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: / opt / td-agent / embedded / lib / ruby / gems / 2.1.0 / gems / ruby-kafka-0.4.2 / lib / kafka /
Filename : consumer.rb
back
require "kafka/consumer_group"
require "kafka/offset_manager"
require "kafka/fetch_operation"

module Kafka

  # 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
  #
  class Consumer

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

      # A list of partitions that have been paused, per topic.
      @paused_partitions = {}

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

      # The maximum number of bytes to fetch from a single partition, by topic.
      @max_bytes = {}
    end

    # 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 `start_from_beginning`
    # to true (the default); in the latter, set it to false.
    #
    # @param topic [String] the name of the topic to subscribe to.
    # @param default_offset [Symbol] whether to start from the beginning or the
    #   end of the topic's partitions. Deprecated.
    # @param start_from_beginning [Boolean] whether to start from the beginning
    #   of the topic or just subscribe to new messages being produced. This
    #   only applies when first consuming a topic partition – once the consumer
    #   has checkpointed its progress, it will always resume from the last
    #   checkpoint.
    # @param max_bytes_per_partition [Integer] the maximum amount of data fetched
    #   from a single partition at a time.
    # @return [nil]
    def subscribe(topic, default_offset: nil, start_from_beginning: true, max_bytes_per_partition: 1048576)
      default_offset ||= start_from_beginning ? :earliest : :latest

      @group.subscribe(topic)
      @offset_manager.set_default_offset(topic, default_offset)
      @max_bytes[topic] = max_bytes_per_partition

      nil
    end

    # Stop the consumer.
    #
    # The consumer will finish any in-progress work and shut down.
    #
    # @return [nil]
    def stop
      @running = false
    end

    # Pause processing of a specific topic partition.
    #
    # When a specific message causes the processor code to fail, it can be a good
    # idea to simply pause the partition until the error can be resolved, allowing
    # the rest of the partitions to continue being processed.
    #
    # If the `timeout` argument is passed, the partition will automatically be
    # resumed when the timeout expires.
    #
    # @param topic [String]
    # @param partition [Integer]
    # @param timeout [Integer] the number of seconds to pause the partition for,
    #   or `nil` if the partition should not be automatically resumed.
    # @return [nil]
    def pause(topic, partition, timeout: nil)
      @paused_partitions[topic] ||= {}
      @paused_partitions[topic][partition] = timeout && Time.now + timeout
    end

    # Resume processing of a topic partition.
    #
    # @see #pause
    # @param topic [String]
    # @param partition [Integer]
    # @return [nil]
    def resume(topic, partition)
      paused_partitions = @paused_partitions.fetch(topic, {})
      paused_partitions.delete(partition)
    end

    # Whether the topic partition is currently paused.
    #
    # @see #pause
    # @param topic [String]
    # @param partition [Integer]
    # @return [Boolean] true if the partition is paused, false otherwise.
    def paused?(topic, partition)
      partitions = @paused_partitions.fetch(topic, {})

      if partitions.key?(partition)
        # Users can set an optional timeout, after which the partition is
        # automatically resumed. When pausing, the timeout is translated to an
        # absolute point in time.
        timeout = partitions.fetch(partition)

        if timeout.nil?
          true
        elsif Time.now < timeout
          true
        else
          @logger.info "Automatically resuming partition #{topic}/#{partition}, pause timeout expired"
          resume(topic, partition)
          false
        end
      end
    end

    # 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.
    #
    # @param min_bytes [Integer] the minimum number of bytes to read before
    #   returning messages from the server; if `max_wait_time` is reached, this
    #   is ignored.
    # @param max_wait_time [Integer, Float] the maximum duration of time to wait before
    #   returning messages from the server, in seconds.
    # @param automatically_mark_as_processed [Boolean] whether to automatically
    #   mark a message as successfully processed when the block returns
    #   without an exception. Once marked successful, the offsets of processed
    #   messages can be committed to Kafka.
    # @yieldparam message [Kafka::FetchedMessage] a message fetched from Kafka.
    # @raise [Kafka::ProcessingError] if there was an error processing a message.
    #   The original exception will be returned by calling `#cause` on the
    #   {Kafka::ProcessingError} instance.
    # @return [nil]
    def each_message(min_bytes: 1, max_wait_time: 5, automatically_mark_as_processed: true)
      consumer_loop do
        batches = fetch_batches(min_bytes: min_bytes, max_wait_time: max_wait_time)

        batches.each do |batch|
          batch.messages.each do |message|
            @instrumenter.instrument("process_message.consumer") do |notification|
              notification.update(
                topic: message.topic,
                partition: message.partition,
                offset: message.offset,
                offset_lag: batch.highwater_mark_offset - message.offset - 1,
                key: message.key,
                value: message.value,
              )

              begin
                yield message
              rescue => e
                location = "#{message.topic}/#{message.partition} at offset #{message.offset}"
                backtrace = e.backtrace.join("\n")
                @logger.error "Exception raised when processing #{location} -- #{e.class}: #{e}\n#{backtrace}"

                raise ProcessingError.new(message.topic, message.partition, message.offset)
              end
            end

            mark_message_as_processed(message) if automatically_mark_as_processed
            @offset_manager.commit_offsets_if_necessary

            @heartbeat.send_if_necessary

            return if !@running
          end
        end

        # We may not have received any messages, but it's still a good idea to
        # commit offsets if we've processed messages in the last set of batches.
        @offset_manager.commit_offsets_if_necessary
      end
    end

    # Fetches and enumerates the messages in the topics that the consumer group
    # subscribes to.
    #
    # Each batch of messages is yielded to the provided block. If the block returns
    # without raising an exception, the batch will be considered successfully
    # processed. At regular intervals the offset of the most recent successfully
    # processed message batch 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.
    #
    # @param min_bytes [Integer] the minimum number of bytes to read before
    #   returning messages from the server; if `max_wait_time` is reached, this
    #   is ignored.
    # @param max_wait_time [Integer, Float] the maximum duration of time to wait before
    #   returning messages from the server, in seconds.
    # @param automatically_mark_as_processed [Boolean] whether to automatically
    #   mark a batch's messages as successfully processed when the block returns
    #   without an exception. Once marked successful, the offsets of processed
    #   messages can be committed to Kafka.
    # @yieldparam batch [Kafka::FetchedBatch] a message batch fetched from Kafka.
    # @return [nil]
    def each_batch(min_bytes: 1, max_wait_time: 5, automatically_mark_as_processed: true)
      consumer_loop do
        batches = fetch_batches(min_bytes: min_bytes, max_wait_time: max_wait_time)

        batches.each do |batch|
          unless batch.empty?
            @instrumenter.instrument("process_batch.consumer") do |notification|
              notification.update(
                topic: batch.topic,
                partition: batch.partition,
                offset_lag: batch.offset_lag,
                highwater_mark_offset: batch.highwater_mark_offset,
                message_count: batch.messages.count,
              )

              begin
                yield batch
              rescue => e
                offset_range = (batch.first_offset .. batch.last_offset)
                location = "#{batch.topic}/#{batch.partition} in offset range #{offset_range}"
                backtrace = e.backtrace.join("\n")

                @logger.error "Exception raised when processing #{location} -- #{e.class}: #{e}\n#{backtrace}"

                raise ProcessingError.new(batch.topic, batch.partition, offset_range)
              end
            end

            mark_message_as_processed(batch.messages.last) if automatically_mark_as_processed
          end

          @offset_manager.commit_offsets_if_necessary

          @heartbeat.send_if_necessary

          return if !@running
        end
      end
    end

    # Move the consumer's position in a topic partition to the specified offset.
    #
    # Note that this has to be done prior to calling {#each_message} or {#each_batch}
    # and only has an effect if the consumer is assigned the partition. Typically,
    # you will want to do this in every consumer group member in order to make sure
    # that the member that's assigned the partition knows where to start.
    #
    # @param topic [String] 
    # @param partition [Integer] 
    # @param offset [Integer] 
    # @return [nil]
    def seek(topic, partition, offset)
      @offset_manager.seek_to(topic, partition, offset)
    end

    def commit_offsets
      @offset_manager.commit_offsets
    end

    def mark_message_as_processed(message)
      @offset_manager.mark_as_processed(message.topic, message.partition, message.offset)
    end

    def send_heartbeat_if_necessary
      @heartbeat.send_if_necessary
    end

    private

    def consumer_loop
      @running = true

      while @running
        begin
          yield
        rescue HeartbeatError, OffsetCommitError
          join_group
        rescue FetchError, NotLeaderForPartition, UnknownTopicOrPartition
          @cluster.mark_as_stale!
        rescue LeaderNotAvailable => e
          @logger.error "Leader not available; waiting 1s before retrying"
          @cluster.mark_as_stale!
          sleep 1
        rescue SignalException => e
          @logger.warn "Received signal #{e.message}, shutting down"
          @running = false
        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.
      make_final_offsets_commit!
      @group.leave rescue nil
      @running = false
    end

    def make_final_offsets_commit!(attempts = 3)
      @offset_manager.commit_offsets
    rescue ConnectionError, EOFError
      # It's important to make sure final offsets commit is done
      # As otherwise messages that have been processed after last auto-commit
      # will be processed again and that may be huge amount of messages
      return if attempts.zero?

      @logger.error "Retrying to make final offsets commit (#{attempts} attempts left)"
      sleep(0.1)
      make_final_offsets_commit!(attempts - 1)
    end

    def join_group
      old_generation_id = @group.generation_id

      @group.join

      if old_generation_id && @group.generation_id != old_generation_id + 1
        # We've been out of the group for at least an entire generation, no
        # sense in trying to hold on to offset data
        @offset_manager.clear_offsets
      else
        # After rejoining the group we may have been assigned a new set of
        # partitions. Keeping the old offset commits around forever would risk
        # having the consumer go back and reprocess messages if it's assigned
        # a partition it used to be assigned to way back. For that reason, we
        # only keep commits for the partitions that we're still assigned.
        @offset_manager.clear_offsets_excluding(@group.assigned_partitions)
      end
    end

    def fetch_batches(min_bytes:, max_wait_time:)
      join_group unless @group.member?

      subscribed_partitions = @group.subscribed_partitions

      @heartbeat.send_if_necessary

      raise NoPartitionsAssignedError if subscribed_partitions.empty?

      operation = FetchOperation.new(
        cluster: @cluster,
        logger: @logger,
        min_bytes: min_bytes,
        max_wait_time: max_wait_time,
      )

      subscribed_partitions.each do |topic, partitions|
        partitions.each do |partition|
          offset = @offset_manager.next_offset_for(topic, partition)
          max_bytes = @max_bytes.fetch(topic)

          if paused?(topic, partition)
            @logger.warn "Partition #{topic}/#{partition} is currently paused, skipping"
          else
            @logger.debug "Fetching batch from #{topic}/#{partition} starting at offset #{offset}"
            operation.fetch_from_partition(topic, partition, offset: offset, max_bytes: max_bytes)
          end
        end
      end

      operation.execute
    rescue OffsetOutOfRange => e
      @logger.error "Invalid offset for #{e.topic}/#{e.partition}, resetting to default offset"

      @offset_manager.seek_to_default(e.topic, e.partition)

      retry
    rescue ConnectionError => e
      @logger.error "Connection error while fetching messages: #{e}"

      raise FetchError, e
    end
  end
end