Create alarms.md

docs/reference/alarms.md

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+# ROS Alarms
+
+In the realm of building dependable control systems, the importance of error detection and effective error-handling mechanisms cannot be overstated. Within this context, MIL presents a robust solution in the form of a live alarm system. This alarm system operates discreetly in the background of both the robot's mission and driver codebases, ready to be activated upon the emergence of errors. Notably, the alarm code doesn't solely serve to identify and address errors; it can also adeptly manage changes or updates that extend beyond error scenarios.
+
+**ROS Alarms: A Service-Oriented Architecture**
+
+The architecture of ROS alarms distinguishes itself by employing a service-oriented model rather than the usual topic-based approach. In ROS, Services act as the conduits for interaction between nodes, functioning in a request-response manner. While ROS topics enable asynchronous data exchange, services facilitate nodes in seeking specific actions or information from other nodes, awaiting a subsequent response before proceeding. This method of waiting before proceeding is known as a synchronous data exchange. This proves especially valuable in tasks that require direct engagement, such as data retrieval or computations.
+
+**Components of a ROS Service**
+
+A ROS service structure is bifurcated into two components:
+
+1. **Server**: This node provides the service and waits for incoming requests from other nodes. In simpler terms, it carries out the operation you have specified.
+
+2. **Client**: This node sends requests to the service server and waits for the response.
+
+Here is the official ROS documentation on services: https://wiki.ros.org/Services
+
+For a concrete illustration, consider the case of a service designed to query and update an array of integers:
+
+1. **Define the Service Messages**
+
+Create two separate service message files in your ROS package's `srv` folder:
+
+- `ArrayUpdate.srv`:
+
+```
+# ArrayUpdate.srv
+int32 index
+int32 value
+---
+bool success
+```
+
+- `ArrayQuery.srv`:
+```
+# ArrayQuery.srv
+int32 index
+---
+int32 value
+```
+
+2. **Implement the Service Server**
+
+Create a Python script named `array_service_server.py`:
+
+```python
+#!/usr/bin/env python
+
+import rospy
+from your_package_name.srv import ArrayUpdate, ArrayQuery
+from std_msgs.msg import Int32MultiArray
+
+class ArrayServiceServer:
+    def __init__(self):
+        self.array_data = [10, 20, 30, 40, 50]
+
+    def handle_array_update(self, request):
+        if 0 <= request.index < len(self.array_data):
+            self.array_data[request.index] = request.value
+            return ArrayUpdateResponse(True)
+        else:
+            return ArrayUpdateResponse(False)
+
+    def handle_array_query(self, request):
+        if 0 <= request.index < len(self.array_data):
+            return ArrayQueryResponse(self.array_data[request.index])
+        else:
+            return ArrayQueryResponse(-1)  # Invalid index
+
+def main():
+    rospy.init_node('array_service_server')
+    server = ArrayServiceServer()
+
+    rospy.Service('array_update', ArrayUpdate, server.handle_array_update)
+    rospy.Service('array_query', ArrayQuery, server.handle_array_query)
+
+    rospy.spin()
+
+if __name__ == '__main__':
+    main()
+```
+
+Replace `your_package_name` with the actual name of your ROS package.
+
+3. **Implement the Service Clients**
+
+Create two separate Python scripts for interacting with the service:
+
+- `array_update_client.py`:
+```python
+#!/usr/bin/env python
+
+import rospy
+from your_package_name.srv import ArrayUpdate
+
+def array_update_client(index, value):
+    rospy.wait_for_service('array_update')
+    try:
+        array_update = rospy.ServiceProxy('array_update', ArrayUpdate)
+        response = array_update(index, value)
+        return response.success
+    except rospy.ServiceException as e:
+        print("Service call failed:", e)
+
+if __name__ == '__main__':
+    rospy.init_node('array_update_client')
+
+    index_to_update = 2
+    new_value = 35
+    success = array_update_client(index_to_update, new_value)
+
+    if success:
+        print(f"Value at index {index_to_update} was updated to {new_value}")
+    else:
+        print(f"Failed to update value at index {index_to_update}")
+```
+
+- `array_query_client.py`:
+```python
+#!/usr/bin/env python
+
+import rospy
+from your_package_name.srv import ArrayQuery
+
+def array_query_client(index):
+    rospy.wait_for_service('array_query')
+    try:
+        array_query = rospy.ServiceProxy('array_query', ArrayQuery)
+        response = array_query(index)
+        return response.value
+    except rospy.ServiceException as e:
+        print("Service call failed:", e)
+
+if __name__ == '__main__':
+    rospy.init_node('array_query_client')
+
+    index_to_query = 2
+    value = array_query_client(index_to_query)
+
+    if value != -1:
+        print(f"Value at index {index_to_query}: {value}")
+    else:
+        print(f"Invalid index {index_to_query}")
+```
+
+Replace `your_package_name` with your actual ROS package name.
+
+**Alarm System Logic**
+
+The alarm system's functionality is more intricate than the preceding example. In this scenario, the server is engineered to manage not numeric calculations, but the tasks of updating, querying, and processing alarm data. Similar to the model above, ROS alarms encompass two distinct types of clients: the alarm broadcaster and the alarm listener. The broadcaster initializes and triggers alarms in response to errors or changes, while the listener monitors the broadcaster's activity and activates designated a callback function when alarms are raised. The callback function should handle the error or change appropriately. 
+
+To peruse the detailed alarm system code, refer to the repository: [https://github.com/uf-mil/mil/tree/master/mil_common/ros_alarms]
+
+To successfully leverage alarms, the initialization of both the broadcaster and listener is needed. The listener should be configured to execute a predefined callback function, addressing errors or changes detected by the broadcaster. Within your codebase, error detection and alarm-raising procedures should be integrated. If orchestrated correctly, the callback function will be automatically invoked, underscoring successful error mitigation.
+
+For a practical example of this workflow, visit: [https://github.com/uf-mil/mil/blob/master/mil_common/ros_alarms/test/rospy/callback_test.py]
+
+**Applications and Context**
+
+The applications of ROS alarms span various contexts, with one notable application residing in the control of the submersible vehicle's thrust and killboard. The thrust and killboard, responsible for the sub's electronic operations, is integrally associated with ROS alarms. Upon the board's activation or deactivation (hard or soft kill), alarms are invoked to apprise users of these changes. The listener's callback function comes into play, ensuring that alarms are updated in alignment with the board's current state. This process, triggered each time the board is deactivated, creates a system whereby users are continually informed about the board's status changes – essentially manifesting a dynamic live alarm system.
+
+To delve into the implementation, visit: [https://github.com/uf-mil/mil/blob/master/SubjuGator/drivers/sub8_thrust_and_kill_board/sub8_thrust_and_kill_board/handle.py]