Scenario 1: Multi-Sensor Monitoring System

Domain

A facility monitoring station collects data from multiple environmental sensors deployed across a building. Each sensor publishes readings at a different rate. A central aggregator node collects all sensor data and publishes a consolidated summary. A watchdog node monitors sensor health by detecting missing data.

System Architecture

Your system must contain the following nodes and topics.

Nodes

Node	Description
temperature_sensor	Publishes std_msgs/msg/Float64 on /sensors/temperature at 2 Hz. Simulates temperature readings between 18.0 and 30.0 using random.uniform().
humidity_sensor	Publishes std_msgs/msg/Float64 on /sensors/humidity at 1 Hz. Simulates humidity readings between 30.0 and 80.0.
battery_sensor	Publishes std_msgs/msg/Int64 on /sensors/battery at 0.5 Hz. Simulates a battery level that decreases by 1 each tick (starting at 100).
aggregator	Subscribes to all three sensor topics. Maintains the latest value from each sensor. Publishes a std_msgs/msg/String summary on /monitoring/summary at 1 Hz containing the latest temperature, humidity, and battery values.
watchdog	Subscribes to /sensors/temperature and /sensors/humidity. Uses a 3-second timer to check whether data has been received from each sensor within the last 3 seconds. Logs a warning if a sensor is silent.
logger (optional, conditional)	Subscribes to /monitoring/summary and logs each message. Started only when a launch argument enable_logger is true.

Topics

Topic	Message Type	QoS
/sensors/temperature	std_msgs/msg/Float64	BEST_EFFORT, VOLATILE
/sensors/humidity	std_msgs/msg/Float64	BEST_EFFORT, VOLATILE
/sensors/battery	std_msgs/msg/Int64	RELIABLE, TRANSIENT_LOCAL
/monitoring/summary	std_msgs/msg/String	RELIABLE, VOLATILE

Specific Requirements

In addition to the common requirements in the main GP 1 page:

1. QoS – TRANSIENT_LOCAL: The battery_sensor must use TRANSIENT_LOCAL durability so that the aggregator receives the last battery reading even if it starts after the battery sensor. Demonstrate this by launching the aggregator 5 seconds after the sensors and verifying the first summary already contains a battery value.

2. QoS – Intentional mismatch: Create a second subscriber in the watchdog node that subscribes to /sensors/battery with RELIABLE reliability but VOLATILE durability. The battery publisher uses TRANSIENT_LOCAL. This is a compatible pairing (subscriber is less strict). Then create a third subscriber with TRANSIENT_LOCAL durability and BEST_EFFORT reliability connecting to the RELIABLE battery publisher – this is the incompatible pairing. Add comments documenting both cases.

3. Aggregator callback groups: The aggregator node must use a MultiThreadedExecutor. The three sensor subscription callbacks must be in a MutuallyExclusiveCallbackGroup (they all write to shared state: the latest sensor values). The summary publisher timer must be in a separate ReentrantCallbackGroup (it only reads the latest values and can run independently).

4. Watchdog: The watchdog node must track the timestamp of the last received message for each sensor. The 3-second timer callback checks whether time.time() - last_received > 3.0 for each sensor and logs a warning using self.get_logger().warn() if true. Use self.get_logger().info() when all sensors are reporting normally.

5. Launch files:

   • system.launch.py: starts all sensor nodes, the aggregator, and the watchdog.

   • enable_logger argument (default false): conditionally starts the logger node.

   • Sensor nodes grouped in a GroupAction.

Expected Behavior

Normal operation:

[INFO] [<timestamp>] [aggregator]: Summary -- temp: 23.4, humidity: 55.2, battery: 97
[INFO] [<timestamp>] [aggregator]: Summary -- temp: 24.1, humidity: 54.8, battery: 96
[INFO] [<timestamp>] [watchdog]: All sensors reporting normally.

Sensor goes silent – to test this, run one sensor separately with ros2 run in its own terminal instead of the launch file, then stop it with Ctrl-C:

# Terminal 1: launch the system without the temperature sensor
ros2 launch group<N>_gp1 system.launch.py start_temperature:=false

# Terminal 2: start the temperature sensor manually
ros2 run group<N>_gp1 temperature_sensor

# Press Ctrl-C in Terminal 2 to simulate a sensor failure

The watchdog should detect the silence and log:

[WARNING] [<timestamp>] [watchdog]: temperature_sensor has not reported in 3.0 s

Note

To support this testing workflow, add a start_temperature launch argument (default true) with an IfCondition on the temperature sensor node. This lets you exclude it from the launch file and run it manually in a separate terminal.

Verification commands:

ros2 launch group<N>_gp1 system.launch.py
ros2 launch group<N>_gp1 system.launch.py enable_logger:=true
ros2 topic list -t
ros2 topic echo /monitoring/summary
ros2 topic hz /sensors/temperature
ros2 topic info /sensors/battery -v
rqt_graph

Scenario 1 Grading Rubric

This rubric details how the 50 points from the common rubric map to Scenario 1 deliverables.

Component	Pts	Criteria
Node Implementation (16 pts)
temperature_sensor	2	Publishes Float64 on /sensors/temperature at 2 Hz with simulated random values. OOP node with timer callback.
humidity_sensor	2	Publishes Float64 on /sensors/humidity at 1 Hz with simulated random values.
battery_sensor	2	Publishes Int64 on /sensors/battery at 0.5 Hz with decrementing counter.
aggregator	4	Subscribes to all three sensor topics. Stores latest values. Publishes String summary on /monitoring/summary at 1 Hz.
watchdog	3	Subscribes to temperature and humidity. Tracks last-received timestamps. Logs warning (get_logger().warn()) if no data in 3 s. Logs info when all sensors are normal.
logger (conditional)	1	Subscribes to /monitoring/summary and logs each message. Only started when enable_logger:=true.
Spinning and lifecycle	2	Proper try/except/finally with rclpy.ok() guard in all entry points. Node classes separated from entry points.
QoS (10 pts)
Sensor topics QoS	3	Temperature and humidity use BEST_EFFORT/VOLATILE. Battery uses RELIABLE/TRANSIENT_LOCAL. Explicit QoSProfile objects (not integer shorthand).
TRANSIENT_LOCAL demo	3	Aggregator started after battery sensor still receives last battery value. Documented in README.md.
Intentional mismatch	4	Compatible pairing (RELIABLE/VOLATILE subscriber to RELIABLE/TRANSIENT_LOCAL publisher) works. Incompatible pairing (BEST_EFFORT/TRANSIENT_LOCAL subscriber to RELIABLE publisher) receives no data. Both documented with comments in code.
Launch Files (10 pts)
system.launch.py	4	Starts all sensor nodes, aggregator, and watchdog. All nodes use output="screen" and emulate_tty=True.
enable_logger argument	2	DeclareLaunchArgument with default false. Logger node uses IfCondition.
start_temperature argument	2	DeclareLaunchArgument with default true. Temperature node uses IfCondition. Enables manual testing of watchdog.
Sensor GroupAction	2	All three sensor nodes grouped in a GroupAction.
Executors and Callback Groups (8 pts)
Aggregator executor	2	Uses MultiThreadedExecutor.
Sensor subscriptions group	3	Three sensor callbacks in a MutuallyExclusiveCallbackGroup with comment explaining shared state protection.
Summary timer group	3	Summary publisher timer in a ReentrantCallbackGroup with comment explaining independence from sensor callbacks.
Documentation and Quality (6 pts)
README.md	3	Group members, scenario summary, node graph, design decisions, build/run instructions.
Code quality	3	Type hints, docstrings, ROS 2 logger with correct severity levels, consistent naming, no linting errors.
TOTAL	50