Roblox experiences are multiplayer by default and run in a client-server model. The Roblox server is the ultimate authority for maintaining the experience's state, and is responsible for keeping all connected clients in sync with the server.
When an experience runs, Roblox copies a version of the "edit" data model that you built and published from Studio and runs it on Roblox servers as the "runtime" data model.
Connected clients also receive a copy of the runtime data model and any initialization of the player occurs, such as initializing a player's backpack (inventory) or local user interface. The client then renders the 3D world and user interface, and begins running any applicable scripts.
The server constantly updates connected clients, keeping everything in sync across the server and clients through a process called replication, which synchronizes the data model, physics simulation, and chat messages. Replication logic exists on both the client and server to ensure synchronization.
Data model changes can occur in a variety of cases, such as when something in the 3D world is created or a property of the 3D world changes. This typically occurs when a script on the server or client makes a change that needs to be reflected on the other side of the client-server boundary. The following diagrams show common scenarios for data replication.
Roblox uses a rigid body physics engine, which is responsible for calculating the movement and interactions of parts in the 3D world. By default, all parts in Roblox are rigid bodies and participate in simulated physics, unless otherwise specified. You can also group multiple parts together into assemblies, which the physics engine treats as a single rigid body.
Roblox replicates physics simulation data between the server and clients when necessary. To assist with simulation performance, Roblox can assign ownership of assemblies to a specific client or server. This means that the client or server can be responsible for simulating the physics of that assembly. Other clients receive updates about the assembly's position and movement from the owning client or server. Ownership typically happens automatically, but you can assign it directly for fine-tuned responsiveness.
Roblox replicates chat messages between the server and client. The server is responsible for filtering chat messages and deciding which messages to replicate to other clients. For example, the server may filter out messages that contain profanity or are too long.