Is a Unity simulation deterministic?

Yhis is something useful mainly for multiplayer RTS games, but could be useful for any other game that needs to be able to execute a game in lock-step on multiple non-identical machines. To be fully deterministic in this sort of sense, the following would all have to be true:

  • RNGs are instantiated, called a consistent number of times in a "turn" (that might be 200ms), and give predictable output during that time period.

  • Floating point math, plus any other math, gives predictable results on all processors (AMD vs Intel, x86 vs x64, etc) and operating systems.

  • There is always a consistent number of simulation cycles per "turn," regardless of the number of frames drawn to the screen.

Basically, I'm wondering if it is possible to build a modern multiplayer RTS in Unity -- obviously single-player is no problem, and other networking models make multiplayer action/FPS/etc games quite possible in Unity, but I've not seen anything relating to simulation determinism in my investigation into Unity so far.

Random numbers can be made 100% predictable cross-platform by using Random.seed. You can set the number of simulation cycles through various methods (FixedUpdate, InvokeRepeating, etc.). The only thing I'm not sure about is the floating point math across different systems. As a side note, the physics engine is not consistent, even on the same machine.

Wow, it’s been quite a while on this question, and I meant to post back with my findings. First of all, I can’t really speak to things like the physics or networking simulation in Unity 3D, because I don’t use any of that. We don’t use GameObject at all, except one for playing back audio.

That said, we have a bunch of scripts that we run via a call stack starting with Update, FixedUpdate, and OnGUI in our main camera’s script. THOSE are definitely fully deterministic, insofar as they would need to be for a multiplayer RTS game.

We still do not use floating point math for any simulation-relevant calculations, but instead use our “Fixed Int” solution. We use the Lidgren library for networking, which is what we used before we ported our games to Unity.

In the end, we have two Steam-published solo/multiplayer games (AI War and Tidalis) running Unity, with north of 100k customers, and they are both running great even with one player on OSX and another on Windows or Linux/WINE.

If I had to guess I would say that the Unity physics simulation is probably NOT fully deterministic in the fashion you would need for an RTS to run in lock-step, but I don’t have proof one way or the other. That just seems like an asynchronous, floating-point sort of bit of calculation, and especially with how unity scales to load that’s going to be inherently non-deterministic from what I can see. For an action game style of networking model that doesn’t matter, but for an RTS that might be what is the cause of the problem roberto_sc is seeing.

It will be quite hard to get this right as when working with floating point numbers there's always the possibility of slight variation between platforms that can play up over time, but worse, timing: FixedUpdate is run at fixed interval except when things go wrong. Switching between applications could result to missing a few update cycles (even when background running is active), or any other instance where CPU is overloaded for a few 100ms.

Hi, I know I’m a little late but here’s a thread for the deterministic physics engine I made: . It comes with a lockstep server and multiplayer setup for RTS games and the like. Sorry for the shameless plug hehe, but I hope this package will be useful for people who are making a lockstep game.