Actions: Goal, Feedback, Result

Services are request/response.

Actions are designed for work that can take time.

The official beginner action tutorial defines three major parts:

GOAL
  ↓
FEEDBACK while work continues
  ↓
RESULT when work completes

Actions are also cancelable.

Turtlesim example

The teleop node can send rotation goals to Turtlesim.

The tutorial demonstrates keys that start rotation goals and a key that cancels a rotation before completion.

This is different from continuously publishing velocity commands.

Client/server roles

ACTION CLIENT
   -- sends goal -->
ACTION SERVER
   -- feedback -->
ACTION CLIENT
   -- final result -->

A server may also reject or abort goals depending on its behavior.

Why not a service?

A service normally gives one response after a request. An action adds a lifecycle around longer-running work and exposes progress/cancel behavior.

Checkpoint

Classify these:

• rotate to a target heading;
• stream wheel velocity continuously;
• clear a screen once.

Explain why each maps to action/topic/service.