Implementation of achievement systems in modern, complex games

Question

Many games that are created these days come with their own achievement system that rewards players/users for accomplishing certain tasks. The badges system here on stackoverflow is exactly the same.

There are some problems though for which I couldn't figure out good solutions.

Achievement systems have to watch out for certain events all the time, think of a game that offers 20 to 30 achievements for e.g.: combat. The server would have to check for these events (e.g.: the player avoided x attacks of the opponent in this battle or the player walked x miles) all time.

• How can a server handle this large amount of operations without slowing down and maybe even crashing?

Achievement systems usually need data that is only used in the core engine of the game and wouldn't be needed out of there anyway if there weren't those nasty achievements (think of e.g.: how often the player jumped during each fight, you don't want to store all this information in a database.). What I mean is that in some cases the only way of adding an achievement would be adding the code that checks for its current state to the game core, and thats usually a very bad idea.

• How do achievement systems interact with the core of the game that holds the later unnecessary information? (see examples above)

• How are they separated from the core of the game?

My examples may seem "harmless" but think of the 1000+ achievements currently available in World of Warcraft and the many, many players online at the same time, for example.

Answer 1

Achievement systems are really just a form of logging. For a system like this, publish/subscribe is a good approach. In this case, players publish information about themselves, and interested software components (that handle individual achievements) can subscribe. This allows you to watch public values with specialised logging code, without affecting any core game logic.

Take your 'player walked x miles' example. I would implement the distance walked as a field in the player object, since this is a simple value to increment and does not require increasing space over time. An achievement that rewards players that walk 10 miles is then a subscriber of that field. If there were many players then it would make sense to aggregate this value with one or more intermediate broker levels. For example, if 1 million players exist in the game, then you might aggregate the values with 1000 brokers, each responsible for tracking 1000 individual players. The achievement then subscribes to these brokers, rather than to all the players directly. Of course, the optimal hierarchy and number of subscribers is implementation-specific.

In the case of your fight example, players could publish details of their last fight in exactly the same way. An achievement that monitors jumping in fights would subscribe to this info, and check the number of jumps. Since no historical state is required, this does not grow with time either. Again, no core code need be modified; you only need to be able to access some values.

Note also that most rewards do not need to be instantaneous. This allows you some leeway in managing your traffic. In the previous example, you might not update the broker's published distance travelled until a player has walked a total of one more mile, or a day has passed since last update (incrementing internally until then). This is really just a form of caching; the exact parameters will depend on your problem.

Answer 2

You can even do this if you don't have access to source, for example in videogame emulators. A simple memory-scan tool can be written to find the displayed score for example. Once you have that your achievement system is as easy as polling that memory location every frame and seeing if their current "score" or whatever is higher than their highest score. The cool thing about videogame emulators is that memory locations are deterministic (no operating system).

Answer 3

There are two ways this is done in normal games.

1. Offline games: nothing as complex as pub/sub - that's massive overkill. Instead you just use a big map / dictionary, and log named "events". Then every X frames, or Y seconds (or, usually: "every time something dies, and 1x at end of level"), you iterate across achievements and do a quick check. When the designers want a new event logged, it's trivial for a programmer to add a line of code to record it.

NB: pub/sub is a poor fit for this IME because the designers never want "when player.distance = 50". What they actually want is "when player's distance as perceived by someone watching the screen seems to have travelled past the first village, or at least 4 screen widths to the right" -- i.e. far more vague and abstract than a simple counter.

In practice, that means that the logic goes at the point where the change happens (before the event is even published), which is a poor way to use pub/sub. There are some game engines that make it easier to do a "logic goes at the point of receipt" (the "sub" part), but they're not the majority, IME.

1. Online games: almost identical, except you store "counters" (int that goes up), and usually also: "deltas" (circular buffers of what's-happened frame to frame), and: "events" (complex things that happened in game that can be hard-coded into a single ID plus a fixed-size array of parameters). These are then exposed via e.g SNMP for other servers to collect at low CPU cost and asynchronously

i.e. almost the same as 1 above, except that you're careful to do two things:

• Fixed-size memory usage; and if the "reading" servers go offline for a while, achievements won in that time will need to be re-won (although you usually can have a customer support person manually go through the main system logs and work out that the achievement "probably" was won, and manually award it)
• Very low overhead; SNMP is a good standard for this, and most teams I know end up using it

Answer 4

If your game architecture is Event-driven, then you can implement achievements system using finite-state machines.