Sides¶

One of the most important concepts that a mod developer should know and understand is the concept of sides.

In Minecraft, we deal with two types of sides: the physical sides and the logical sides.

Physical Sides¶

The physical sides refer to the actual files which contain the game code, as distributed by Mojang. A physical side defines what program the user is actually running, either through the Minecraft Launcher, through the command line, or some other method.

There are two physical sides:

The physical client, also known as the Minecraft client, is the program ran by players of Minecraft. This is shipped with the libraries which are used to render the graphics, play the audio, and receive inputs from the user.
The physical server, also known as the dedicated server, is a JAR file which contains all the necessary dependencies to run a copy of the Minecraft server, to which players (through their physical clients) can connect to and play on.

Logical Sides¶

If physical sides refer to the files which contain the game code (or difference thereof), the logical sides refer to the actual game code or logic being run.

As with the physical sides, there are lso two logical sides:

The logical client is the game code responsible for handling player inputs and communicating with the server both to inform it of the inputs from the play and to be informed about changes in the world.
The logical server is the game code responsible for maintaining the authoritative copy of the game world, running the ticking logic -- as in, actually advancing the time and actions in-game -- and communicating with clients to know player inputs and to respond appropriately.

Logicals within Physicals¶

One of the key sub-concepts within the concept of sides is that logical sides are present within physical sides. When you load either the Minecraft client or the dedicated server, it will load one or both of the logical sides, depending on what the physical side is being loaded.

The two key points are that:

The logical client is only present on the physical client.
The logical server is present on both the physical client and physical server.

That may seem confusing at first, but you have to keep in mind the architecture of Minecraft and how the logical sides relate to them.

When a player is playing a game, both in singleplayer or in multiplayer, there are at least two parts: the client, by which the player is controlling their character and observing the world, and the server, which does processing for what happens within the world -- including the action taken by the player. These are what we refer to as logical sides.

In a multiplayer environment, this is easily understandable: the Minecraft client provides the logical client, while the dedicated server provides the logical server.

However, in a singleplayer environment, who is hosting the server? In fact, it is the same Minecraft client! When the Minecraft client loads up a world, it creates a local server, known as the integrated server, which provides the logical server. The Minecraft client still provides the logical client, but it also manages the logical server for the world.

Note

While it may be made for the purpose of a singleplayer world, Tte integrated server is still a server in its own right, and can even allow other clients to connect to the same world!

This is known as publishing the server (the /publish command) or opening the server to LAN (which is why the pause menu shows the Open to LAN button). In this situation, it just so happens that the Minecraft client creates and hosts the same server to which it itself connects to.

Classes across Sides¶

As of writing and current and past observations of the codebase, the Minecraft client classes are known to be a superset of the dedicated server classes. This means that they both share a large portion of code which is used for the logical server implementation, as well as common objects between the logical server and the logical client.

Unsurprisingly, the majority of classes which are only present in the Minecraft client are those under the net.minecraft.client subpackage, a majority of which relate to rendering the world and in-game screens on the user's screen.

Warning

One of the common mistakes of Minecraft modding, which plagues both beginners and experienced modders alike, is accidentally referencing a client-only class from a place where it may be possible for the dedicated server to run into.

In the Forge development environment, this manifests as the well-known error message of Attempted to load class <class name> for invalid dist DEDICATED_SERVER. In the production or end-user environment, this manifests as an ClassNotFoundException.

Mod developers should be alert when directly referencing a client-only class in common code (that is, reachable in both the Minecraft client and the dedicated server). It is also recommended practice to test mods while running the dedicated server, to catch these errors within the developer's environment before they cause a problem for end-users.

Trivia¶

Because the physical client contains all the server classes, one can possibly craft a command which runs the dedicated server using the Minecraft client's JAR. While fascinating, it wouldn't be viable to run the dedicated server, as the command would have to reference the whole classpath required to run the client, as opposed to the dedicated server which packs all its dependencies into the same JAR.