Implementing Client-Side Prediction for Movement

In this lesson, you will learn how to eliminate perceived input lag by simulating movement locally on the client while simultaneously requesting authorization from the server.

The Problem: The Latency Gap

In a networked environment, if you wait for the server to confirm your movement before updating your character’s position, you experience "input lag"—the time it takes for your packet to reach the server and the response to return. At 100ms latency, your character would feel sluggish and unresponsive.

The Solution: Local Simulation

Client-side prediction solves this by running the movement logic on the client immediately when an input occurs, regardless of the server’s status. We treat our local client as an "optimistic" authority. When the player presses 'W' to move forward, we update the Transform immediately.

void Update() {
    if (IsOwner) {
        Vector3 input = GetInputVector();
        // Predict movement locally immediately
        transform.position += input * moveSpeed * Time.deltaTime;
        
        // Send request to server to validate the move
        RequestMoveServerRpc(input, transform.position);
    }
}

The Workflow

1. Predict: When the client moves, apply the change to the Transform component instantly.
2. Buffer: Store the input and the resulting state (position, rotation) in a local buffer, tagged with a "tick" or timestamp.
3. Reconcile: When the server eventually sends back the authoritative state, compare it to your buffered record. If the server says you are at (10, 0, 5) but your local prediction said (10, 0, 5.5), you "snap" or smoothly interpolate to the server's authoritative position to correct the error.

By running the physics locally, the game feels instant. Because you keep a record of your inputs, you can "replay" them if the server disagrees with your position, ensuring that the game remains fair while feeling snappy. You aren't replacing server authority; you are simply masking the latency by running a temporary local simulation that gets verified after the fact.

Key Takeaway: Client-side prediction masks network latency by simulating movement locally the moment input is provided, then reconciling with the server's authoritative state to ensure consistency.