YAML file lists the observers to be initialized

[hsd-dev]

This can also include input params like link to the ROS models in case of ROSGraphObserver.

[hsd-dev]

http://wiki.ros.org/roslaunch/XML/rosparam#Examples

in launch file:

<rosparam command="load" file="$(find rosparam)/example.yaml" />

to access in python code:

ros::NodeHandle nh;
double number_to_get;
nh.getParam("/custom_prefix/number_float", number_to_get);

Another option is to set param as path to YAML file and use YAML module in python to load and read file:
https://github.com/udacity/CarND-Capstone/blob/master/ros/src/camera_info_publisher/yaml_to_camera_info_publisher.py

[hsd-dev]

Example file:

observers:
  - name: QualityObserver
      - topic: /speed
        type: std_msgs/Float64
      - topic: /accel
        type: std_msgs/Int32
      - hz: 100
  - name: ROSGraphIbserver
      - model: resources/talker_listener.rossystem
      - hz: 10