Peer-to-peer synchronized simulations

It seems it's been a while since I wrote anything about distributed systems. The next few posts should hopefully make up for that.

To kick things off with some light observations, I've recently (thanks to a whole bunch of people in my del.icio.us network) come across this 2001 article describing the multiplayer system of Age of Empires.

The system is a peer-to-peer simulation where input from each player in the form of commands is synchronized across all nodes in discrete "game turn" steps, whose real-time duration is calculated as a function of latency between the nodes and the framerate at which the nodes can render the simulation.

It's interesting to compare the Age of Empires system to that of Croquet (I've blogged about Croquet before). Like Age of Empires, Croquet is a peer-to-peer system. The TeaTime architecture behind Croquet can be thought of as a distributed object simulation where messages between objects are what gets synchronized across nodes.

Where Croquet and the AoE multiplayer system differ is in how time and synchronization is handled. TeaTime extends Reed's multi-version timestamping with the notion of pseudo-real time and uses that notion to enable deadline-based scheduling for message processing, with synchronization being achieved via two-phase commit. This allows the decoupling of simulation rendering speed from message synchronization. Unlike the AoE system, this approach is unaccomodating to high-latency nodes.

One thing I found interesing about the AoE article is the large number of synchronization bugs (a lot of them relating to PRNG generation) that the developers encountered.