Handling Client Authoritative Movement

In this lesson, you will learn how to balance local responsiveness with server-side integrity by implementing client-side prediction alongside server reconciliation for player movement.

The Problem: Latency vs. Trust

If you rely purely on the server to move a character, your players will feel a frustrating "lag" between pressing a key and seeing their character move. Conversely, if you let the client fully dictate their position (Client Authoritative Movement), a cheater can easily modify their transform values to teleport across the map.

The Solution: Predict and Correct

To solve this, we split movement into two halves: Prediction and Validation.

1. Client-Side Prediction: When a player presses 'W', the client immediately moves the local character. Crucially, the client keeps a local buffer (a history) of these inputs and the resulting positions, tagged with a "Tick" number.
2. Server Validation: The client sends the input to the server. The server runs the same physics logic. If the server’s calculated position matches the client’s predicted position for that Tick, all is well.
3. Server Reconciliation: If the server detects a discrepancy (due to a collision or a speed hack), it sends a "Correction" packet back to the client. The client must then snap to the server’s authoritative position and "re-play" all subsequent inputs stored in the buffer to catch up to the current time.

Implementation Snippet

Your movement script needs to keep track of its history:

public struct PlayerState {
    public int Tick;
    public Vector3 Position;
    public Vector3 Input;
}

// Inside your NetworkBehaviour
void FixedUpdate() {
    if (IsOwner) {
        var input = GetPlayerInput();
        var predictedPos = SimulateMovement(input);
        
        // Store in buffer for future reconciliation
        inputBuffer.Add(new PlayerState(currentTick, predictedPos, input));
        
        // Send input to server to be validated
        ServerMoveRPC(input, currentTick);
    }
}

By allowing the client to "guess" their future position while holding the server as the final judge, you eliminate perceived latency while ensuring that illegal movements are instantly overwritten by the server's master state.

Key Takeaway: Never trust the client’s reported position; allow them to predict movement for responsiveness, but always force a reconciliation when the server’s authoritative simulation disagrees with the client’s history.