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DownFlux Client Design Doc

Client-Facing Engine Design

Status Draft
Author(s) minke.zhang@gmail.com
Last Updated 2020-10-23


Outline the core mechanics necessary for rendering the server-calculated game state to the players.


DownFlux is a collaborative RTS.


The client will consist of two parts – the actual rendering engine (e.g. drawing tanks and units on screen) and the client API component that does the lower level communications with the server.

Detailed Design


The API client is tasked with the lower-level communications with the server, and forwards the transformed user inputs given by the Renderer. The client also runs a daemon thread to process incoming data provided by the StreamCurves endpoint.

class APIClient {
  public string ID;  // Client ID used to identify the player to the server.

  // Invokes AddClient and announces to server the current tick of the client
  // (in case of reconnection)
  public string Connect(string tickID);

  // Returns the current buffer of recieved data from server stream. Will clear
  // buffer after invocation.
  public StreamData Data;

  public void Move(
    string tickID,
    List<string> entityIDs,
    Position destination,
    MoveType moveType,

  public async Task StreamCurvesLoop(string tickID);


The rendering engine will process actual user input (key presses, mouse clicks, etc.), transform them into useful game intents, and forward them via the API client to the server.

The engine will also be tasked with the core loop of processing game state and displaying that in some form to the user.

Core Loop

using EntityID = string;
using CurveID = string;

public Dictionary<EntityID, Entity> EntityLookup;
public Dictionary<CurveID, Curve> CurveLookup;

public Queue<PlayerAction> Actions;
Tick Rate

The server and rendering engine will run at differing tick rates – because we aim to minimize the network traffic for communicating game state, the data sent to the client will be much slower than the rate at which information needs to be redrawn. For example, the canonical server tick rate is at ~10Hz (from server design), but obviously for games, the renderer needs to draw at a rate of 30 - 60Hz, if not higher.

To account for this tick discrepancy, the renderer will interpolate the server curve data for the relevant partial server tick times.

The renderer may also need different curve rates for different phases in the core loop – for example, if the server only broadcasts game state at 10Hz, the renderer doesn’t need to poll for every frame (60Hz).

Server Reconciliation
public static TickRate = 10;

The renderer will need to query the API client regularly to update its internal curve state and for new entity announcements. The API client holds the actual server stream.

If any new data is provded from the API client (whether new entity announcements or curve updates), the renderer will update EntityLookup and CurveLookup, either with Curve.ReplaceTail or by creating a new entity row.

N.B.: EntityLookup and CurveLookup are add only sets. If an entity should no longer be rendered, it will be marked as tombstoned instead.


The renderer will iterate through the set of curves and draw appropriate data for the values interpolated at the current client tick time.

The server will also need to iterate through the Actions queue and render any client-only changes.

Process Player Input

TODO(minkezhang): Is this async?

At this phase, the renderer will take note of the current player actions (e.g. physical clicks, mouse drags, etc.), transforms them into a usable struct, and append them to the PlayerAction queue.

Process Player Actions

Depending on the action specified (e.g. Move or ScrollViewport), the may need to communicate with the server – this phase is leaving room for calling out action-specific handlers

The renderer will call the server (via the API client) with appropriate commands at this time.

See Also

Minke Zhang
Minke is a software engineer based in the US. He enjoys running, climbing, photography, and banana-related facts. He works on DownFlux in his spare time. Minke prefers spaces over tabs in Python.