Allow extra_consumer_args option to pass arguments to consumer modu…

…le init

lib/kafka_ex/gen_consumer.ex

@@ -137,6 +137,10 @@ defmodule KafkaEx.GenConsumer do
       {:ok, %State{}}
     end
 
+    def init(_topic, _partition, extra_args) do
+      {:ok, %State{}}
+    end
+
     def handle_message_set(message_set, state) do
       {:async_commit, %{state | messages: state.messages ++ message_set}}
     end
@@ -204,6 +208,7 @@ defmodule KafkaEx.GenConsumer do
           {:commit_interval, non_neg_integer}
           | {:commit_threshold, non_neg_integer}
           | {:auto_offset_reset, :none | :earliest | :latest}
+          | {:extra_consumer_args, map()}
 
   @typedoc """
   Options used when starting a `KafkaEx.GenConsumer`.
@@ -227,6 +232,28 @@ defmodule KafkaEx.GenConsumer do
   @callback init(topic :: binary, partition :: non_neg_integer) ::
               {:ok, state :: term}
 
+  @doc """
+  Invoked when the server is started. `start_link/5` will block until it
+  returns.
+
+  `topic` and `partition` are the arguments passed to `start_link/5`. They
+  identify the Kafka partition that the `KafkaEx.GenConsumer` will consume from.
+
+  `extra_args` is the value of the `extra_consumer_args` option to `start_link/5`. 
+
+  The default implementation of this function calls `init/2`.
+
+  Returning `{:ok, state}` will cause `start_link/5` to return `{:ok, pid}` and
+  the process to start consuming from its assigned partition. `state` becomes
+  the consumer's state.
+
+  Any other return value will cause the `start_link/5` to return `{:error,
+  error}` and the process to exit.
+  """
+  @callback init(topic :: binary, partition :: non_neg_integer, extra_args :: map()) ::
+    {:ok, state :: term}
+
+
   @doc """
   Invoked for each message set consumed from a Kafka topic partition.
 
@@ -285,6 +312,10 @@ defmodule KafkaEx.GenConsumer do
         {:ok, nil}
       end
 
+      def init(topic, partition, _extra_args) do
+        init(topic, partition)
+      end
+
       def handle_call(msg, _from, consumer_state) do
         # taken from the GenServer handle_call implementation
         proc =
@@ -336,7 +367,7 @@ defmodule KafkaEx.GenConsumer do
         {:noreply, consumer_state}
       end
 
-      defoverridable init: 2, handle_call: 3, handle_cast: 2, handle_info: 2
+      defoverridable init: 2, init: 3, handle_call: 3, handle_cast: 2, handle_info: 2
     end
   end
 
@@ -392,6 +423,11 @@ defmodule KafkaEx.GenConsumer do
   * `:fetch_options` - Optional keyword list that is passed along to the
     `KafkaEx.fetch` call.
 
+  * `:extra_consumer_args` - Optional parameter that is passed along to the
+    `GenConsumer.init` call in the consumer module. Note that if `init/3` is not 
+    implemented, the default implementation calls to `init/2`, dropping the extra
+    arguments.
+
   Both `:commit_interval` and `:commit_threshold` default to the application
   config (e.g., `Application.get_env/2`) if that value is present, or the
   stated default if the application config is not present.
@@ -482,7 +518,13 @@ defmodule KafkaEx.GenConsumer do
         Application.get_env(:kafka_ex, :auto_offset_reset, @auto_offset_reset)
       )
 
-    {:ok, consumer_state} = consumer_module.init(topic, partition)
+    extra_consumer_args = 
+      Keyword.get(
+        opts,
+        :extra_consumer_args
+      )
+
+    {:ok, consumer_state} = consumer_module.init(topic, partition, extra_consumer_args)
     worker_opts = Keyword.take(opts, [:uris])
 
     {:ok, worker_name} =