gistfile1.hrl

handle_call({insert, Value}, _From, State) ->
  NextNode = node_manager:next_node(?METHOD),
  NextDb = db_manager:next_db_node(NextNode),
  db_node:insert_value(Value),

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On the db_manager module:
next_db_node(DbManagerPid) ->
  gen_server:call(DbManagerPid, {next_db, ?METHOD}).

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On the db_node module:
insert_value(DbPid, Value) ->
  gen_server:call(DbPid, {insert, null, Value}).