CS333 Lab T-1:
Configuring Asynchronous Systems

Goals of this lab:


An asynchronous distributed system consists of a set of processors that run at their own speeds, do not share a common clock, do not have synchronized clocks, and do not share any memory. All communication among the processors occurs by message-passing, and there is no predictable upper bound on the time it takes for the communication network to deliver a message.

With The Programmers' Playground, we construct distributed applications from collections of modules running on possibly different processors. Each module has an an external data interface called the presentation through which it interacts with its environment (the outside world).

Communication among Playground modules is established by a configuration consisting of logical connections among the presentation items of different modules. If a module writes a value into one of its presentation entries, then the value is sent to each connected module in accordance with the logical configuration and the presentation entries at those modules are updated accordingly. The configuration is dynamic, meaning that logical connections may be added and removed while the modules are running.


Read over the entire assignment before starting. Then, follow the instructions step by step.

  1. Getting Started: As a first-time Playground user, you should follow the set-up instructions before proceeding with Step 2.

  2. Starting the Connection Manager: At the beginning of each Playground session, you will start up a Playground connection manager graphical interface. If you haven't already done so,
    1. Create an xterm window.
    2. In the other xterm window, type PGcmgui to start up the connection manager front-end.
    An empty window should pop up on your screen. It is empty because you have no other Playground modules running.

  3. Starting Some Playground Modules: In order to experiment with configurations, you need to start up some Playground modules. We have provided several Playground modules that do some simple calculations. Create several more xterm windows (possibly running on various machines). Start each of the following Playground applications running by typing their names in different xterm windows.
    This module has one input/output presentation entry that is incremented approximately once per second.
    This module has one input presentation item. It just prints the value of the variable to the terminal whenever the value is changed externally.
    This module reads let's you enter non-negative integer values and stores them into a single output presentation variable.
    This module has one input presentation variables in and one output presentation variable out, both integers. Whenever the input is changed externally, the output value is updated to be the square the input.
    This module has two input presentation variables i1 and i2, and one output presentation variable out, all integers. Whenever the input numbers are changed externally, the output value is updated to be the product of the inputs.

  4. Configuration: At this point, you should see boxes in the graphical connection manager front-end corresponding to the modules you started up in Step 3. Notice that each module has a name and one or more presentation entries. However, nothing interesting will happen until logical connections are established so that the modules can begin communicating with each other.

    To create a logical connection in the connection manager user interface, just drag an arrow from the output port (circle) at the presentation entry of one module to the name of the presentation entry of the other module while holding the left mouse button. A bidirectional connection is created the same way, but the middle mouse button is held. To delete a connection, pick on the connection line with the right mouse button. Modules may be repositioned on the display by dragging, and may be rotated by clicking on their names.

    Complete each of the following configuration exercises. Keep lab notes that describe what you did and what happened to the next class. (You will hand in all of your notes, including anything that didn't go as you had expected.)

    1. Create a connection from the counter to the consumer. Look at the shell in which the consumer is running. Write down what is happening. Now delete the connection and write down what happens. Try putting it back again. Does computation continue even when there are no connections?
    2. Create a connection from the constant to the counter. Enter a number into the constant module. (To edit the number, double click on it. Then you'll see a blinking edit cursor for editing the usual way. To unselect stop editing the number, press return or click elsewhere in the window.) What happens in the consumer shell? Offer an explanation.
    3. Using the square module, create a configuration that causes the consumer to print out all the squares of the positive integers. Draw the configuration in your notes.
    4. Now, instead of using the square module, try to achieve the same effect (as in exercise 3 above) by using two counter modules, a constant module (connected to both), a product module, and a consumer module. Draw the configuration before you try it. Does it work? Try it with a single counter and a product module. Does that work? Offer a possible explanation for the behaviors you have observed.

Turn in your lab notes to receive credit for completing this tutorial.