Game Logic Scripting and Networking

I've been very distracted from working on my project, and this blog, since the holiday season. Various circumstances are resolving themselves, finally, and work will resume. I've also been somewhat stalled out trying to wrap my brain around the topic of this post, and thought it wise to take something of a break from wracking my brain in pursuit of the 'ultimate solution'.

One of the important features of the engine is that it should be easily moddable. My goal is to not only produce a game for people to play, but also a game they can manipulate and customize to further derive enjoyment from. This is also something that I feel affords me maximum engine re-usability insofar as creating and releasing another game is concerned. I have a serious aversion to hard-coding game-specific behavior and logic, because it always gets tangled up in the rest of the engine code, making it a mess to change certain aspects of the engine when trying to build a new game out of it.

The top priority is allowing the people who host game servers online to customize the game in any way they like without players being required to manually download and install anything externally just to play. Server admins should be able to customize everything about the game that people experience when they join their game. Players should be able to see all game servers running on the same engine, and choose between the different games/mods that each server is running. Being that virtually all of the assets and resources used for generating the game experience are scripted procedurally, clients quickly download these procedures and 'rules' upon connecting and the entire game experience they encounter is dictated by the scripted configuration of the game on the server.

Games that are almost entirely hard-coded into the engine usually feature customization of the constant values for things like weapon damage amounts, and other little nuanced values like this, but the behavior of the game itself is otherwise 'stuck' the way that it is. Typically they have some sort of text file where the configuration exists, delineating variables and their values for controlling physics and game behaviors. This is simple enough, and plenty sufficient for smaller projects of a less serious nature.

Most games utilize some form of a scripting language to accomplish the de-coupling of game logic from the game engine itself. There are others which simply incorporate the use of an external compiled binary, e.g. a DLL file. Having a background in reverse engineering and 'hacking' games, I can say that using a DLL is probably the most insecure thing a programmer could do. Operating systems are equipped with all sorts of debugging APIs and features that enable hackers to have a field day with such games.

Another top priority alongside game customization is the quality of multiplayer networking and the resultant online gameplay. It's pretty standard now to just design a server/client model using all the usual tricks that have been around for the past decade to mitigate internet latency and packet loss, to smooth out the appearance of the gameplay that is actually occurring on the host machine that is being simulated remotely. Everything you see on the screen is a virtual lie, and the typical bag of tricks are designed with the intent to please the player with promises that can't always be kept.

It is my opinion that the existing techniques are sub-par and that it is time we begin to explore other options, and come up with new ideas. For my project I am turning conventional networking strategy on its head. In my networking model the client has equal authority as the server and other clients as far as the game state is concerned. The server merely maintains the game rules and authority over who can be connected and participating in the game. It also serves to route the game state between clients as it evolves. No single machine retains the absolute state of the game, and all machines are participating equally in the progress and simulation of the game state as it unfolds.

To make all of this possible, combining a sort of peer-based game state simulation along with client/server networking model, as well as keeping the system for user-made mods in a manageable and user-friendly state, I have opted to use a console-scripted system that is made up of a handful of smaller 'systems' of commands. Everything in the engine is scripted using sets of commands in this fashion.

There are three components to this setup. At the core we have entity 'types', which are a set of parameters that define a specific entity. Properties that don't change about a type of game object are represented as a 'type'. Things like a model, conditional logic, physics behaviors, etc. Properties that are consistent across all instances of an entity type are thus considered aspects of that type.

Secondly, we have entity 'functions'. These are small sets of 'operations' to perform on a given entity. Things like playing sounds, spawning particles, or entities, inflicting damage, etc. These functions are referenced by an entity type's conditional logic definitions. Conditional logic is hard-coded into the engine, there are only a certain set of conditions which the engine detects about an entity and, in turn, executes any logic for those conditions as defined in the entity's type. Conditions such as when an entity touches the world, or another entity, or gets damaged or killed, for example.

Functions can perform a number of operations, but they cannot change anything about the original entity type it is executing upon. However, if something is to change about a specific entity's type, it can simply be changed to a new type with different static properties. If a player is supposed to go from a walking physics to a ragdoll physics, simply change the player entity's type from "player" to "deadplayer", where the physics settings differ accordingly.

This makes the process and/or job of designing entities pretty simple and painless. They can be edited in notepad, and reloaded in a snap. It becomes easy to create variations of the default game.

It also simplifies the networking model. The typical setup most games use for networking the game state do so by 'delta-compressing' entity updates, comparing an older state to the present state to determine what aspects or properties changed and need to be transmitted. This allows developers to define any number of entity changes to occur over the evolution of the game state, and have everything reach from one machine to the other over a network connection.

My implementation boils this same design down around the fact that a lot of times there are many entities which have properties that never change over the course of their existence. These properties can all be lumped together and conveyed in a minimal number of packet bytes by simply indicating which entity should be which type, when that entity becomes that type.

The actual networking system continuously relays 'events' to the other side. The game logic, in the form of entity functions, is responsible for invoking events which have a networked component to them. Events like particles, sounds, etc. all are 'networkable events' - in that they should be seen by other participants in the game. These are queued up to be transmitted in the next outgoing update. An entity's type being set is an example of an event that is serialized and queued up for network transmission.

Not all entity function operations have a networked component. Some things are meant to only occur on the local machine, and even networked operations will stay local if the entity type is defined as being local-only (eg: client-side detail entities). If an entity changes into something completely different, and everything about it changes, this is not a large update. The local machine simply indicates which type the entity is now.

Along with the events queue is a prioritized list of entity positions, velocities, angles, etc.. All the location information about an entity gets tacked onto the update after all the events. Positional updates are 'optional', in that they don't always need to get to the other side the way events are supposed to. Entity positions are prioritized by the entity's proximity to the client's player entity. Entities within a certain distance of the player have their positional information included with every update being sent to them. Once entities become further out, the number of updates they are included in per second begins to lessen down to a bare minimum.

This is a screenshot of the ill-fated Revolude game, circa 2010.

Now one idea I had, back in the Revolude days, was to perform a similar network conveyance of entity properties, and logic, by sending game logic function indices to clients, telling them what functions to execute to bring an entity's state "up to speed". This made sense in my head, but in practice there was an issue between preventing functions from overlapping or overwriting eachother's changes.

The solution was to divide up the game logic into a server-logic and client-logic. Sometimes the two had the same pieces of code but used it in different ways. The server's job was to control the actual state of the game, and direct how clients should be simulating their end, which entities are where, and what functions they are executing.

It never worked out, fully. The Revolude build I still have is wrought with networking bugs. A poorly thought-out event networking system wasn't ensuring all events made it across, in order. Objects can be seen turning topsy-turvy, appearing and disappearing, or never existing (but leaving evidence that they did). It's a nightmare I am happy to never return to.

The networking in BITPHORIA is awesome, though. I am very happy with how the game is coming along.

BITPHORIA, in its current form.