distributed/

README

Eventing-Kafka Distributed Channel

This is the Kafka Channel implementation, originally contributed by SAP's Kyma project.

See https://github.com/knative/eventing-contrib/issues/1070 for discussion of the donation process.

This repo falls under the Knative Code of Conduct

This project is a Knative Eventing implementation of a Kafka backed channel which provides a more granular architecture as an alternative to what the original "consolidated" implementation offers. Specifically it deploys a single/separate Receiver, and one Dispatcher per KafkaChannel.

Rationale

The Knative "consolidated" KafkaChannel already provides a Kafka backed Channel implementation, so why invest the time in building another one? At the time this project was begun, and still today, the reference Kafka implementation does not provide the scaling characteristics required by a large and varied use case with many Topics and Consumers. That implementation is based on a single choke point that could allow one Topic's traffic to impact the throughput of another Topic.

We also had the need to support a variety of Kafka providers, including Azure EventHubs in Kafka compatibility mode as well as exposing the ability to customize the Kafka Topic management. Finally, the ability to expose Kafka configuration was very limited, and we needed the ability to customize certain aspects of the Kafka Topics / Producers / Consumers.

Status

Significant work has recently gone into aligning the two implementations from a CRD, configuration, authorization, and code-sharing perspective, in order to standardize the user experience as well as maximize code reuse. While the runtime architectures will always be different (the "raison d'etre" for having multiple implementations), the goal is to continue this sharing. The eventual goal might be to have a single KafkaChannel implementation that can deploy either runtime architecture as desired.

Architecture

As mentioned in the "Rationale" section above, the desire was to implement different levels of granularity to achieve improved segregation and scaling characteristics. Our original implementation was extremely granular in that there was a separate Channel/Producer Deployment for every KafkaChannel (Kafka Topic), and a separate Dispatcher/Consumer Deployment for every Knative Subscription. This allowed the highest level of segregation and the ability to tweak K8S resources at the finest level.

The downside of this approach, however, is the large resource consumption related to the sheer number of Deployments in the K8S cluster, as well as the inherent inefficiencies of low traffic rate Channels / Subscriptions being underutilized. Adding in a service-mesh (such as Istio) further exacerbates the problem by adding side-cars to every Deployment. Therefore, we've taken a step back and aggregated the Channels/Producers together into a single Deployment per Kafka authorization, and the Dispatchers/Consumers into a single Deployment per KafkaChannel (Topic). The implementations of each are horizontally scalable which provides a reasonable compromise between resource consumption and segregation / scaling.

Project Structure

The "distributed" KafkaChannel consists of three distinct runtime K8S deployments as follows...

• controller - This component implements the KafkaChannel Controller. It is using the current knative-eventing "Shared Main" approach based directly on K8S informers / listers. The controller is using the shared KafkaChannel CRD, apis/, and client implementations in this repository.

• dispatcher - This component runs the Kafka ConsumerGroups responsible for processing messages from the corresponding Kafka Topic. This is the "Consumer" from the Kafka perspective. A separate dispatcher Deployment will be created for each unique KafkaChannel (Kafka Topic), and will contain a distinct Kafka Consumer Group for each Subscription to the KafkaChannel.

• receiver - The event receiver to which all inbound messages are sent. An HTTP server which accepts messages that conform to the CloudEvent specification, and then writes those messages to the corresponding Kafka Topic. This is the "Producer" from the Kafka perspective. A single receiver Deployment is created to service all KafkaChannels in the cluster.

• config - Eventing-kafka ko installable YAML files for installation.

Control Plane

The control plane for the Kafka Channels is managed by the eventing-kafka-controller which is installed in the knative-eventing namespace. KafkaChannel Custom Resource instances can be created in any user namespace. The eventing-kafka-controller will guarantee that the Data Plane is configured to support the flow of events as defined by Subscriptions to a KafkaChannel. The underlying Kafka infrastructure to be used is defined in a specially labeled K8S Secret in the knative-eventing namespace. Eventing-kafka supports several Kafka (and Kafka-like) infrastructures .

Data Plane

The data plane for all KafkaChannels runs in the knative-eventing namespace. There is a single deployment for the receiver side of all channels which accepts CloudEvents and writes them to Kafka Topics. Each KafkaChannel uses one Kafka Topic. This deployment supports horizontal scaling with linearly increasing performance characteristics through specifying the number of replicas.

Each KafkaChannel has one deployment for the dispatcher side which reads from the Kafka Topic and sends to subscribers. Each subscriber has its own Kafka consumer group. This deployment can be scaled up to a replica count equalling the number of partitions in the Kafka Topic.

Messaging Guarantees

An event sent to a KafkaChannel is guaranteed to be persisted and processed if a 202 response is received by the sender.

The CloudEvent is partitioned based on the CloudEvent partitioning extension field called partitionkey. If the partitionkey is not present, then the subject field will be used. Finally, if neither is available, it will fall-back to random partitioning.

Events in each partition are processed in order, with an at-least-once guarantee. If a full cycle of retries for a given subscription fails, the event is ignored, or sent to the DLQ according to the Subscription's DeliverySpec and processing continues with the next event.

Offset Repositioning

The ConsumerGroup Offsets of a specific Knative Subscription can be repositioned (backwards or forwards within the Topic's retention window) via the ResetOffset Custom Resource, to allow events to be "replayed" in failure recovery scenarios.

Installation

For installation and configuration instructions please see the config files README.