Class: Kafka::Consumer

Inherits:
Object
  • Object
show all
Defined in:
lib/kafka/consumer.rb

Overview

Note:

This code is still alpha level. Don't use this for anything important. The API may also change without warning.

A client that consumes messages from a Kafka cluster in coordination with other clients.

A Consumer subscribes to one or more Kafka topics; all consumers with the same group id then agree on who should read from the individual topic partitions. When group members join or leave, the group synchronizes, making sure that all partitions are assigned to a single member, and that all members have some partitions to read from.

Example

A simple producer that simply writes the messages it consumes to the console.

require "kafka"

kafka = Kafka.new(seed_brokers: ["kafka1:9092", "kafka2:9092"])

# Create a new Consumer instance in the group `my-group`:
consumer = kafka.consumer(group_id: "my-group")

# Subscribe to a Kafka topic:
consumer.subscribe("messages")

# Loop forever, reading in messages from all topics that have been
# subscribed to.
consumer.each_message do |message|
  puts message.topic
  puts message.partition
  puts message.key
  puts message.value
  puts message.offset
end

Instance Method Summary collapse

Constructor Details

#initialize(cluster:, logger:, group:, offset_manager:, session_timeout:) ⇒ Consumer

Returns a new instance of Consumer.



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# File 'lib/kafka/consumer.rb', line 46

def initialize(cluster:, logger:, group:, offset_manager:, session_timeout:)
  @cluster = cluster
  @logger = logger
  @group = group
  @offset_manager = offset_manager
  @session_timeout = session_timeout

  # Send two heartbeats in each session window, just to be sure.
  @heartbeat_interval = @session_timeout / 2

  # Whether or not the consumer is currently consuming messages.
  @running = false
end

Instance Method Details

#each_batch ⇒ Object 



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# File 'lib/kafka/consumer.rb', line 145

def each_batch
  loop do
    begin
      fetch_batches.each do |batch|
        unless batch.empty?
          Instrumentation.instrument("process_batch.consumer.kafka") do |notification|
            notification.update(
              topic: batch.topic,
              partition: batch.partition,
              highwater_mark_offset: batch.highwater_mark_offset,
              message_count: batch.messages.count,
            )

            yield batch
          end

          mark_message_as_processed(batch.messages.last)
        end

        @offset_manager.commit_offsets_if_necessary

        send_heartbeat_if_necessary
      end
    rescue ConnectionError => e
      @logger.error "Connection error while sending heartbeat; rejoining"
      join_group
    rescue UnknownMemberId
      @logger.error "Kicked out of group; rejoining"
      join_group
    rescue RebalanceInProgress
      @logger.error "Group is rebalancing; rejoining"
      join_group
    rescue IllegalGeneration
      @logger.error "Group has transitioned to a new generation; rejoining"
      join_group
    end
  end
ensure
  # In order to quickly have the consumer group re-balance itself, it's
  # important that members explicitly tell Kafka when they're leaving.
  @offset_manager.commit_offsets
  @group.leave
end

#each_message {|message| ... } ⇒ nil

Fetches and enumerates the messages in the topics that the consumer group subscribes to.

Each message is yielded to the provided block. If the block returns without raising an exception, the message will be considered successfully processed. At regular intervals the offset of the most recent successfully processed message in each partition will be committed to the Kafka offset store. If the consumer crashes or leaves the group, the group member that is tasked with taking over processing of these partitions will resume at the last committed offsets.

Yield Parameters:

Returns:

  • (nil) 


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# File 'lib/kafka/consumer.rb', line 91

def each_message
  @running = true

  while @running
    begin
      fetch_batches.each do |batch|
        batch.messages.each do |message|
          Instrumentation.instrument("process_message.consumer.kafka") do |notification|
            notification.update(
              topic: message.topic,
              partition: message.partition,
              offset: message.offset,
              offset_lag: batch.highwater_mark_offset - message.offset,
              key: message.key,
              value: message.value,
            )

            yield message
          end

          @offset_manager.commit_offsets_if_necessary

          send_heartbeat_if_necessary
          mark_message_as_processed(message)

          break if !@running
        end
      end
    rescue ConnectionError => e
      @logger.error "Connection error while sending heartbeat; rejoining"
      join_group
    rescue UnknownMemberId
      @logger.error "Kicked out of group; rejoining"
      join_group
    rescue RebalanceInProgress
      @logger.error "Group is rebalancing; rejoining"
      join_group
    rescue IllegalGeneration
      @logger.error "Group has transitioned to a new generation; rejoining"
      join_group
    end
  end
ensure
  # In order to quickly have the consumer group re-balance itself, it's
  # important that members explicitly tell Kafka when they're leaving.
  @offset_manager.commit_offsets
  @group.leave
  @running = false
end

#stop ⇒ Object 



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# File 'lib/kafka/consumer.rb', line 141

def stop
  @running = false
end

#subscribe(topic, default_offset: :earliest) ⇒ nil

Subscribes the consumer to a topic.

Typically you either want to start reading messages from the very beginning of the topic's partitions or you simply want to wait for new messages to be written. In the former case, set default_offsets to :earliest (the default); in the latter, set it to :latest.

Parameters:

  • topic (String) —

    the name of the topic to subscribe to.

  • default_offset (Symbol) (defaults to: :earliest) —

    whether to start from the beginning or the end of the topic's partitions.

Returns:

  • (nil) 


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# File 'lib/kafka/consumer.rb', line 71

def subscribe(topic, default_offset: :earliest)
  @group.subscribe(topic)
  @offset_manager.set_default_offset(topic, default_offset)

  nil
end