Wednesday, November 13, 2013

Game Architecture and Networking, Part 2

In this post I will talk in more detail about my game architecture and how the networking is realized in it. For the basics of my architecture, see these two old blog posts from last year:
Parallel Game Architecture, Part 1
Parallel Game Architecture, Part 2
In short, my architecture consists of 6 subsystems which all work on their own copy of a subset of game data. Because they work on a copy, they can be updated concurrently. If a subsystem changes its game data copy, it has to queue the change up in the change manager. After all subsystems are finished updating, the change manager distributes the changes to the other subsystems. Right now my engine has the following 6 subsystems:
  • Input: responsible for reading the input from the gamepad or keyboard
  • Physic: collision detection and physical simulation of the world (uses Jitter Physics Engine)
  • Graphic and Audio: uses the SunBurn Engine and Mercury Particle Engine
  • LevelLogic: responsible for checking winning/losing conditions, creating/destroying objects, and so on
  • Networking: communicate with other peers over the network
  • AI: calculates the InputActions for not human controlled game entities (allies, enemies, homing missiles, ...)
In the picture below you can see how a player controllable game entity (for example the player's space ship) is represented in the game architecture. Every box equates to an object in the specific subsystem.
Representation of a player controllable entity on the host and a client system. The numbers in the white circles show the latency in number of frames.
In the input system the pressed buttons are read and translated to InputActions (Accelerate, Rotate, Fire Weapon, ...). Those InputActions are interpreted by the phyics-system which is the central subsystem of the engine. It informs the other system about the physical state of all the game entities like the current position, current acceleration, collisions and so on. These information are needed in the graphics and audio system to render the game entities models at the right position, play a sound and/or trigger a particle effect.

Host Side Networking
The networking system has to send the physical state of the game entities to all clients. For important game entities like player ships, this is done a few times per second, for ai objects much more infrequently, because ai objects behave more or less deterministically (iff the client and host are in sync!). There are some important physical state changes which trigger an immediate network send. For example, if the game entity is firing, we want to create the fired projectile on the clients as fast as possible, to minimize the divergence between the host-world and the client worlds.

Client Side Networking
On the client side, the networking system receives the physical state and calculates a prediction, based on the latency, the received velocity and current acceleration. In the next change distribution phase the current physical state in the physic-system is overwritten by the predicted physical state. Then it takes one more frame till the information is really visible on the client side. On the Xbox my game runs on average at 60 frames per second. This means, if a player fires her weapon this is seen at the earliest 66 ms (4 frames) + X ms later on another peer. X depends on the latency L and the point in time the network packet is received. If the packet is received after the networking system has already finished updating, the packet has to wait for the next frame till it gets processed.

The latency is a disadvantage of the game architecture. But it could be improved for example by merging the input system into the physics-system. A separate input system does not improve performance. But merging the two system would reduce the maintainability, therefor I have no plans doing this in the near future. Separation of concerns between the systems is really a big plus. You could even replace a system completely without affecting the other systems.
On importent part of my networking code is not finished yet: the interpolation. Right now the old physical state is simply overwritten by the new one received from the host. If the two states differ much, the movement of the game entity looks jerky. To solve this, the difference from the old to the new physical state can be evenly spread and applied over the next few frames.

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