivered over HTTP and transform the event, returning 0 or 1 new events in the HTTP response. These returned events may be further processed in the same way that events from an external event source are processed.

Event channels and subscriptions

Knative Eventing also defines a single event forwarding and persistence layer, called a Channel. Messaging implementations may provide implementations of Channels via the ClusterChannelProvisioner object. Events are delivered to Services or forwarded to other channels (possibly of a different type) using Subscriptions. This allows message delivery in a cluster to vary based on requirements, so that some events might be handled by an in-memory implementation while others would be persisted using Apache Kafka or NATS Streaming.

Future design goals

The focus for the next Eventing release will be to enable easy implementation of event sources. Sources manage registration and delivery of events from external systems using Kubernetes Custom Resources. Learn more about Eventing development in the Eventing work group.

Installation

Knative Eventing currently requires Knative Serving and Istio version 1.0 or later installed. Follow the instructions to install on the platform of your choice.

Many of the sources require making outbound connections to create the event subscription, and if you have any functions that make use of any external (to cluster) services, you must enable it also for them to work. Follow the instructions to configure outbound network access.

Install the core Knative Eventing (which provides an in-memory ChannelProvisioner) and the core sources (which provides the Kubernetes Events, GitHub, and “Container” Sources) with the following commands:

kubectl apply --filename https://github.com/knative/eventing/releases/download/v0.4.0/release.yaml
kubectl apply --filename https://github.com/knative/eventing-sources/releases/download/v0.4.0/release.yaml

In addition to the core sources, there are other sources that you can install.

This document will be updated as additional sources (which are custom resource definitions and an associated controller) and channels (ClusterChannelProvisioners and controllers) become available.

Check out the Configuration section to learn more about operating Knative Eventing.

Architecture

The eventing infrastructure supports two forms of event delivery at the moment:

  1. Direct delivery from a source to a single Service (an Addressable endpoint, including a Knative Service or a core Kubernetes Service). In this case, the Source is responsible for retrying or queueing events if the destination Service is not available.
  2. Fan-out delivery from a source or Service response to multiple endpoints using Channels and Subscriptions. In this case, the Channel implementation ensures that messages are delivered to the requested destinations and should buffer the events if the destination Service is unavailable.

Control plane object model

The actual message forwarding is implemented by multiple data plane components which provide observability, persistence, and translation between different messaging protocols.

Data plane implementation

Sources

Each source is a separate Kubernetes custom resource. This allows each type of Source to define the arguments and parameters needed to instantiate a source. Knative Eventing defines the following Sources in the sources.eventing.knative.dev API group. Types below are declared in golang format, but may be expressed as simple lists, etc in YAML. All Sources should be part of the sources category, so you can list all existing Sources with kubectl get sources. The currently-implemented Sources are described below:

Want to implement your own source? Check out the tutorial.

KubernetesEventSource

The KubernetesEventSource fires a new event each time a Kubernetes Event is created or updated.

Spec fields:

  • namespace: string The namespace to watch for events.
  • serviceAccountname: string The name of the ServiceAccount used to connect to the Kubernetes apiserver.
  • sink: ObjectReference A reference to the object that should receive events.

See the Kubernetes Event Source example.

GitHubSource

The GitHubSource fires a new event for selected GitHub event types.

Spec fields:

  • ownerAndRepository: string The GitHub owner/org and repository to receive events from. The repository may be left off to receive events from an entire organization.
  • eventTypes: []string A list of event types in “Webhook event name” format (lower_case).
  • accessToken.secretKeyRef: SecretKeySelector containing a GitHub access token for configuring a GitHub webhook. One of this or secretToken must be set.
  • secretToken.secretKeyRef: SecretKeySelector containing a GitHub secret token for configuring a GitHub webhook. One of this or accessToken must be set.
  • serviceAccountName: string The name of the ServiceAccount to run the container as.
  • sink: ObjectReference A reference to the object that should receive events.
  • githubAPIURL: string Optional field to specify the base URL for API requests. Defaults to the public GitHub API if not specified, but can be set to a domain endpoint to use with GitHub Enterprise, for example, https://github.mycompany.com/api/v3/. This base URL should always be specified with a trailing slash.

See the GitHub Source example.

GcpPubSubSource

The GcpPubSubSource fires a new event each time a message is published on a Google Cloud Platform PubSub topic.

Spec fields:

  • googleCloudProject: string The GCP project ID that owns the topic.
  • topic: string The name of the PubSub topic.
  • serviceAccountName: string The name of the ServiceAccount used to access the gcpCredsSecret.
  • gcpCredsSecret: ObjectReference A reference to a Secret which contains a GCP refresh token for talking to PubSub.
  • sink: ObjectReference A reference to the object that should receive events.

See the GCP PubSub Source example.

AwsSqsSource

The AwsSqsSource fires a new event each time an event is published on an AWS SQS topic.

Spec fields:

  • queueURL: URL of the SQS queue to pull events from.
  • awsCredsSecret: credential to use to poll the AWS SQS queue.
  • sink: ObjectReference A reference to the object that should receive events.
  • serviceAccountName: string The name of the ServiceAccount used to access the awsCredsSecret.

ContainerSource

The ContainerSource will instantiate a container image which can generate events until the ContainerSource is deleted. This may be used (for example) to poll an FTP server for new files or generate events at a set time interval.

Spec fields:

  • image (required): string A docker image of the container to be run.
  • args: []string Command-line arguments. If no --sink flag is provided, one will be added and filled in with the DNS address of the sink object.
  • env: map[string]string Environment variables to be set in the container.
  • serviceAccountName: string The name of the ServiceAccount to run the container as.
  • sink: ObjectReference A reference to the object that should receive events.

CronJobSource

The CronJobSource fires events based on given Cron schedule.

Spec fields:

  • schedule (required): string A Cron format string, such as 0 * * * * or @hourly.
  • data: string Optional data sent to downstream receiver.
  • serviceAccountName: string The name of the ServiceAccount to run the container as.
  • sink: ObjectReference A reference to the object that should receive events.

See the Cronjob Source example.

Getting Started

Configuration

  • Default Channels provide a way to choose the persistence strategy for Channels across the cluster.

Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License.