Allow workers to be configured per consumer

The same application may have differing requirements in terms of load,
so it doesn't make that much sense to have a global configuration for
this

lib/racecar.rb

@@ -54,7 +54,7 @@ def self.instrumenter
   def self.run(processor)
     runner = Runner.new(processor, config: config, logger: logger, instrumenter: instrumenter)
 
-    if config.parallel_workers > 1
+    if config.parallel_workers && config.parallel_workers > 1
       ParallelRunner.new(runner: runner, config: config, logger: logger).run
     else
       runner.run

lib/racecar/config.rb

@@ -168,11 +168,10 @@ class Config < KingKonf::Config
           If this is not set to greater than 1, no extra processes will be generated"
     integer :parallel_workers, default: 1
 
-
     # The error handler must be set directly on the object.
     attr_reader :error_handler
 
-    attr_accessor :subscriptions, :logger
+    attr_accessor :subscriptions, :logger, :parallel_workers
 
     def statistics_interval_ms
       if Rdkafka::Config.statistics_callback
@@ -225,6 +224,7 @@ def load_consumer_class(consumer_class)
         consumer_class.name.gsub(/[a-z][A-Z]/) { |str| "#{str[0]}-#{str[1]}" }.downcase,
       ].compact.join
 
+      self.parallel_workers = consumer_class.parallel_workers
       self.subscriptions = consumer_class.subscriptions
       self.max_wait_time = consumer_class.max_wait_time || self.max_wait_time
       self.pidfile ||= "#{group_id}.pid"

lib/racecar/consumer.rb

@@ -9,7 +9,7 @@ class Consumer
     class << self
       attr_accessor :max_wait_time
       attr_accessor :group_id
-      attr_accessor :producer, :consumer
+      attr_accessor :producer, :consumer, :parallel_workers
 
       def subscriptions
         @subscriptions ||= []
@@ -27,7 +27,15 @@ def subscriptions
       # @param additional_config [Hash] Configuration properties for consumer.
       #   See https://github.com/edenhill/librdkafka/blob/master/CONFIGURATION.md
       # @return [nil]
-      def subscribes_to(*topics, start_from_beginning: true, max_bytes_per_partition: 1048576, additional_config: {})
+      def subscribes_to(
+        *topics,
+        start_from_beginning: true,
+        max_bytes_per_partition: 1048576,
+        additional_config: {},
+        parallel_workers: nil
+      )
+        self.parallel_workers = parallel_workers
+
         topics.each do |topic|
           subscriptions << Subscription.new(topic, start_from_beginning, max_bytes_per_partition, additional_config)
         end

spec/integration/consumer_spec.rb

@@ -15,7 +15,7 @@ class NoProcessConsumer < Racecar::Consumer
 RSpec.describe "running a Racecar consumer", type: :integration do
   context "when an error occurs trying to start the runner" do
     context "when there are no subscriptions, and no parallelism" do
-      before { Racecar.configure { |c| c.parallel_workers = 1 } }
+      before { NoSubsConsumer.parallel_workers = nil }
 
       it "raises an exception" do
         expect do
@@ -25,7 +25,7 @@ class NoProcessConsumer < Racecar::Consumer
     end
 
     context "when there are no subscriptions, and parallelism" do
-      before { Racecar.configure { |c| c.parallel_workers = 3 } }
+      before { NoSubsConsumer.parallel_workers = 3 }
 
       it "raises an exception" do
         expect do
@@ -35,7 +35,7 @@ class NoProcessConsumer < Racecar::Consumer
     end
 
     context "when there is no process method, and no parallelism" do
-      before { Racecar.configure { |c| c.parallel_workers = 1 } }
+      before { NoProcessConsumer.parallel_workers = nil }
 
       it "raises an exception" do
         expect do
@@ -45,7 +45,7 @@ class NoProcessConsumer < Racecar::Consumer
     end
 
     context "when there is no process method, and parallelism" do
-      before { Racecar.configure { |c| c.parallel_workers = 3 } }
+      before { NoSubsConsumer.parallel_workers = 3 }
 
       it "raises an exception" do
         expect do
@@ -72,11 +72,9 @@ def process(message)
     end
 
     before do
-      Racecar.configure { |c| c.parallel_workers = parallelism }
-
       create_topic(topic: input_topic, partitions: topic_partitions)
 
-      consumer_class.subscribes_to(input_topic)
+      consumer_class.subscribes_to(input_topic, parallel_workers: parallelism)
       consumer_class.output_topic = output_topic
 
       publish_messages!(input_topic, input_messages)
@@ -94,7 +92,7 @@ class EchoConsumer1 < mock_echo_consumer_class
 
       let(:input_messages) { [{ payload: "hello", key: "greetings", partition: nil }] }
       let(:topic_partitions) { 1 }
-      let(:parallelism) { 1 }
+      let(:parallelism) { nil }
 
       it "can consume and publish a message" do
         in_background(cleanup_callback: -> { Process.kill("INT", Process.pid) }) do