Multiplayer for an already created game?

Question

Java

Recently I wanted to create a multiplayer for an already created game (not mine) and use JAVA.
I started looking for various articles on this topic on the Internet, but in most cases they write about creating a multiplayer based on UNITY, UE4 ...
Tell me, please, what do you need to learn?

Answer 1

Even if you start from the wrong side, there is a server right there with the game that you can launch yourself and probably do anything.
Only for a separate launch, for example, on a remote server for 24/7, as far as I understand from light googling, you will need your own copy of the game.
So no "multiplayer for an already created game" is needed, it is there out of the box ...

Answer 2

If you do everything yourself, then you need to be familiar with debugging techniques and architectural patterns of multithreaded applications; clearly understand the architecture of game engines - because it will need to be broken and rebuilt for multiplayer, even if it is splitscreen / hotseat; to cut in network interaction: both interfaces, and the device of libraries (in C); C/C++ and Java interop.
Well, and of course, methods of dealing with network delays, reconnects.
If the organization of the multiplayer is centralized, then in addition it would be necessary to design a meta-game, methods for organizing anti-cheats, decide on a selection algorithm, develop a horizontal scaling strategy, understand Kubernetes and database clustering.
And all those "based" solutions are designed for customers on the same "base", because it is impossible to supply serial solutions to the above issues for a completely custom code.

Answer 3

Making multiplayer for any game is much more difficult than just redrawing some picture. In FS19, as far as I understand from half a minute of googling, there is some possibility for developing mods, but how deeply it allows you to integrate into the game is still a big question. And in any case, you will have to write a lot, the client library as a mod for the game and the multiplayer game server itself with the interaction protocol.