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...

Thursday, August 1, 2013

Harmonic: Game replication and git?

So I took some time off of trying to implement game replication. Instead, I did some other stuff (like university), and gathered some experience with git. With time, I realized that git, and distributed version control in general, provides exactly those concepts needed for the model of replication I wanted. Looking back at an earlier post on that topic, I can't really say how I expected it to work out. In the end, it turned out that transmitting "unfinished transactions" was a really bad idea; in the end, the very essence of a transaction is that it executes either as a whole or not at all.

Well, anyway, now I had experience with git, and it contained exactly those concepts I needed for my replication as well:
  • a repository of files (objects) in which changes are tracked
  • a history of commits, where commits have a parent/child relation. The main attribute of that relation is what changes happened between those commits
  • branches which point at specific commits and provide a comfortable way of traversing and manipulating the history
  • definition of remote repositories for synchronization of branches
In contrast to the transaction approach I pursued earlier, where the parent/child relationship is in regard to application logic and does not really add to the synchronization capabilities of the framework, this version control approach is based around a tree of commits, where the parent/child relationship is a temporal one, and essential to the framework. The framework is less cluttered with concepts that are not important to it.

Of course, a few things are different: a git repository manages files, essentially byte sequences, while a replication framework manages objects. While these could be interpreted as byte sequences, it doesn't really add to the usability, because objects are usually manipulated on the level of fields and methods. Furthermore, files are organized in a file system with human-chosen names for every single file. Objects simply reside at some location in memory not even the program can control; some other sort of identification is needed. And finally, for file versioning, a human user can try to resolve conflicts. For replication, this is not possible. Conflicts can't be resolved automatically, and as different object store different data, even manual merging could result in inconsistent state.

Luckily, besides the last problem, these can be solved. And for the last problem, usually it shouldn't be a problem in the first place, at least when talking about Magic. Magic is not a game of reaction time, where all players try to do something at the same time, and in the end someone was the earliest. Magic's system of priority defines very clearly which player may take an action at what point, so the game logic can ensure that conflicts do not arise, simply by obeying the rules.

You can take a look at Harmonic if you want. I'm not happy with the name, but I couldn't come up with anything better. I wanted it to sound like multiple independent systems are in sync, like vibrating at the same frequency, or looking the same; I don't think I succeeded, but that's okay as long as it works.

An Entity compares to a file. Entities are objects managed by Harmonic and are identified by a numeric ID. IDs are created by simply incrementing a counter, so creating IDs in different instances of Harmonic is consistent.

An Engine compares to a repository; it is a store of entities. An Engine also manages the different states (analogous to commits), and its current state: in a repository, the files always reflect one specific commit. In the engine, the entities always reflect one state, and the engine knows how to move between the states. Engines are identified with random IDs; as engines are generated on different machines, there is no way to ensure that engine IDs are always different. So, random IDs are used.

States, as mentioned above, compare to commits. Git uses random commit IDs - at least they look random. In Harmonic, the engine ID is part of the state ID. Assuming the engine IDs are different, it's enough to simply increase the ID for each subsequently created state.

Last but not least, Actions and Modifications correspond to the changes that make up a commit. A modification is a single change that is easily reverted, like setting a variable (or creating a new entity ID). An action is a sequence of modifications, so the action can be reverted by reverting the modifications making it up.

In this model, states, containing actions, are transmitted between engines, but modifications are not. Actions are defined by the application, so they may be as small as needed and as big as possible. The modifications making up the action are only created during execution, so they don't need to be transmitted, making large actions more efficient. For example, instead of transmitting that a card is tapped and mana is added to a mana pool, it's now enough to transmit that a mana ability of a land was played.

I like the direction where this is going. I think distributed source control is an elegant concept, and I don't think I did a bad job at catching its essence, so this should turn out to be pretty usable. In fact, while developing Harmonic, I also wrote a game of Tic Tac Toe that uses it, so I have a proof of concept that Harmonic really works as intended in a turn based game.