Sarama Cluster

Cluster extensions for Sarama, the Go client library for Apache Kafka 0.9 (and later).

Documentation

Documentation and example are available via godoc at http://godoc.org/github.com/bsm/sarama-cluster

Examples

Consumers have two modes of operation. In the default multiplexed mode messages (and errors) of multiple topics and partitions are all passed to the single channel:

package main

import (
	"fmt"
	"log"
	"os"
	"os/signal"

	cluster "github.com/bsm/sarama-cluster"
)

func main() {

	// init (custom) config, enable errors and notifications
	config := cluster.NewConfig()
	config.Consumer.Return.Errors = true
	config.Group.Return.Notifications = true

	// init consumer
	brokers := []string{"127.0.0.1:9092"}
	topics := []string{"my_topic", "other_topic"}
	consumer, err := cluster.NewConsumer(brokers, "my-consumer-group", topics, config)
	if err != nil {
		panic(err)
	}
	defer consumer.Close()

	// trap SIGINT to trigger a shutdown.
	signals := make(chan os.Signal, 1)
	signal.Notify(signals, os.Interrupt)

	// consume errors
	go func() {
		for err := range consumer.Errors() {
			log.Printf("Error: %s\n", err.Error())
		}
	}()

	// consume notifications
	go func() {
		for ntf := range consumer.Notifications() {
			log.Printf("Rebalanced: %+v\n", ntf)
		}
	}()

	// consume messages, watch signals
	for {
		select {
		case msg, ok := <-consumer.Messages():
			if ok {
				fmt.Fprintf(os.Stdout, "%s/%d/%d\t%s\t%s\n", msg.Topic, msg.Partition, msg.Offset, msg.Key, msg.Value)
				consumer.MarkOffset(msg, "")	// mark message as processed
			}
		case <-signals:
			return
		}
	}
}

Users who require access to individual partitions can use the partitioned mode which exposes access to partition-level consumers:

package main

import (
  "fmt"
  "log"
  "os"
  "os/signal"

  cluster "github.com/bsm/sarama-cluster"
)

func main() {

	// init (custom) config, set mode to ConsumerModePartitions
	config := cluster.NewConfig()
	config.Group.Mode = cluster.ConsumerModePartitions

	// init consumer
	brokers := []string{"127.0.0.1:9092"}
	topics := []string{"my_topic", "other_topic"}
	consumer, err := cluster.NewConsumer(brokers, "my-consumer-group", topics, config)
	if err != nil {
		panic(err)
	}
	defer consumer.Close()

	// trap SIGINT to trigger a shutdown.
	signals := make(chan os.Signal, 1)
	signal.Notify(signals, os.Interrupt)

	// consume partitions
	for {
		select {
		case part, ok := <-consumer.Partitions():
			if !ok {
				return
			}

			// start a separate goroutine to consume messages
			go func(pc cluster.PartitionConsumer) {
				for msg := range pc.Messages() {
					fmt.Fprintf(os.Stdout, "%s/%d/%d\t%s\t%s\n", msg.Topic, msg.Partition, msg.Offset, msg.Key, msg.Value)
					consumer.MarkOffset(msg, "")	// mark message as processed
				}
			}(part)
		case <-signals:
			return
		}
	}
}

Running tests

You need to install Ginkgo & Gomega to run tests. Please see http://onsi.github.io/ginkgo for more details.

To run tests, call:

$ make test

Troubleshooting

Consumer not receiving any messages?

By default, sarama's Config.Consumer.Offsets.Initial is set to sarama.OffsetNewest. This means that in the event that a brand new consumer is created, and it has never committed any offsets to kafka, it will only receive messages starting from the message after the current one that was written.

If you wish to receive all messages (from the start of all messages in the topic) in the event that a consumer does not have any offsets committed to kafka, you need to set Config.Consumer.Offsets.Initial to sarama.OffsetOldest.