Why actions for long-running tasks in ROS - Quick Recap

Recall & Review

beginner

What is the main reason to use actions in ROS for long-running tasks?

Actions allow a robot to perform tasks that take time while still providing feedback and the ability to cancel or monitor progress.

beginner

How do actions differ from services in ROS?

Services are for quick, immediate requests and responses, while actions handle tasks that take longer and need ongoing feedback or cancellation.

beginner

What feature of ROS actions helps users know the progress of a task?

Actions provide feedback messages during execution to show the current status or progress of the task.

intermediate

Why is the ability to cancel a task important in long-running ROS actions?

It lets the system stop a task if conditions change or if the task is no longer needed, saving time and resources.

intermediate

What happens if you try to use a service for a long-running task in ROS?

The system may block or freeze waiting for the service to finish, making it hard to get updates or cancel the task.

What ROS feature is best for tasks that take a long time and need progress updates?

A Actions

B Services

C Topics

D Parameters

Actions support long-running tasks with feedback and cancellation, unlike services or topics.

Which of these is NOT a benefit of using ROS actions for long tasks?

A Ability to cancel the task

B Feedback on task progress

C Immediate response without waiting

D Handling tasks asynchronously

Actions do not provide immediate completion; they handle tasks over time with feedback.

If you want to stop a running task in ROS before it finishes, what should you use?

A Stop the service call

B Change a parameter

C Unsubscribe from the topic

D Cancel the action goal

Actions allow canceling goals to stop long-running tasks safely.

Why might using a service for a long task cause problems?

A It automatically cancels tasks

B It blocks until the task finishes

C It cannot send requests

D It provides too much feedback

Services block the caller until the task completes, which is not good for long tasks.

What kind of message do ROS actions send to show task progress?

A Feedback messages

B Result messages

C Request messages

D Parameter updates

Feedback messages keep the client informed about the ongoing task status.

Explain why ROS actions are preferred over services for long-running tasks.

Think about how you want to control and monitor a task that takes time.

Describe how feedback and cancellation improve the use of long-running tasks in ROS actions.

Imagine you are waiting for a robot to finish a job and want to know how it is doing or stop it if needed.

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