Why actions for long-running tasks in ROS - The Real Reasons

The Big Idea

Discover how to keep your robot in control even during long jobs!

The Scenario

Imagine you have a robot that needs to move to a far location. You send a command and wait without knowing if it is still moving or stuck.

The Problem

Manually tracking progress is hard. You might not know if the robot is still working, paused, or failed. This causes delays and confusion.

The Solution

ROS actions let you send a goal and get updates while the robot works. You can see progress, cancel tasks, and get results when done.

Before vs After

✗ Before

send_goal(); wait_for_result(); // no feedback or cancel

✓ After

send_goal(); on_feedback(update); on_result(done); cancel_goal();

What It Enables

It enables smooth control and monitoring of tasks that take time, making robots smarter and more responsive.

Real Life Example

A delivery robot navigating a building can report its location continuously and stop if obstacles appear.

Key Takeaways

Manual waiting hides progress and errors.

Actions provide feedback, results, and cancel options.

This improves control over long tasks in robotics.

Practice

1. Why are actions used in ROS for long-running tasks instead of simple service calls?

easy

A. Because actions do not require a goal to be set

B. Because actions are faster than service calls

C. Because actions only run once and then stop automatically

D. Because actions provide feedback and allow preemption during the task

Solution

1. Step 1: Understand the difference between service calls and actions

   Service calls in ROS are blocking and do not provide feedback during execution, making them unsuitable for long tasks.

2. Step 2: Identify the benefits of actions for long tasks

   Actions allow sending a goal, receiving continuous feedback, and can be preempted or canceled, which is essential for long-running tasks.

3. Final Answer:

   Because actions provide feedback and allow preemption during the task -> Option D

4. Quick Check:

   Actions give feedback and control = C [OK]

Hint: Actions = feedback + control for long tasks [OK]

Common Mistakes:

• Thinking actions are faster than services
• Believing actions don't need goals
• Assuming actions run only once without control

2. Which of the following is the correct ROS action message structure for a long-running task?

easy

A. Goal, Feedback, Result

B. Request, Response

C. Publisher, Subscriber

D. Service, Client

Solution

1. Step 1: Recall the components of a ROS action

   A ROS action consists of three parts: Goal (what to do), Feedback (progress updates), and Result (final outcome).

2. Step 2: Compare with other ROS communication types

   Request/Response is for services, Publisher/Subscriber is for topics, and Service/Client is a pair for services, not actions.

3. Final Answer:

   Goal, Feedback, Result -> Option A

4. Quick Check:

   Action message parts = Goal, Feedback, Result = A [OK]

Hint: Actions = Goal + Feedback + Result [OK]

Common Mistakes:

• Confusing actions with services
• Mixing up topics and actions
• Ignoring the feedback part

3. Consider this ROS action client code snippet:

client.send_goal(goal_msg)
while not client.wait_for_result(timeout=1.0):
    print('Working...')
result = client.get_result()
print(result)

What will this code do during the long-running task?

medium

A. Raise an error because wait_for_result does not accept timeout

B. Block immediately until the task finishes, then print the result

C. Print 'Working...' repeatedly until the task finishes, then print the result

D. Print 'Working...' once and then stop without printing the result

Solution

1. Step 1: Analyze the wait_for_result call with timeout

   The method waits up to 1 second for the result; if not ready, it returns False, so the loop continues printing 'Working...'.

2. Step 2: Understand the loop behavior

   The loop keeps printing 'Working...' every second until the action finishes, then it exits and prints the final result.

3. Final Answer:

   Print 'Working...' repeatedly until the task finishes, then print the result -> Option C

4. Quick Check:

   wait_for_result with timeout loops = D [OK]

Hint: wait_for_result(timeout) loops until done [OK]

Common Mistakes:

• Assuming wait_for_result blocks fully without timeout
• Expecting only one 'Working...' print
• Thinking wait_for_result with timeout causes error

4. You wrote this ROS action client code but it never prints the result:

client.send_goal(goal_msg)
client.wait_for_result()
print('Result:', client.get_result())

What is a likely reason for this behavior?

medium

A. The client must call send_goal repeatedly

B. The action server is not running or not responding

C. wait_for_result() requires a timeout argument

D. get_result() must be called before wait_for_result()

Solution

1. Step 1: Understand wait_for_result behavior

   wait_for_result() blocks until the server sends a result or the server is unavailable.

2. Step 2: Identify why it never prints

   If the action server is not running or not responding, wait_for_result() blocks forever, so print never executes.

3. Final Answer:

   The action server is not running or not responding -> Option B

4. Quick Check:

   Server missing causes wait block = B [OK]

Hint: No server = wait_for_result blocks forever [OK]

Common Mistakes:

• Thinking send_goal needs repeating
• Believing wait_for_result needs timeout
• Calling get_result too early

5. You want to implement a ROS action for a robot that cleans a room. The cleaning can be paused or stopped anytime, and you want to show progress updates. Which approach best uses ROS actions for this long-running task?

hard

A. Define an action with a goal to start cleaning, send periodic feedback with percent done, and allow preemption to pause or stop

B. Use a service call that blocks until cleaning finishes, then returns success

C. Publish a topic with cleaning commands and ignore feedback

D. Use a timer to run cleaning steps without feedback or control

Solution

1. Step 1: Identify requirements for the cleaning task

   The task is long-running, needs progress updates, and must support pause/stop (preemption).

2. Step 2: Match ROS communication types to requirements

   Actions support goals, feedback, and preemption, making them ideal for this use case. Services block and lack feedback, topics alone don't handle goals or preemption well.

3. Final Answer:

   Define an action with a goal to start cleaning, send periodic feedback with percent done, and allow preemption to pause or stop -> Option A

4. Quick Check:

   Actions fit long tasks with feedback and control = A [OK]

Hint: Use actions for control, feedback, and preemption [OK]

Common Mistakes:

• Using services for long tasks needing feedback
• Ignoring feedback and control with topics
• Not supporting preemption in design