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In most MMOs you control a character, interact with other players and group up with other players to clear dungeons. Here's a few networking considerations for anyone creating something similar.
In a MMO the world needs to be persistent. Any given user can join and leave a world at any given time and we want their changes to be persistent. Which NPCs was killed, which treasures were looted, which items are available on the action house, etc. In order to achieve this, you need to run a World Replication Server in the coherence Cloud which will make sure the world state is saved even if no players are logged in.
Given that there can be a large amount of players distributed over a very large area, we don't really care about the ones in all the different areas. By not sending information about players far from the player itself, we can significantly limit the amount of data sent over the network. When using coherence you can use the LiveQuery Component to set a bounding box in which we replicate data from networked entities - anything outside of it is ignored.
Even within a LiveQuery bounding box there might be further room to optimize. When having a large amount of networked entities, you might want to prioritize those that are closer. Our solution to this is called Level Of Detail (LOD). Using this you are able to control values such compression, value range and sample rate in order to hit the optimization sweet spot.
To make sure that all users experience an identical game world at any given time, we need a Simulator to be responsible for taking decisions for the AI, triggering events, etc. In coherence we support launching your game with any number of Simulators taking responsibility for the various parts of your game.
A common part of all MMO's are instanced areas where a smaller group of players enters together and completes a set of tasks. It does not make much sense to run these instances as a part of the World Replication Server. The idea is that it can be possible to spin up a separate server for each group who enters such an instance. In coherence we offer this through the Room Replication Server. This allows you to have a shared world, as well as any number of instances for a specific area for a subset of players.