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Running the Distributed Algorithms

Overview

The distributed algorithms can be run in four ways:

  • Browser
  • Appletviewer
  • Networked computers
  • Loopback on a single computer using address 127.0.0.1

The Java applets are identical in all modes and only the HTML files vary between modes. Sample HTML files are included in the distribution.

These applets are not animations; rather, they are interactive state machines that display the execution of distributed algorithms. The content of each applet is identical:

  • Two lines of data for node represented by this applet. The first line displays the state of the node and the second line lists prompts for outstanding actions that must be performed.
  • Two similar lines of data for each of the other nodes.
  • A sequence of buttons for choosing a step of the algorithm.
  • The nodes are color-coded: the color of the first pair of lines of an applet containing the data of the node that it represents is also used to color the pairs of lines in other nodes containing data of the node.

Interactive execution of an applet

Warning: If the applets do not start executing or crash while running, it is almost certainly the result of a bug or incompatibility in the compiler, appletviewer or browser. Try using an alternate mode (appletviewer instead of browser, or conversely), or a different version of the JDK or browser.

The sample HTML files start applets for each of four nodes. The file for the browser creates a 2x2 table and runs an applet in each cells. The file for the appletviewer creates a separate window for each of the four applets, and it is convenient to tile the screen 2x2.

The reset button in the top right corner will return all the applets to their initial state. (This has been implemented only for the non-distributed mode of execution.) When running with the appletviewer you can terminate the applets by interrupting the appletviewer (ctrl-C from the command window), or by choosing Quit on one of the applet menus.

To perform a step of the algorithm, click on buttons in one of the applets. This is how you specify the interleaved sequence of instruction execution. Note:

  1. As the state of the algorithm at a node changes, an appropriate set of buttons is displayed.
  2. An instruction may require multiple clicks to perform.
  3. Some buttons may not be active. It is a test of your understanding of the algorithm that you not click on a non-active node, though if you do so, nothing happens and you can analyze why the button is not relevant.
  4. There are prompts to remind you what messages to send.
  5. Even some of the accepted transitions may not be "correct".

We illustrate each of these items with reference to the Byzantine Generals algorithm:

  1. A node must initially choose attack or retreat before participating in the exchange of messages. Only then do the buttons for message passing appear.
  2. When sending a message from one node to another, you must specify: (i) the address of the destination node, (ii) the name of the general "about whom" the node is expressing an opinion, and (iii) the opinion attack or retreat. Thus each step of the algorithm requires three clicks.
  3. You can only send an opinion about another node if you have already received that node's plan. If not, the address button will be inactive.
  4. The prompts list which messages must be sent or relayed to other nodes.
  5. The BG algorithm works correctly only if there is at most one traitor. However, you can have two nodes give traitorous messages and specify an execution sequence that causes an incorrect state to be reached when the algorithm terminates.

HTML parameters
An HTML file must contain an applet invocation for each node in the algorithm. The required Name parameter assigns a sequence number to each applet, and it is recommended that you specify the standard parameters WIDTH and HEIGHT so that the applets will fit comfortably on your screen.

<HTML>
<TITLE>Byzantine Generals</TITLE>
<BODY>
  <APPLET CODE="BG.class" NAME="0" WIDTH=350 HEIGHT=200></APPLET>
  <APPLET CODE="BG.class" NAME="1" WIDTH=350 HEIGHT=200></APPLET>
  <APPLET CODE="BG.class" NAME="2" WIDTH=350 HEIGHT=200></APPLET>
  <APPLET CODE="BG.class" NAME="3" WIDTH=350 HEIGHT=200></APPLET>
</BODY>
</HTML>

The following parameters are optional:

Parameter name     Meaning Default
NUMBER Total number of applets 4
DISTRIBUTED Networked communications    false
FONTSIZE Size of font 12
NODEi Name of node i John, Zoe, Leo, Basil
COLORi Color of node i cyan, green, yellow, lightgray
IPi IP address of node i 127.0.0.1

Usually it is easier to change the source code and recompile than to change the HTML parameters of all the nodes!
For example, to use white background for all nodes just change one line (supplied as a comment) in the source code of method getParms in class DistAlg.