Homework 1

Many of today's Internet of Things (IoT) devices and applications rely heavily on cloud-based servers. This exercise studies the potential use of such servers in embedded systems. To simplify experimentation, we use a server hosted by Yahoo that provides stock tick information.

The concrete deliverables to be uploaded to bCourses are highlighted below.

1. Download and install Ptolemy II from the SVN repository (do not use the 10.0 release, because I will be updating the software for use in this course throughout the semester, but if you have trouble with the SVN repository, you can do this assignment with the 10.0 release, which is trivial to install).  Instructions here.
2. Read Chapter 1 of Ptolemaeus, System Design, Modeling, and Simulation, a free PDF download and low-cost paperback. Pay particular attention to section 1.8 on models of computation.
3. Start Ptolemy II, browse the demos, and get familiar with the user interface. Chapter 2 of Ptolemaeus is a tutorial that may be useful.
4. Build a Ptolemy II model with an instance of the InteractiveShell actor and an instance of the StockTick accessor (see the accessor page Links to an external site. for background and the vision paper Download vision paper
   on accessors). To instantiate the StockTick accessor in Ptolemy II, in the File menu, choose Import Accessor and select the StockTick accessor, which gets downloaded from the accessor web page Links to an external site.. Set up this model so that you can enter a stock symbol in the interactive shell and display its current stock price.
5. Build at least two versions of this model with different directors, at a minimum SDFDirector and DEDirector. Annotate your models with comments on the particular features that are needed to work with these directors. Turn in your models. (Upload the XML files).
6. Examine how the StockTick accessor works. Specifically, double click on the accessor, select Preferences, and enable Expert Mode. Then double click on the accessor again and read and understand the script that the accessor runs. This is written in JavaScript, but you probably do not need any prior experience with JavaScript to understand it. Turn in a paragraph explaining how it works.
7. The StockTick accessor, when it fires, blocks until a web server somewhere in the cloud responds. Build a model that measures the response time of the server. Measure the response time over some reasonable interval and construct a histogram of response times. Be careful to not overwhelm the server with repeated requests, or you will skew your data. The synchronizeToRealTime parameter of the director that you choose can be used to ensure that the model makes queries to the server at well-defined intervals. Turn in your model and your histogram plot (well labeled, as a PDF file). Annotate your model with a brief explanation of how it works and any key features. The WallClockTime and HistogramPlotter actors might be useful. Compare the StockTick accessor to a similar model using the Weather accessor available at the same site.
8. The blocking nature of this accessor is potentially problematic. Any thread that fires this accessor cannot do anything else while it waits for a response. Suppose for example your model includes another submodel that responds to user input or performs some function every 50 milliseconds. Write a paragraph or two analyzing this problem. Does the choice of director provide a possible solution? Could the script in this accessor be modified in some way to work better? You may want to look at the documentation for the PNDirector and the ThreadedComposite actor for ideas, though these by no means represent the only possible approaches.