Implement Kafka middleware for CloudEvents and raw message handling

[Copilot]

Adds Kafka consumer middleware following the existing patterns from cloudevents and http packages.

Implementation

Core Package (kafka/)

• service.go - Kafka consumer with automatic CloudEvents detection (structured/binary mode) and raw message handling
• instance.go - Handler interfaces supporting both Handle(context.Context, event.Event) and Handle(context.Context, kafka.Message) signatures
• logs.go - Logging configuration matching existing packages
• service_test.go - Tests for lifecycle, configuration parsing, and handler detection
• mock/function.go - Test mock avoiding import cycles
• README.md - Comprehensive documentation for the kafka package

Example Commands

Following the repository pattern of separate commands for different handler types:

cmd/fcekafka/main.go - CloudEvents handler example (similar to fce)

type MyFunction struct{}

func (f *MyFunction) Handle(ctx context.Context, e event.Event) (*event.Event, error) {
    fmt.Println("Instanced CloudEvents handler invoked")
    fmt.Println(e)
    return &e, nil
}

func main() {
    kafka.Start(New())
}

cmd/frawkafka/main.go - Raw message handler example (similar to fhttp)

type MyFunction struct{}

func (f *MyFunction) Handle(ctx context.Context, msg kafka.Message) error {
    fmt.Printf("Topic: %s, Partition: %d, Offset: %d\n", msg.Topic, msg.Partition, msg.Offset)
    fmt.Printf("Key: %s, Value: %s\n", string(msg.Key), string(msg.Value))
    return nil
}

func main() {
    kafka.Start(New())
}

Configuration

Environment variables:

• KAFKA_BROKERS - Comma-separated broker list (default: localhost:9092)
• KAFKA_TOPICS - Comma-separated topics (default: func-topic)
• KAFKA_CONSUMER_GROUP - Consumer group ID (default: func-go-consumer)

Dependencies

Added github.com/segmentio/[email protected] - standard Go Kafka client with no known vulnerabilities.

• Fixes Kafka middleware #139

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Kafka middleware</issue_title>
<issue_description>Implement Kafka middleware.

With CloudEvents: similar to https://github.com/knative-extensions/func-go/blob/main/cmd/fce/main.go
With raw messages: similar to https://github.com/knative-extensions/func-go/blob/main/cmd/fhttp/main.go</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Kafka middleware #139

---

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[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Copilot
Once this PR has been reviewed and has the lgtm label, please assign matzew for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
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[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Copilot
Once this PR has been reviewed and has the lgtm label, please assign matzew for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[aliok]

@copilot you should create 2 mains

• cmd/fcekafka/main.go
• cmd/frawkafka/main.go

[Copilot]

@copilot you should create 2 mains

• cmd/fcekafka/main.go
• cmd/frawkafka/main.go
  ...

Done. Created separate commands:

• cmd/fcekafka/main.go for CloudEvents handler (similar to fce)
• cmd/frawkafka/main.go for raw message handler (similar to fhttp)

Removed the old cmd/fkafka directory and updated .gitignore accordingly.

Commit: 2ba569f

[knative-prow-robot]

PR needs rebase.

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