How do you design a scalable and reliable ROS communication architecture?

1 What is ROS communication?

ROS communication is based on the publish-subscribe pattern, where nodes can publish data to topics, and subscribe to topics to receive data. Topics are named channels that carry messages, which are data structures that define the content of the communication. For example, a node can publish a sensor_msgs/Image message to the /camera/image_raw topic, and another node can subscribe to that topic to receive the image data. ROS communication is asynchronous, meaning that the publisher and subscriber do not need to be synchronized or aware of each other.

2 What are ROS services?

ROS services are another way of communication in ROS, where nodes can request and provide services to each other. Services are similar to topics, but they use a different message type, called srv, which defines the request and response fields of the service. For example, a node can request a service to move the robot to a certain position, and another node can provide that service by sending back a confirmation. ROS services are synchronous, meaning that the requester and provider need to be connected and wait for each other.

3 What are ROS actions?

ROS actions are a higher-level form of communication in ROS, where nodes can perform long-running tasks that can be monitored and canceled. Actions are similar to services, but they use a different message type, called action, which defines the goal, feedback, and result fields of the action. For example, a node can send a goal to move the robot to a certain position, and another node can execute that action by sending back feedback and result messages. ROS actions are asynchronous, meaning that the action client and server do not need to block each other.

4 How to use namespaces and remapping in ROS?

Namespaces and remapping are two techniques that can help you organize and customize your ROS communication system. Namespaces are prefixes that can be added to topic, service, and action names to create hierarchical structures and avoid name collisions. For example, you can use the /robot1 namespace to group all the topics, services, and actions related to the first robot. Remapping is a way of changing the name of a topic, service, or action at runtime, without modifying the code. For example, you can remap the /camera/image_raw topic to /robot1/camera/image_raw to make it consistent with the namespace.

5 How to use multimaster in ROS?

Multimaster is an advanced feature of ROS that allows you to run multiple ROS masters on different machines or networks, and synchronize them using multimaster tools. A ROS master is a central node that manages the registration and lookup of topics, services, and actions. By default, ROS only supports one master per system, which can limit the scalability and reliability of your ROS communication system. Multimaster can help you overcome this limitation by enabling distributed and redundant communication across multiple masters. For example, you can use multimaster to connect two robots that run their own masters, and share data and services between them.