Thursday, August 8, 2013


If you read my previous post and/or looked at my GitHub account, you already know that I'm programming a small game of TicTacToe to demonstrate and debug my state synchronization library, Harmonic. At first, I had two engines, where one actually played the game, and the other simply listened. Both engines were located in the same JVM, so identifying them was easy.

At that time, I didn't even have states yet, so the communication exchanged the actual action. This was a problem, because although both engines executed the same actions, there was no way that the application could really know. Only later, when the communication consisted of states, this could really be called synchronization.

The real fun began when I added a notion of branches. A branch points at a state, and can move to other states as well. An engine can publish changes to a branch, and the receiving engine will then ask for the missing states. Branches also make it possible to delve into alternate histories, for example for AI.

Although states and actions are defined as protobuf messages, nothing I described so far contained any networking or IO code. The BranchManager class contains three methods that do communication, but all they do is call a callback method and leave the actual networking to whatever the implementation of that callback sees fit. I added an implementation using JGroups; the same library I used in my last attempt.

And now, I have a graphical multiplayer Tic Tac Toe. Granted, it does not enforce that every client plays only his own pieces, but that's not the point and would be easy to do; it just requires a more sophisticated new-game negotiation. What does work out of the box is spectating. A newly started client does not yet ask for the current state, but as soon as one of the playing clients performs an action, the spectator is also updated.

Right now, to try it, you have to build Polybuf, Harmonic and TicTacToe yourself, which is not optimal. TicTacToe is meant to help me experiment, find out what's good practice and what needs improvement. When it comes to distribution, applications have different requirements than libraries, and fulfilling these requirements is the next thing on my schedule.

Okay, that's fixed now! I have added a git repository that contains a Maven repository. That means you can reference my libraries (for example, in building TicTacToe), and the dependencies can be downloaded from there, without the need for you to build them yourself.

The repository also contains a TicTacToe-0.1.0-jar-with-dependencies.jar, which... well. It also has a MainClass attribute, so it should be reasonably double-clickable. I have only tested it on one machine, and I don't know how the default JGroups-Stack behaves in LANs and WANs, but fixing issues arising from that is a matter of passing a configuration to a constructor...

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