State Synchronization vs RPCs in Multiplayer Games

In multiplayer game development, managing how data is shared between the server and clients is critical to creating a smooth, desync-free experience. The two primary methods for distributing this data are state synchronization and Remote Procedure Calls (RPCs). While state synchronization continuously aligns the ongoing state of game objects across all players, RPCs are used to trigger specific, one-time events on remote machines. Understanding when to use each method is essential for optimizing network bandwidth and ensuring gameplay consistency.

What is State Synchronization?

State synchronization, often referred to as “state sync” or “replication,” is the process of automatically sharing the properties of game objects across the network. Instead of sending actions, the server periodically sends the current “truth” of the game world to the clients.

For example, if a player is moving, state synchronization ensures that their coordinates (position and rotation) are continuously updated on everyone else’s screen. If a new player joins the match late, state synchronization automatically ensures they see the world exactly as it is at that moment, including correct player health bars, door states, and item placements.

Best used for: * Continuous data like player position, velocity, and rotation. * Persistent variables like player health, ammo count, and team scores. * Any game state that late-joining players must be aware of.

What are Remote Procedure Calls (RPCs)?

A Remote Procedure Call (RPC) is a method that allows one machine to execute a function on another machine across the network. RPCs are event-driven and transient, meaning they are used to signal that a specific event has occurred rather than to maintain an ongoing state.

For example, if a player presses a button to honk a car horn, an RPC is sent to play the audio clip on all other clients. The sound plays once and finishes. If a player joins the game five seconds after the horn was honked, they do not need to know it happened, making an RPC the perfect tool for this scenario.

Best used for: * One-off visual or audio effects (e.g., explosions, particle triggers, sound effects). * Sending text messages in a chat window. * Requesting actions from the server (e.g., a client requesting to buy an item from an in-game shop).

Key Differences

1. Persistence and Late Joins

• State Synchronization: This data is persistent. When a player connects to a game mid-match, the network library automatically synchronizes the current state of all replicated variables to that player.
• RPCs: These events are transient. Unless specifically designed with “buffered” RPCs (which can be expensive and difficult to manage), late-joining players will miss any RPCs sent before they connected.

2. Network Efficiency

• State Synchronization: It can consume significant bandwidth because updates are sent repeatedly at a set tick rate. Developers must optimize this by only syncing variables when they change or by using network relevancy limits (only syncing data to players close enough to see it).
• RPCs: These are highly efficient because they are only sent when an event actually occurs. However, spamming RPCs (such as sending one every frame) can easily clog the network and crash the connection.

3. Execution Control

• State Synchronization: The flow is declarative. You define what the state should be, and the network engine handles making sure everyone matches that state.
• RPCs: The flow is imperative. You explicitly command when and where a function should run (e.g., Server-to-Client, Client-to-Server, or Client-to-All).

Summary

Choosing between state synchronization and RPCs depends on whether you are managing state or events. Use state synchronization to ensure everyone agrees on the current, ongoing reality of the game world. Use RPCs to trigger temporary, instant actions that do not permanently alter the game state. Balancing these two tools is the key to building a responsive, reliable multiplayer game.