ROS Publishers

Before you see the code for simple_mover , it may be helpful to see how ROS Publishers work in Python.

Publishers allow a node to send messages to a topic, so that data from the node can be used in other parts of the ROS system. In Python, ROS publishers typically have the following definition format, although other parameters and arguments are possible:

pub1 = rospy.Publisher("/topic_name", message_type, queue_size=size)

The "/topic_name" indicates which topic the publisher will be publishing to. The message_type is the type of message being published on "/topic_name".

ROS publishing can be either synchronous or asynchronous:

• Synchronous publishing means that a publisher will attempt to publish to a topic but may be blocked if that topic is being published to by a different publisher. In this situation, the second publisher is blocked until the first publisher has serialized all messages to a buffer and the buffer has written the messages to each of the topic's subscribers. This is the default behavior of a rospy.Publisher if the queue_size parameter is not used or set to None .
• Asynchronous publishing means that a publisher can store messages in a queue until the messages can be sent. If the number of messages published exceeds the size of the queue, the oldest messages are dropped. The queue size can be set using the queue_size parameter.

Once the publisher has been created as above, a message with the specified data type can be published as follows:

pub1.publish(message)

For more information about ROS publishers, see the documentation here .

Assume that a queued message is typically picked up in an average time of 1/10th of a second with a standard deviation of 1/20th of a second, and your publisher is publishing at a frequency of 10Hz. Of the options below, which would be the best setting for queue_size ?

queue_size=None

queue_size=2

queue_size=10

queue_size=0

SOLUTION:

`queue_size=2`

Let's get started with simple_mover !