design question: topic vs service?

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I tend to consider Topics for use in any communication that is high speed or synchronous in nature. That communication can be either data or commands. But only commands that don't need a success/fail/etc. type of response.

I tend to consider Services only for commands that require an explicit response that needs to be acted upon. The tutorial for Services actually adds two numbers and returns the answer, so it is very useful in situations where you want a subroutine or a function that is standalone and callable by any local or remote ROS node. I would avoid using a service for commands that are high speed, though.

From the data receiver's perspective, topics are fire and forget communication, and are asynchronous by default - that is why callbacks need to be setup at receiving nodes.

Services are synchronous - data is requested, and then the receiver blocks until the data is received. While potentially this could be used for commands (with the data representing success and failure), the blocking nature makes this awkward in most use cases.

Basic commands, with no feedback required, are implemented using topics for their simplicity. Commands that require feedback/state tracking, and take a non-trivial time to complete, are implemented using actionlib.

Take a look at move_base in the navigation stack - a 'client' requesting the robot move would likely want to know the robot's progress throughout the move action, but would not want to halt operation (block) while waiting, hence actionlib. For a fire-and-forget movement command, the move_base_simple topic exists, but here you would have no idea if the robot ever successfully moves or not, and is not suitable for the complexities of most high-level planners.