support for dynamically subscribing to new topics after start

lib/kaffe/group_member/manager/group_manager.ex

@@ -18,13 +18,19 @@ defmodule Kaffe.GroupManager do
       subscriber_name: nil,
       consumer_group: nil,
       topics: nil,
-      offset: nil
+      offset: nil,
+      worker_manager_pid: nil
   end
 
   def start_link() do
     GenServer.start_link(__MODULE__, [self()], name: name())
   end
 
+  # to manually subscribe to new topics (not from config), at any point in time
+  def subscribe_topics(topics) do
+    GenServer.call(name(), {:subscribe_topics, topics})
+  end
+
   ## Callbacks
 
   def init([supervisor_pid]) do
@@ -61,7 +67,26 @@ defmodule Kaffe.GroupManager do
         topic)
     end)
 
-    {:noreply, state}
+    {:noreply, %State{state|worker_manager_pid: worker_manager_pid} }
+  end
+
+  def handle_call({:subscribe_topics, new_topics}, _from, %State{topics: topics} = state) do
+    subscribed_topics = Enum.flat_map(new_topics, fn(new_topic) ->
+      if Enum.member?(topics, new_topic) do
+        Logger.info "Not subscribing to #{new_topic}, already subscribed"
+        []
+      else
+        Logger.debug "Starting group member for topic: #{new_topic}"
+        {:ok, _pid} = GroupMemberSupervisor.start_group_member(
+          state.supervisor_pid,
+          state.subscriber_name,
+          state.consumer_group,
+          state.worker_manager_pid,
+          new_topic)
+        [new_topic]
+      end
+    end)
+    {:reply, {:ok, subscribed_topics}, %State{state|topics: subscribed_topics ++ topics} }
   end
 
   defp kafka do