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Chapter 2. Discover

2.1. About OpenShift Serverless

OpenShift Serverless provides Kubernetes native building blocks that enable developers to create and deploy serverless, event-driven applications on OpenShift Container Platform. OpenShift Serverless is based on the open source Knative project, which provides portability and consistency for hybrid and multi-cloud environments by enabling an enterprise-grade serverless platform.

2.1.1. Knative Serving

Knative Serving supports developers who want to create, deploy, and manage cloud-native applications. It provides a set of objects as Kubernetes custom resource definitions (CRDs) that define and control the behavior of serverless workloads on an OpenShift Container Platform cluster.

Developers use these CRDs to create custom resource (CR) instances that can be used as building blocks to address complex use cases. For example:

  • Rapidly deploying serverless containers.
  • Automatically scaling pods.

2.1.1.1. Knative Serving resources

Service
The service.serving.knative.dev CRD automatically manages the life cycle of your workload to ensure that the application is deployed and reachable through the network. It creates a route, a configuration, and a new revision for each change to a user created service, or custom resource. Most developer interactions in Knative are carried out by modifying services.
Revision
The revision.serving.knative.dev CRD is a point-in-time snapshot of the code and configuration for each modification made to the workload. Revisions are immutable objects and can be retained for as long as necessary.
Route
The route.serving.knative.dev CRD maps a network endpoint to one or more revisions. You can manage the traffic in several ways, including fractional traffic and named routes.
Configuration
The configuration.serving.knative.dev CRD maintains the desired state for your deployment. It provides a clean separation between code and configuration. Modifying a configuration creates a new revision.

2.1.2. Knative Eventing

Knative Eventing on OpenShift Container Platform enables developers to use an event-driven architecture with serverless applications. An event-driven architecture is based on the concept of decoupled relationships between event producers and event consumers.

Event producers create events, and event sinks, or consumers, receive events. Knative Eventing uses standard HTTP POST requests to send and receive events between event producers and sinks. These events conform to the CloudEvents specifications, which enables creating, parsing, sending, and receiving events in any programming language.

Knative Eventing supports the following use cases:

Publish an event without creating a consumer
You can send events to a broker as an HTTP POST, and use binding to decouple the destination configuration from your application that produces events.
Consume an event without creating a publisher
You can use a trigger to consume events from a broker based on event attributes. The application receives events as an HTTP POST.

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

Addressable resources
Able to receive and acknowledge an event delivered over HTTP to an address defined in the status.address.url field of the event. The Kubernetes Service resource also satisfies the addressable interface.
Callable resources
Able to receive an event delivered over HTTP and transform it, returning 0 or 1 new events in the HTTP response payload. These returned events may be further processed in the same way that events from an external event source are processed.

You can propagate an event from an event source to multiple event sinks by using:

2.1.3. Supported configurations

The set of supported features, configurations, and integrations for OpenShift Serverless, current and past versions, are available at the Supported Configurations page.

2.1.4. Scalability and performance

OpenShift Serverless has been tested with a configuration of 3 main nodes and 3 worker nodes, each of which has 64 CPUs, 457 GB of memory, and 394 GB of storage each.

The maximum number of Knative services that can be created using this configuration is 3,000. This corresponds to the OpenShift Container Platform Kubernetes services limit of 10,000, since 1 Knative service creates 3 Kubernetes services.

The average scale from zero response time was approximately 3.4 seconds, with a maximum response time of 8 seconds, and a 99.9th percentile of 4.5 seconds for a simple Quarkus application. These times might vary depending on the application and the runtime of the application.

2.1.5. Additional resources

2.2. About OpenShift Serverless Functions

OpenShift Serverless Functions enables developers to create and deploy stateless, event-driven functions as a Knative service on OpenShift Container Platform. The kn func CLI is provided as a plug-in for the Knative kn CLI. OpenShift Serverless Functions uses the CNCF Buildpack API to create container images. After a container image is created, you can use the kn func CLI to deploy the container image as a Knative service on the cluster.

Important

OpenShift Serverless Functions is a Technology Preview feature only. Technology Preview features are not supported with Red Hat production service level agreements (SLAs) and might not be functionally complete. Red Hat does not recommend using them in production. These features provide early access to upcoming product features, enabling customers to test functionality and provide feedback during the development process.

For more information about the support scope of Red Hat Technology Preview features, see https://access.redhat.com/support/offerings/techpreview/.

2.2.1. Included runtimes

OpenShift Serverless Functions provides templates that can be used to create basic functions for the following runtimes:

2.2.2. Next steps

2.3. Event sources

A Knative event source can be any Kubernetes object that generates or imports cloud events, and relays those events to another endpoint, known as a sink. Sourcing events is critical to developing a distributed system that reacts to events.

You can create and manage Knative event sources by using the Developer perspective in the OpenShift Container Platform web console, the kn CLI, or by applying YAML files.

Currently, OpenShift Serverless supports the following event source types:

API server source
Brings Kubernetes API server events into Knative. The API server source sends a new event each time a Kubernetes resource is created, updated or deleted.
Ping source
Produces events with a fixed payload on a specified cron schedule.
Kafka event source
Connects a Kafka cluster to a sink as an event source.

You can also create a custom event source.

2.4. Channels and subscriptions

Channels are custom resources that define a single event-forwarding and persistence layer. After events have been sent to a channel from an event source or producer, these events can be sent to multiple Knative services or other sinks by using a subscription.

Channel workflow overview

You can create channels by instantiating a supported Channel object, and configure re-delivery attempts by modifying the delivery spec in a Subscription object.

After you create a Channel object, a mutating admission webhook adds a set of spec.channelTemplate properties for the Channel object based on the default channel implementation. For example, for an InMemoryChannel default implementation, the Channel object looks as follows:

apiVersion: messaging.knative.dev/v1
kind: Channel
metadata:
  name: example-channel
  namespace: default
spec:
  channelTemplate:
    apiVersion: messaging.knative.dev/v1
    kind: InMemoryChannel

The channel controller then creates the backing channel instance based on the spec.channelTemplate configuration.

Note

The spec.channelTemplate properties cannot be changed after creation, because they are set by the default channel mechanism rather than by the user.

When this mechanism is used with the preceding example, two objects are created: a generic backing channel and an InMemoryChannel channel. If you are using a different default channel implementation, the InMemoryChannel is replaced with one that is specific to your implementation. For example, with Knative Kafka, the KafkaChannel channel is created.

The backing channel acts as a proxy that copies its subscriptions to the user-created channel object, and sets the user-created channel object status to reflect the status of the backing channel.

2.4.1. Channel implementation types

InMemoryChannel and KafkaChannel channel implementations can be used with OpenShift Serverless for development use.

The following are limitations of InMemoryChannel type channels:

  • No event persistence is available. If a pod goes down, events on that pod are lost.
  • InMemoryChannel channels do not implement event ordering, so two events that are received in the channel at the same time can be delivered to a subscriber in any order.
  • If a subscriber rejects an event, there are no re-delivery attempts by default. You can configure re-delivery attempts by modifying the delivery spec in the Subscription object.

For more information about Kafka channels, see the Knative Kafka documentation.

2.4.2. Next steps