How to manage the game state in face of the EDT?

Question

I'm developing a real time strategy game clone on the Java platform and I have some conceptional questions about where to put and how to manage the game state. The game uses Swing/Java2D as rendering. In the current development phase, no simulation and no AI is present and only the user is able to change the state of the game (for example, build/demolish a building, add-remove production lines, assemble fleets and equipment). Therefore, the game state manipulation can be performed in the event dispatch thread without any rendering lookup. The game state is also used to display various aggregated information to the user.

However, as I need to introduce simulation (for example, building progress, population changes, fleet movements, manufacturing process, etc.), changing the game state in a Timer and EDT will surely slow down the rendering.

Lets say the simulation/AI operation is performed in every 500ms and I use SwingWorker for the computation of about 250ms in length. How can I ensure, that there is no race condition regarding the game state reads between the simulation and the possible user interaction?

I know that the result of the simulation (which is small amount of data) can be efficiently moved back to the EDT via the SwingUtilities.invokeLater() call.

The game state model seems to be too complex to be infeasible for just using immutable value classes everywhere.

Is there a relatively correct approach to eliminate this read race condition? Perhaps doing a full/partial game state cloning on every timer tick or change the living space of the game state from EDT into some other thread?

Update: (from the comments I gave) The game operates with 13 AI controlled players, 1 human player and has about 10000 game objects (planets, buildings, equipment, research, etc.). A game object for example has the following attributes:

World (Planets, Players, Fleets, ...)
Planet (location, owner, population, type, 
    map, buildings, taxation, allocation, ...)
Building (location, enabled, energy, worker, health, ...)

In a scenario, the user builds a new building onto this planet. This is performed in EDT as the map and buildings collection needs to be changed. Parallel to this, a simulation is run on every 500ms to compute the energy allocation to the buildings on all game planets, which needs to traverse the buildings collection for statistics gathering. If the allocation is computed, it is submitted to the EDT and each building's energy field gets assigned.

Only human player interactions have this property, because the results of the AI computation are applied to the structures in EDT anyway.

In general, 75% of the object attributes are static and used only for rendering. The rest of it is changeable either via user interaction or simulation/AI decision. It is also ensured, that no new simulation/AI step is started until the previous one has written back all changes.

My objectives are:

• Avoid delaying the user interaction, e.g. user places the building onto the planet and only after 0.5s gets the visual feedback
• Avoid blocking the EDT with computation, lock wait, etc.
• Avoid concurrency issues with collection traversal and modification, attribute changes

Options:

• Fine grained object locking
• Immutable collections
• Volatile fields
• Partial snapshot

All of these have advantages, disadvantages and causes to the model and the game.

Update 2: I'm talking about this game. My clone is here. The screenshots might help to imagine the rendering and data model interactions.

Update 3:

I'll try to give a small code sample for clarify my problem as it seems from the comments it is misunderstood:

List<GameObject> largeListOfGameObjects = ...
List<Building> preFilteredListOfBuildings = ...
// In EDT
public void onAddBuildingClicked() {
    Building b = new Building(100 /* kW */);
    largeListOfGameObjects.add(b);
    preFilteredListOfBuildings.add(b);
}
// In EDT
public void paint(Graphics g) {
    int y = 0;
    for (Building b : preFilteredListOfBuildings) {
        g.drawString(Integer.toString(b.powerAssigned), 0, y);
        y += 20;
    }
}
// In EDT
public void assignPowerTo(Building b, int amount) {
    b.powerAssigned = amount;
}
// In simulation thread
public void distributePower() {
    int sum = 0;
    for (Building b : preFilteredListOfBuildings) {
        sum += b.powerRequired;
    }
    final int alloc = sum / (preFilteredListOfBuildings.size() + 1);
    for (final Building b : preFilteredListOfBuildings) {
        SwingUtilities.invokeLater(=> assignPowerTo(b, alloc));            
    }
}

So the overlapping is between the onAddBuildingClicked() and distributePower(). Now imagine the case where you have 50 of these kind of overlappings between various parts of the game model.

Answer 1

This sounds like it could benefit from a client/server approach:

The player is a client - interactivity and rendering happen on that end. So the player presses a button, the request goes to the server. The reply from the server comes back, and the player's state is updated. At any point between these things happening, the screen can be re-painted, and it reflects the state of the game as the client currently knows it.

The AI is likewise a client - it's the equivalent of a bot.

The simulation is the server. It gets updates from its clients at various times and updates the state of the world, then sends out these updates to everyone as appropriate. Here's where it ties in with your situation: The simulation/AI requires a static world, and many things are happening at once. The server can simply queue up change requests and apply them before sending the updates back to the client(s). So as far as the server's concerned, the game world isn't actually changing in real time, it's changing whenever the server darn well decides it is.

Finally, on the client side, you can prevent the delay between pressing the button and seeing a result by doing some quick approximate calculations and displaying a result (so the immediate need is met) and then displaying the more correct result when the server gets around to talking to you.

Note that this does not actually have to be implemented in a TCP/IP over-the-internet sort of way, just that it helps to think of it in those terms.

Alternately, you can place the responsibility for keeping the data coherent during the simulation on a database, as they're already built with locking and coherency in mind. Something like sqlite could work as part of a non-networked solution.

Answer 2

Not sure I fully understand the behavior you are looking for, but it sounds like you need something like a state change thread/queue so all state changes are handled by a single thread.

Create an api, maybe like SwingUtilities.invokeLater() and/or SwingUtilities.invokeAndWait() for your state change queue to handle your state change requests.

How that is reflected in the gui I think depends on the behavior you are looking for. i.e. Can't withdraw money because current state is $0, or pop back to the user that the account was empty when the withdraw request was processed. (probably not with that terminology ;-) )

Answer 3

The easiest approach is to make the simulation fast enough to run in the EDT. Prefer programs that work!

For the two-thread model, what I suggest is synchronise the domain model with a rendering model. The render model should keep data on what came from the domain model.

For an update: In the simulation thread lock the render model. Traverse the render model updating where things are different from what is expected update the render model. When finished traversing, unlock the render model and schedule a repaint. Note that in this approach you don't need a bazillion listeners.

The render model can have different depths. At one extreme it might be an image and the update operation is just to replace a single reference with the new image object (this wont handle, for instance, resizing or other superficial interaction very well). You might not bother checking whether an item has change and just update eveything.

Answer 4

If changing the game state is fast (once you know what to change it to) you can treat the game state like other Swing models and only change or view the state in the EDT. If changing the game state is not fast, then you can either synchronize state change and do it in swing worker/timer (but not the EDT) or you can do it in separate thread that you treat similarly to the EDT (at which point you look at using a BlockingQueue to handle change requests). The last is more useful if the UI never has to retrieve information from the game state but instead has the rendering changes sent via listeners or observers.

Answer 5

Is it possible to incrementally update the game state and still have a model that is consistent? For example recalculate for a subset of planet/player/fleet objects in between renders/user updates.

If so, you could run incremental updates in the EDT that only calculate a small part of the state before allowing the EDT to process user inputs and render.

Following each incremental update in the EDT you would need to remember how much of the model remains to be updated and schedule a new SwingWorker on the EDT to continue this processing after any pending user inputs and rendering has been performed.

This should allow you to avoid copying or locking the game model while still keeping the user interactions responsive.

Answer 6

I think you shouldn't have World store any data or make changes to any objects itself, it should only be used to maintain a reference to an object and when that object needs to be changed, have the Player making the change change it directly. In this event, the only thing you need to do is synchronize each object in the game world so that when a Player is making a change, no other Player can do so. Here's an example of what I'm thinking:

Player A needs to know about a Planet, so it asks World for that Planet (how is dependent upon your implementation). World returns a reference to the Planet object Player A asked for. Player A decides to make a change, so it does so. Let's say it adds a building. The method to add a building to the Planet is synchronized so only one player can do so at a time. The building will keep track of its own construction time (if any) so the Planet's add building method would be freed up almost immediately. This way multiple players can ask for information on the same planet at the same time without affecting each other and players can add buildings almost simultaneously without much appearance of lag. If two players are looking for a place to put the building (if that is part of your game), then checking the suitability of a location will be a query not a change.

I'm sorry if this doesn't answer you're question, I'm not sure if I understood it correctly.

Answer 7

How about implementing a pipes and filters architecture. Pipes connect filters together and queue requests if the filter is not fast enough. Processing happens inside filters. The first filter is the AI engine while the rendering engine is implemented by a set of subsequent filters.

On every timer tick, the new dynamic world state is computed based on all the inputs (Time is also an input) and a copy inserted into the first pipe.

In the simplest case your rendering engine is implemented as a single filter. It just takes the state snapshots from the input pipe and renders it together with the static state. In a live game, the rendering engine may want to skip states if there are more than one in the pipe while if you're doing a benchmark or outputting a video you'll want to render every one.

The more filters you can decompose your rendering engine into, the better the parallelism will be. Maybe it is even possible to decompose the AI engine, e.g. you may want to separate dynamic state into fast changing and slow changing state.

This architecture gives you good parallelism without a lot of synchronization.

A problem with this architecture is that garbage collection is going to run frequently freezing all the threads every time, possible killing any advantage gained from multi-threading.

Answer 8

It looks like you need a priorityqueue to put the updates to the model on, in which updates frmo the user have priority over the updates from the simulation and other inputs. What I hear you saying is that the user always needs immediate feedback over his actions wheras the other inputs (simulation, otherwise) could have workers that may take longer than one simulation step. Then synchronize on the priorityqueue.