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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.
Knative Eventing supports multiple modes of usage. The following scenarios are well-supported by the existing components; since the system is modular, it’s also possible to combine the components in novel ways.
I just want to consume events like X, I don’t care how they are published. Use a Trigger to consume events from a Broker based on CloudEvents attributes. Your application will receive the events as an HTTP POST.
I want to transform events through a series of steps. Use Channels and Subscriptions to define complex message-passing topologies. For simple pipelines, the Sequence automates construction of Channels and Subscriptions between each stage.
Knative also supports some additional patterns such as Parallel fanout of events, and routing response events from both Channels and Brokers.
Knative Eventing is designed around the following goals:
- 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).
- Event producers and event consumers 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.
- 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.
- Ensure cross-service interoperability. Knative Eventing is consistent with the CloudEvents specification that is developed by the CNCF Serverless WG.
To enable delivery to multiple types of Services, Knative Eventing defines two generic interfaces that can be implemented by multiple Kubernetes resources:
- Addressable objects are able to receive and acknowledge an event
delivered over HTTP to an address defined in their
status.address.urlfield. As a special case, the core Kubernetes Service object also fulfils the Addressable interface.
- 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 brokers and triggers
Broker and Trigger objects make it easy to filter events based on event attributes.
A Broker provides a bucket of events which can be selected by attribute. It
receives events and forwards them to subscribers defined by one or more matching
Triggers. Since a Broker implements Addressable, event senders can submit events
to the Broker by POSTing the event to the Broker’s
A Trigger describes a filter on event attributes which should be delivered to an Addressable. You can create as many Triggers as necessary.
For most use cases, a single bucket (Broker) per namespace is sufficient, but there are serveral use cases where multiple buckets (Brokers) can simplify architecture. For example, separate Brokers for events containing Personally Identifiable Information (PII) and non-PII events can simplify audit and access control rules.
Knative Eventing defines an EventType object to make it easier for consumers to discover the types of events they can consume from Brokers.
The registry consists of a collection of event types. The event types stored in the registry contain (all) the required information for a consumer to create a Trigger without resorting to some other out-of-band mechanism.
To learn how to use the registry, see the Event Registry documentation.
Simplify event delivery
The SinkBinding custom object supports decoupling event production from delivery addressing.
When you create a SinkBinding, you reference an Addressable and a Kubernetes
object which provides a PodTemplateSpec. The SinkBinding will inject environment
$K_SINK for the destination URL) into the PodTemplateSpec so that
the application code does not need to interact with the Kubernetes API to locate
the event destination.
Event channels and subscriptions
Knative Eventing also defines an event forwarding and persistence layer, called a Channel. Each channel is a separate Kubernetes Custom Resource. 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.
See the List of Channel implementations.
Higher Level eventing constructs
There are cases where you may want to utilize a set of co-operating functions together and for those use cases, Knative Eventing provides two additional resources:
- Sequence provides a way to define an in-order list of functions.
- Parallel provides a way to define a list of branches for events.
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.
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.
All Sources should be part of the
sources category, so you can list all existing Sources with
kubectl get sources.
In addition to the sources explained below, there are other sources that you can install. If you need a Source not covered by the ones mentioned below nor by the other available implementations, there is a tutorial on writing a Source with a Receive Adapter.
If your code needs to send events as part of its business logic and doesn’t fit the model of a Source, consider feeding events directly to a Broker.
These are the core sources that come out-of-the-box when installing Knative Eventing.
The APIServerSource fires a new event each time a Kubernetes resource is created, updated or deleted.
See the Kubernetes API Server Source example for more details.
The PingSource fires events based on given Cron schedule.
See the Ping Source example for more details.
The ContainerSource will instantiate container image(s) that 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.
Refer to the Container Source example for more details.
The SinkBinding can be used to author new event sources using any of the familiar compute abstractions that Kubernetes makes available (e.g. Deployment, Job, DaemonSet, StatefulSet), or Knative abstractions (e.g. Service, Configuration).
See the SinkBinding example for more details.
Eventing Contrib Sources
This is a non-exhaustive list of Sources supported by our community and maintained in the Knative Eventing-Contrib Github repo.
The GitHubSource fires a new event for selected GitHub event types.
See the GitHub Source example for more details.
The GitLabSource creates a webhooks for specified event types, listens for incoming events and passes them to a consumer.
See the GitLab Source example for more details.
The AwsSqsSource fires a new event each time an event is published on an AWS SQS topic.
The KafkaSource reads events from an Apache Kafka Cluster, and passes these to a Knative Serving application so that they can be consumed.
See the Kafka Source example for more details.
A CamelSource is an event source that can represent any existing Apache Camel component that provides a consumer side, and enables publishing events to an addressable endpoint. Each Camel endpoint has the form of a URI where the scheme is the ID of the component to use.
Google Cloud Sources
In order to consume events from different GCP services, Knative-GCP supports different GCP Sources.
The CloudPubSubSource fires a new event each time a message is published on a Google Cloud Platform PubSub topic.
See the CloudPubSubSource example for more details.
Registers for events of specific types on the specified Google Cloud Storage bucket and optional object prefix. Brings those events into Knative.
See the CloudStorageSource example.
Creates, updates, and deletes Google Cloud Scheduler Jobs. When those jobs are triggered, receive the event inside Knative.
See the CloudSchedulerSource example for further details.
Registers for events of specific types on the specified Google Cloud Audit Logs. Brings those events into Knative.
Refer to the CloudAuditLogsSource example for more details.