#1: Side concepts

The client-server architecture on JamiesWhiteShirt's Ramblings

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When discussing Minecraft’s architecture, the terms “server” and “client” may be ambiguous. This resource explains some necessary concepts to get started.

Logical sides - The separation of concerns

The Minecraft game has two logical sides. A logical side refers to a specific part of the game logic that has certain responsibilities. The two logical sides are separated but connected through a very primitive interface and work in unison. We call this a “logical” side because it describes what kind of logic is applied.

First, some definitions:

• The game state contains things such as entities, tile entities, the placement of blocks and various properties of the world.
• Game events are events such as monsters spawning, blocks changing and entities moving. Game events lead to changes in the game state.
• Player actions are actions such as walking, jumping, hitting blocks and clicking buttons in GUIs. Player actions lead to game events.

A logical server is responsible for being the authoritative source of truth about the game state and game events.

• Is connected to one or multiple logical clients.
• Holds a complete representation of the game state. Connected clients are selectively informed about this state.
• Decides which, when, and how game events happen. Connected clients are selectively informed about it these events.
• Receives player actions from clients. Decides the effective outcome of the actions.

A logical client is responsible for collecting keyboard/mouse input and providing video/audio output.

• Is connected to one logical server.
• Holds an incomplete representation of the game state. Only knows for certain what the server has informed it about.
• Is informed by the server about some game events that happen when they happen.
• Informs the server of its player actions. The effective outcome of player actions can only be predicted by the client.

Physical sides - Minecraft distributions

The use of the word logical is a necessity for disambiguation. A logical side is not the same as a physical side. A physical side refers to a specific type of Minecraft distribution. We call this a “physical” side because it describes what systems are “physically” present in the software environment.

The concepts are still related. Logical sides are hosted as in the table below:

	Logical server	Logical client
Physical client	Integrated/LAN server	Yes
Physical server	Dedicated server	No

A physical client is a Minecraft client distribution. It hosts an integrated server when in singleplayer or acting as a LAN server. When connected to a remote server, the physical client does not host the logical server. The logical client can connect to the local logical server, or connect remotely to either a physical server or another physical client via LAN.

A physical server is a Minecraft server distribution. It only hosts a dedicated server that logical clients from physical clients can connect to.

The universal client-server architecture

It should be obvious that this architecture is not strictly necessary in a single player situation. However, Minecraft applies this architecture universally. The client is either connected to an integrated server or a remote server.

The universal client-server architecture turns out to have some very useful properties. It allows reusing a significant amount of code across the Minecraft client and the Minecraft server distribution. In fact, the physical client and physical server are different distributions of the same codebase.