implement unsubscribing from a topic. And getting current topic

lib/kaffe/group_member/manager/group_manager.ex

@@ -25,6 +25,21 @@ defmodule Kaffe.GroupManager 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
+
+  # to manually unsubscribe from existing topics (not from config), at any point in time
+  def unsubscribe_topics(topics) do
+    GenServer.call(name(), {:unsubscribe_topics, topics})
+  end
+
+  # returns the current subscribed topics
+  def get_topics() do
+    GenServer.call(name(), {:get_topics})
+  end
+
   ## Callbacks
 
   def init([supervisor_pid]) do
@@ -64,6 +79,25 @@ defmodule Kaffe.GroupManager do
     {:noreply, state}
   end
 
+  def handle_call({:unsubscribe_topics, old_topics}, _from, %State{topics: topics} = state) do
+    unsubscribed_topics = Enum.flat_map(old_topics, fn(old_topic) ->
+      if ! Enum.member?(topics, old_topic) do
+        Logger.info "Not unsubscribing from #{old_topic}, already unsubscribed"
+        []
+      else
+        :ok = GroupMemberSupervisor.stop_group_member(state.supervisor_pid, state.subscriber_name, old_topic)
+        Logger.debug "Unsubscribed from topic: #{old_topic}"
+        [old_topic]
+      end
+    end)
+    {:reply, {:ok, unsubscribed_topics}, %State{state|topics: topics -- unsubscribed_topics } }
+  end
+
+
+  def handle_call({:get_topics}, _from, %State{topics: topics} = state) do
+    {:reply, {:ok, topics}, state}
+  end
+
   defp kafka do
     Application.get_env(:kaffe, :kafka_mod, :brod)
   end

lib/kaffe/group_member/manager/group_member_supervisor.ex

@@ -22,6 +22,13 @@ defmodule Kaffe.GroupMemberSupervisor do
       id: :"group_member_#{subscriber_name}_#{topic}"))
   end
 
+  def stop_group_member(supervisor_pid, subscriber_name, topic) do
+    case Supervisor.terminate_child(supervisor_pid, "group_member_#{subscriber_name}_#{topic}") do
+      :ok -> :ok
+      {:error, :not_found} -> :ok
+    end
+  end
+
   def init(:ok) do
     Logger.info "event#starting=#{__MODULE__}"