Exercise - Network Communication

For this exercise, we will explore a method for coordinating several Snap sketches using communication over a network.

Coordinating Multiple Robots

Programming multiple robots to work together requires coordinating their actions. This can be as simple as taking turns or as complex as combining sensor information from multiple robots. Timing is essential to dramatic performance, so it can be important to synchronize multiple robot actors tightly to move together or precisely take turns.

The simplest way to accomplish coordination is for humans to act as the eyes and ears of the robots and puppet them using keyboard commands, but it would be useful to automate elements of a performance. The Finch has very limited capabilities to physically communicate from robot to robot, but the control computers can much more easily pass messages back and forth using the network.

Try it out!

Before we go further in the discussion, let’s try this. The first example sketch demonstrates communication to control graphic sprites without needing any robots. Please read the comment in the Snap sketch and try out the steps to see how network communication works.

Discussion

If you have followed the notes in the sketch, then you should have been able to see that a set of (key, value) pairs can be shared among multiple computers using a server as an intermediary.

The complications of controlling robots with this approach revolve around carefully choosing what kind of data will be sent and by which robots. It takes time for each query, so the server can only be polled at a modest rate, no more than a few times a second. The amount of data is limited to a few sets of numbers or strings.

However, the robots can each be preprogrammed with script sequences, so the actual movements don’t need to be sent. The controlling computer need only act as a kind of conductor or director and indicate only which section to run and when. Since each computer has a local clock, this instructions can also specify actions to take place in the near future so each robot has time to fetch the current directions. We will see some of this in the next exercise with actual robots.

Terminology

There are several terms which will help us in future discussion.

Key and Value. A key is a kind of name, a text string used to identify a piece of data, i.e. a value, in a database. This approach is commonly called a key-value store or key-value database. In this example, the ‘value’ is a text string which can encode a set of numbers or strings.

Blackboard. A physical blackboard has the property than anyone in the room can write on it, and anyone can read it. A blackboard system is a programming idiom for using a common memory for coordinating multiple computers or programs. The communication system in this exercise can be considered a kind of online blackboard.

Code Commentary

It is instructive to examine the details of the [process message] command block. If you right-click on an instance, you can select edit.. to see the code.

The main structure is a for-each-of loop which iterates once for each line of text from the server response. The split operator is used to divide the response text into a list of strings, one per line. On each iteration of the loop, the line script variable holds one text line.

This line is divided again with split, this time dividing the line at each space into a list of strings, and storing the result in record. The first item in this list is the key string, stored in key. The list of one or more value items is stored in value. The number of value items depends upon the data format for the given key; in this example, most values have only a single item, but the xy_position value is a pair of numbers.

Following this data dissection is a set of if-then-else commands to determine what to do with the key-value pair. In this example, each outcome is simply to store the value data in a particular global variable, but other actions are also possible such as immediately issuing a robot command.

Note that the keys can come from the server in any order and may include unrecognized new keys. This scheme handles these cases without breaking.

Challenges

1. Can you add a new key-value pair to the blackboard? Be sure to update the [process message] block to correctly interpret the value.

Garth Zeglin, Personal Robotics Lab, Carnegie Mellon University