Using throttling to reduce network errors

Recently we managed to reduce the number of network errors in Awesomenauts by 10% by improving our throttling algorithm. While automatic throttling by a router is killing, smart throttling by the game itself can be a good tool to make the game work better on crappy internet connections. Today I would like to explain what throttling is and how we approached this topic.

(Note that the throttling improvements were in Awesomenauts patch 2.5.3. This post is unrelated to the bandwidth optimisations in patch 2.5.4 that are currently being tested in beta.)

The basic idea of throttling is that if you detect that an internet connection cannot handle as much as you are sending, then you start sending less. This can happen on the side of the game, or on the side of the connection itself (by the modem or router, for example). If we keep sending more than the connection can handle, then either we lose a lot of packets or, even worse, the internet connection is lost altogether, causing a network error. Throttling is intended to keep this from happening.



The basic idea may sound simple enough, but actually implementing this is a lot more difficult. There are two problems: how can we reduce bandwidth dynamically, and how can we detect whether we need to throttle?

Lets start with how to reduce bandwidth. Modems usually have an approach to this that is simple enough: they just throw away some packets. This is a very efficient way of reducing bandwidth, but completely killing to any game. If a packet with important data is dropped then it needs to be resent. We cannot do this immediately as the internet connection doesn't notify the game that a packet was dropped. The only thing we can do is just wait for the acknowledgement to come in. If after a while we still haven't received an acknowledgement, then we conclude that the packet has probably been lost and needs to be resent.

Resending based on acknowledgements is the best we can do, but it is a pretty imperfect solution: the acknowledgement might still be under way. We cannot know whether it is, so we just need to pick a duration and decide to resend if that much time has passed. If we set this duration too long, then it takes very long before the packet is resent, causing extra delay in the gameplay if the packet was really dropped. If we choose a very short resending duration, then we will probably be sending a lot of data double that had actually already arrived. This wastes a lot of bandwidth and is not an option either.



Since we cannot pick such a short resending time, we need to wait a little while before resending. This means that dropped packets arrive in the long run, but with an enormous delay. If for example the packet contained information on a player's death then he might die one second too late, which is really bad for the gameplay experience.

In other words: we never ever want the modem to throttle. We want to decide ourselves what gets thrown away, so that we can make sure that the really important packets are never dropped. If we need to throttle, then we want to at least throttle data that is less important. The problem with this is that if we could get away with sending less, then we would always do that instead of only when throttling. After all, an important goal in multiplayer programming is to use as little bandwidth as possible. This means that throttling comes with a drawback and we don't want to do it unless necessary.

In Awesomenauts we reduce bandwidth and packet count when throttling by sending less position updates. During normal gameplay Awesomenauts will send the position of a character 30 times per second. This way if a player turns around quickly, other players will know about it as soon as possible. If we send position updates less often, then we essentially add a bit of lag: we don't send the latest information as soon as we know it. We would prefer not doing that of course. However, if the alternative is that the modem is going to throttle by randomly throwing away packets that might be important, then our hand is forced and we prefer sending less position updates.



Now that we know how to throttle we get to the much more important question: when to throttle. How can we know whether we need to throttle? It is not possible to just ask an internet connection how much bandwidth it can handle. Nor do you get notifications when the connection starts dropping packets because it is too much. We therefore can never know for sure whether throttling is needed and have to deduce this somehow.

Our initial approach to this was to throttle if the ping was too high. The idea is that if a connection cannot handle the packets it needs to send, then latency will increase and we can detect this. This works fine for connections that normally have low ping: if the standard ping is 50ms and suddenly it rises to 300ms, then it is extremely likely that we are sending too much and need to throttle to keep the connection from being lost altogether.

This approach is too simplistic however: internet connections are a very complex topic and can have all kinds of properties. Some people might indeed have a fast connection and a painfully low maximum bandwidth. However, if an Australian and a European player are playing together and they both have a really good internet connection, then their ping will still be high because the distance is so large. In this case throttling won't help at all. In fact, since our throttling essentially increases lag by sending less often, throttling in this case will actually decrease the quality of the connection!

This brings us to the change we recently made in Awesomenauts patch 2.5.3. Instead of looking at ping, we now look at packet loss. Awesomenauts uses UDP packets and we have our own manual reliability system, since various parts of the game require various degrees of reliability. This means that we send and receive our own acknowledgements and thus know exactly how many packets are lost. This is a much better indicator of connection problems than ping. If a lot of packets are dropped by the connection, then apparently we need to throttle to keep from sending too much over a limited internet connection.

It doesn't end there though. I already mentioned that internet connections are a complex topic, and this new plan too is thwarted. Some internet connections are just inherently lossy. For example, maybe someone is playing on a wireless connection and has a wall in between the computer and the modem. Maybe this causes 10% of all packets to be lost, no matter how many packets are sent. I don't know whether wireless routers actually work like that, but we have definitely seen connections that always drop a percentage of the packets, no matter how few we send. Since throttling increases lag we only want to do it when it significantly improves the internet connection. If, like in this case, throttling does not reduce the number of dropped packets, then we do not want the throttle.

Ronimo programmer Maarten came up with a nice approach to solve this problem. His throttling algorithm is based on letting the game perform little experiments. If a player has high packet loss, then the game enables throttling and starts sending less. Then it measures the packet loss again. If packet loss decreased significantly, then we keep throttling. If packet loss remains roughly the same, then we stop throttling and start sending at maximum sending rate again.



The result of this approach is that we only throttle if it actually improves the internet connection. If throttling does not help, then we only throttle shortly during those experiments. These experiments take place automatically during gameplay, but are short and subtle enough that players won't actually notice this happening. If the connection is really good, then we never ever throttle: we don't even do those experiments.

The result of adding this throttling algorithm is that network errors due to losing the connection have been reduced by 10%. This is not a spectacular improvement that many players will have noticed directly, but it is definitely significant enough that we are happy with this result.

In conlusion I would like to stress that internet connections are extremely unpredictable. We have seen all kinds of weird situations: connections that are really fast but stop for a few seconds every couple of minutes, connections that send packets in groups instead of immediately, connections that have low ping but also low bandwidth capacity, and many other combinations of properties. The big lesson we have learned from this is to not make assumptions about properties of internet connections, and to assume any random weirdness can happen for anyone's internet connection. This why I like the approach with the experiments so much: instead of assuming throttling works, it just tries it.

Evolving level art from Swords & Soldiers 1 to 2

In Swords & Soldiers 2 we are stepping up the quality of everything compared to the original. Level art is one of the areas where we are improving the game a lot. With our improved tools we can make them look a lot more detailed and varied. We also approach level art in a very different way now. Today I would like to discuss how the approaches in both games differ and why we switched to different techniques.



In the original Swords & Soldiers the levels were mostly procedurally generated. The only parts that were placed by hand were the ground itself and the small props on it, like grass and flowers. The foreground and background layers were randomly filled with objects. Since this is essentially a 1D game I simply wrote an algorithm that placed mountains (or trees, or houses) at varying distances from each other. Each layer had a bunch of different textures and settings for how densely it should be filled with objects.



There are many arguments in favour of procedurally placing objects like that, but at the time only one really mattered: we didn't have any tools to place objects by hand. I was the only full-time programmer on Swords & Soldiers 1 and I had to build all the gameplay plus the entire engine, so there was no time to create such tools. I did have a couple of interns helping with programming, but in total there was just way too little time to implement any real tools for level creation.

Levels therefore had to be created in Notepad, as I previously described in this blogpost. This worked surprisingly fast and simple for our artists and designers, but of course precise control over graphics is not possible this way.



Our artists had a limited palette of options to differentiate levels and create varied places. They made six different art sets they could use for parts of levels: desert, snow, grass, rocks, jungle and cherry blossom. They could set a gradient for the sky colour and a second gradient to modify the colour of background props. They could set the weather to rain or snow, modify the density of background props, foreground props and clouds and place smaller props on the landscape by hand.

While all of these options together may sound like a decent set of tools to create unique levels, in practice this is not the case. Levels in Swords & Soldiers 1 quickly blend together and start to look similar. One reason for this is that levels often contain more than one environment type. They might for example vary between rocks and desert. An environment type always randomly varies between all the textures it has. The result is that in only a few levels we can have seen nearly all level art in the game. In fact, the very long Berserker Run level contains all six settings and thus contains nearly all level art the game has.



Another problem is that randomly placed objects may stand in a different spot in every level, but they all feel the same. No matter at what distance the same set of mountains is positioned, it always feels like the same place. This could of course be done better with a more advanced procedural algorithm and a smarter set of textures, but in practice procedural content nearly always suffers from this.

This problem even shows in a game like Diablo III, which has high quality procedural world generation at its core. Blizzard spent enormous amounts of time on creating sophisticated algorithms and tons of art assets for random world generation, and yet when I played through Diablo III it felt like I was constantly visiting the same places. Either a place is uninteresting, with some random trees and bushes, or it is unique with some special big props. The number of unique props in Diablo III is large but not infinite, so they start to feel repetitive quickly.

I have not seen a single game with procedural worlds where the visuals did not become repetitive really quickly. In that sense I am slightly afraid for No Man's Sky: the game looks mindblowingly amazing, but I can hardly imagine they solved this problem well enough to keep planet discovery visually interesting. (I sure hope they did though, because that game looks soooooo good!)

In Swords & Soldiers 2 we therefore concluded that we did not want to use this approach anymore. We also don't need to work around a lack of tools anymore, since we now have the impressive toolset we created for Awesomenauts. All of that is at our disposal for Swords & Soldiers 2 as well.

By placing most objects by hand our artists can now create real compositions. The way trees are arranged can create forests, small clearings and green fields. This way even without additional assets it is possible to create places that feel much more real and unique than any procedural algorithm can. Of course one could add procedural rules to create such places, but there are limits to how much can be done this way, especially because every rule that creates a specific kind of place can be used only a couple of items if repetitiveness is to be avoided. With a good artist there really are no limits to how much variation and uniqueness can be created. Most of the levels in Swords & Soldiers 2 are filled with art by Ronimo artist Ralph and he is a true master at creating interesting compositions and little micro-stories through object placement.



Another big difference between Swords & Soldiers 1 and 2 is that in 2 we have much bigger props. In the original game mountains and trees were relatively small, so you always saw a lot of them. In the sequel we have scaled them much larger, so you might see only one or two really large mountains in the background, instead of dozens. This makes it all feel a lot larger, drawing the player more into the world instead of looking at funny miniatures. At the same time this makes the backgrounds look less repetitive: if you don't see ten mountains at the same time, then it is also much less obvious that several of them use the same texture.

This is strengthened by another big improvement in Swords & Soldiers 2: there are many more unique assets for specific levels. For example, one level plays in a Viking golf court and there are golf related objects everywhere. These props are not used in any of the other levels, making this level unique and memorable. This way the player will remember more specific things than in Swords & Soldiers 1, where all the levels looked quite similar. A level doesn't even need all that many unique props: since background objects are so much larger now, having one really big prop can sometimes already define a level.



Another improvement that I would like to mention today is the art style. The original game had a very simple cartoony look, while the sequel is much more detailed and painterly. Our artists went for a style reminiscent of French animation and it works beautifully. Most of the level art in Swords & Soldiers 2 is drawn by Adam Daroszewski. He worked as a freelancer for us and did an amazing job at creating a painterly and coherent look.

I have spent most of this blogpost explaining how Swords & Soldiers 2's level art is better than the original. There is however also one big downside: it is much more work to make. With the lack of proper tools and the simple style we had when we made Swords & Soldiers 1 it took very little time to decorate more levels. In part 2 everything requires much more work to do. It really shows, but it is an important trade-off to be aware of. Not having proper tools means that there are a lot of things that artists simply cannot do and this saves huge amounts of time.

Limitations like that are a very effective way of limiting the amount of time it takes to make a game. The better your tools, the bigger the risk of using them too much. We have already spent much more time making Swords & Soldiers 2 than originally planned and the game isn't even done yet. At the same time it is looking so good that spending so much time is totally worth it. I hope that when the game launches you will be as enthusiastic about it as we are! ^_^

In conclusion, I would like to say that I think procedural level generation is great for games that evolve around it, but for a designed experience having hand-made levels works much better. Levels in a game like Swords & Soldiers 2 look better when made by hand than when generated.

Why composition is often better than inheritance

An important question in code structure is whether to make classes work together through composition or inheritance. The "has a" relationship versus the "is a" relationship. For example: a seat has a cushion and a seat is a piece of furniture. While in this example the difference is really obvious, there are in practice many cases where both could make sense. Does a character in a game have a collision box, or is it a collidable object? These are not the same, but each can be used as the main structure for collision handling (or even both together) and it is not always clear which is better. In my experience intuition often favours inheritance, but it gives so many problems that in many cases composition is better.

Let's first have a look at an example of how the same problem can often be solved in both ways. In Awesomenauts we have a separate class that handles the 'physics' of a character. This class handles things like gravity, knockback, sliding and jumping. Since a character is a physics object, it would make sense to say that Character inherits from PhysicsObject. It is also possible to say that a character has a PhysicsObject that handles its collision. Instead of has a we might also word this as uses a.



Let's see what this relation would look like in code. This is a highly simplified version, but it shows the basic concepts nicely:

class PhysicsObject { Vector2D position; void updatePhysics(); void applyKnockback(Vector2D force); Vector2D getPosition() const; }; //Using PhysicsObject through inheritance class CharacterInheritance: PhysicsObject { void update() { updatePhysics(); } }; //Using PhysicsObject through composition class CharacterComposition { PhysicsObject physicsObject; void update() { physicsObject.updatePhysics(); } void applyKnockback(Vector2D force) { physicsObject.applyKnockback(force); } void getPosition() const { return physicsObject.getPosition(); } };

As you can see in this code, CharacterInheritance is shorter. It also feels more natural, since we don't have to write these extra accessor functions for applyKnockback and getPosition. However, after years of creating both of these kinds of structures I have learned that in a situation like this, using composition is actually more flexible, less sensitive to bugs and even more understandable than using inheritance.

Flexibility

Let's start with the flexibility argument. What if we want to make an enemy that consists of two blocks linked by an energy chain. This enemy does damage to anyone who touches the chain. The blocks can move separately, making for some interesting positional gameplay when fighting this enemy. The two blocks this object consists of move entirely separately and each have their own physics. Knocking back one block does not influence the other block. Yet they really are one character: they have one healthbar, one AI, one entry on the minimap and can only exist together.



With a composition structure it would be pretty straightforward to create this enemy, since we could just create a character that has several PhysicsObjects. With inheritance however we cannot make this as a single character, since we cannot inherit from PhysicsObject twice. We could probably work around this somehow and make it work with inheritance, but it quickly becomes much less simple and intuitive.

If you read this and think this is a far-fetched situation and not all that relevant, then you have probably never been in a project of significant size where gameplay was king. Game designers constantly come up with game mechanics that are exceptions to what you already programmed. Saying no to these just because your code structure cannot handle them will seriously damage your game quality, since in the end the only argument should be whether it is fun to play (okay, and maybe whether it is achievable within the scope of the project).

Just take a look at the diversity of the hundreds of upgrades in Awesomenauts and you will realise how many of those were likely exceptions to what our code could do. Our designers came up with those upgrades and the code had to make it work somehow. An important goal in game programming is flexibility: making your code in such a way that it is relatively easy to add whatever weird whim the game designers come up with today. In most cases composition is much more flexible than inheritance.

Readability

My next argument against inheritance is readability. "Readability" is always accomponied by "sensitivity to bugs", since if a programmer does not really understand how something works, then he will likely break it when working on it.

At Ronimo one of the more important rules in our coding standard is that we strive to keep the size of all classes below 500 lines of code. We don't always see ways to keep to that, but the goal is clear: keep classes relatively short so that they are easy to understand and a programmer can fit the workings of the entire class in his head.

Let's say our code has grown over time as more and more features were added to our game, and Character and PhysicsObject have both grown to 500 lines of code each. We have also added two more classes that use PhysicsObject: Pickup and Projectile. These are also 500 lines of code each.



In such a situation inheritance will usually make all of this very confusing. We might have strived to keep as much private as possible, but in the end inheritance almost always introduces a bunch of virtual and protected functions. In other words: PhysicsObject has become pretty intertwined with its three child classes Character, Pickup and Projectile. In my experience this nearly always happens: inherited classes work together to create complex behaviour and intertwine more and more over time.

By itself this might not be that much of a problem, but to really understand any of these classes we now need to know them all. If we restructure something in PhysicsObject because Projectile needs a new feature, then Character and Pickup will also be involved. To understand the entire situation, the programmer now needs to think about four different classes and fit all 2000 lines of code in her head simultaneously. In my experience this quickly becomes impossible: this is just too much code to really grasp it all at once without starting to mix things up. The result is that readability decreases and the programmer becomes more likely to introduce bugs because she overlooked something.

Of course a composition based structure does not magically fix all of this, but it does help a lot in keeping this structure simple and understandable. With composition there is no virtual or protected, so the separation between PhysicsObject on the one side and Character, Projectile and Pickup on the other is much clearer. This keeps the classes from intertwining over time and makes it easier to keep them truly separate. While they could in theory also have been kept separate with inheritance, in my experience this is too difficult to maintain, while composition enforces it. We have numerous cases in our code where an inheritance structure of classes that all grew too big is hardly understandable any more.

Diamond problem

Any discussion of inheritance versus composition is incomplete without mentioning the famous diamond problem. What happens when a class A inherits from two classes B and C that both inherit from a single parent D? A now has a D twice and chaos ensues.



There are several solutions to the diamond problem in C++. For example, you might just accept that A has double D. (Haha, double D! Ahum.) Or you might use virtual inheritance to solve it. However, both solutions cause all kinds of problems and potential bugs, so just avoiding the diamond problem altogether is highly preferable.

This problem is usually not around in the initial design of a gameplay code structure, but as features are added it might pop up occasionally. The problem is that when it does, it can often be very difficult to come up with a good solution for how to get rid of it without doing a lot of refactoring.

Nevertheless, the diamond problem has only popped up a couple of times in my 12 years of object oriented programming, so I don't think the diamond problem is a very strong argument against inheritance.

(By the way, even without the diamond problem complex multiple inheritance structures tend to get a little sleazy, as I explained in this previous blogpost about why this is not always this.)

Use inheritance, but use it less

Does all of this mean that I am against inheritance in general? Nope, absolutely not! Inheritance is very useful for a lot things, for example in polymorphism and in design patters like listeners and factories. My point is just that inheritance is not as generally useful as it might seem. There are lots of cases where inheritance may seem the cleanest solution, while in practice composition would be better. So use inheritance, but don't overuse it.