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Expand Up @@ -5,150 +5,10 @@ In the realm of building dependable control systems, the importance of error det
**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](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.

If you are not familiar with ROS Services it is highly recommended that you take a look at [our page on ROS services]().
**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.
The alarm system's functionality is more intricate than that of a typical ROS service, which usually manages operations of base types (ints, strings, etc). In this scenario, the alarm's service server is engineered to manage the tasks of updating, querying, and processing alarm data. 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](https://github.com/uf-mil/mil/tree/master/mil_common/ros_alarms)

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