Add multiplayer networking architecture document

docs/architecture/networking.md

@@ -0,0 +1,286 @@
+# Multiplayer Networking Architecture
+
+> **Status:** Proposed
+> **Owner:** Platform team
+> **Related:** Server authority + client prediction
+
+## 1. Overview
+
+This document specifies the multiplayer networking architecture for **agp-game**,
+a Godot-based game built by autonomous agents. The architecture follows a
+**server-authoritative model with client-side prediction and reconciliation**,
+which is the industry standard for fast-paced, competitive multiplayer games.
+
+The goals of this architecture are:
+
+- **Fairness & anti-cheat:** The server is the single source of truth for all
+  gameplay state. Clients never directly mutate world state.
+- **Responsiveness:** Client-side prediction hides network latency so local input
+  feels immediate.
+- **Determinism:** The server runs a fixed-timestep simulation so that state is
+  reproducible and testable.
+- **Scalability:** A clear separation between transport, simulation, and
+  replication layers allows the server to be tuned independently.
+
+---
+
+## 2. Transport Choice: ENet over WebSocket
+
+### 2.1 Decision
+
+We choose **ENet** as the primary transport for gameplay traffic.
+
+### 2.2 Justification
+
+| Criterion            | ENet                                   | WebSocket (TCP)                     |
+|----------------------|----------------------------------------|-------------------------------------|
+| **Reliability model**| Reliable + unreliable channels         | Reliable only (TCP)                 |
+| **Latency**          | Low; unreliable packets skip retransmit| Head-of-line blocking on loss       |
+| **Packet ordering**  | Per-channel ordering                   | Global strict ordering              |
+| **Congestion control**| Lightweight, game-oriented            | TCP-style (slow start, backoff)     |
+| **Godot support**    | First-class (`ENetConnection`, `MultiplayerPeerENet`) | Supported but TCP-based |
+| **Use case fit**     | Real-time action game                  | Chat, lobby, HTTP-style messaging   |
+
+**Rationale:**
+
+- **Movement & state replication** are latency-sensitive and tolerate occasional
+  packet loss. ENet's *unreliable* channel lets us send position snapshots at high
+  frequency without head-of-line blocking.
+- **RPCs that must be delivered** (e.g., interactions, spawns) use ENet's
+  *reliable* channel, giving us exactly the semantics we need without TCP's
+  retransmission stalls.
+- Godot's `MultiplayerPeerENet` is mature and integrates directly with the
+  high-level multiplayer API (`MultiplayerAPI`, `rpc()`).
+
+### 2.3 WebSocket as a Secondary Transport
+
+WebSocket remains available as a **secondary transport** for:
+
+- **Lobby / matchmaking** messaging (low-frequency, must-be-reliable).
+- **Browser builds** where raw UDP/ENet is unavailable.
+- **Admin / telemetry** channels.
+
+The transport layer is abstracted behind an interface so gameplay code does not
+depend on the underlying protocol.
+
+---
+
+## 3. Server-Side Game Loop
+
+### 3.1 Tick Rate
+
+- **Simulation tick rate:** **30 Hz** (33.33 ms per tick).
+- **Replication / snapshot rate:** **30 Hz**, aligned with the simulation tick.
+
+**Justification:**
+
+- 30 Hz is sufficient for smooth movement and interaction in a top-down / 2D
+  action game while keeping CPU and bandwidth costs predictable.
+- A fixed timestep (rather than delta-time) makes the simulation deterministic
+  and testable.
+- The tick rate is a **configuration constant** so it can be raised (e.g., to
+  60 Hz) for competitive modes without architectural changes.
+
+### 3.2 Authority Model
+
+- **Server is authoritative** for:
+  - Player position, velocity, and physics state.
+  - Health, damage, and death.
+  - Item pickups, inventory, and world-object state.
+  - Spawning and despawning of entities.
+  - Game rules, scoring, and win/lose conditions.
+- **Client is authoritative** for:
+  - Its own *input* (which keys/buttons are pressed).
+  - Local camera and UI.
+  - Cosmetic / non-gameplay presentation.
+
+### 3.3 Server Loop Pseudocode
+
+```
+while server_running:
+    process_network_inbound()      # read client inputs & RPCs
+    for _ in range(ticks_to_advance):
+        simulate_tick()            # fixed 30 Hz step
+    broadcast_snapshots()          # replicate authoritative state
+    process_network_outbound()     # flush reliable/unreliable channels
+```
+
+The server accumulates real elapsed time and advances the simulation in fixed
+`TICK_DURATION` increments, decoupling simulation from frame rate.
+
+---
+
+## 4. RPC Call Patterns
+
+RPCs are grouped by direction and reliability. All gameplay RPCs are routed
+through the high-level `MultiplayerAPI` with explicit `rpc_id` targeting.
+
+### 4.1 Client → Server (Input & Requests)
+
+| RPC                          | Channel   | Payload                                   | Notes |
+|------------------------------|-----------|-------------------------------------------|-------|
+| `request_move_input`         | unreliable| `{ seq, input_state, timestamp }`         | Batched per tick; drives prediction |
+| `request_interact`           | reliable  | `{ target_id }`                           | Open door, pick up item, use object |
+| `request_attack`             | reliable  | `{ target_id, aim_dir }`                  | Combat action |
+| `request_join`               | reliable  | `{ player_name, profile }`                | Session setup |
+| `request_respawn`            | reliable  | `{}`                                      | After death |
+
+**Design notes:**
+
+- **Input is batched** per client tick and sent with a monotonically increasing
+  `seq` so the server can detect gaps and the client can reconcile.
+- **Interactions are reliable** because they are discrete, must-happen events
+  (opening a door, picking up an item). Losing them would corrupt world state.
+
+### 4.2 Server → Client (State & Events)
+
+| RPC                          | Channel   | Payload                                   | Notes |
+|------------------------------|-----------|-------------------------------------------|-------|
+| `snapshot_state`             | unreliable| `{ tick, entities[], player_states[] }`   | Periodic authoritative state |
+| `spawn_entity`               | reliable  | `{ entity_id, type, transform, props }`   | New entity enters world |
+| `despawn_entity`             | reliable  | `{ entity_id, reason }`                   | Entity removed |
+| `interact_result`            | reliable  | `{ request_seq, success, result }`        | Acknowledges interaction |
+| `damage_event`               | reliable  | `{ target_id, amount, source_id }`        | Combat feedback |
+| `game_event`                 | reliable  | `{ type, payload }`                       | Score, win, round start/end |
+
+### 4.3 Broadcast vs. Targeted
+
+- **Broadcast (all clients):** `snapshot_state`, `spawn_entity`,
+  `despawn_entity`, `game_event`.
+- **Targeted (single client):** `interact_result`, `damage_event` (to affected
+  players), and any player-private data.
+
+---
+
+## 5. Client-Side Prediction & Reconciliation
+
+### 5.1 Prediction
+
+- The client runs a **local copy of the movement simulation** using the same
+  physics constants as the server.
+- When the player provides input, the client:
+  1. Applies the input to its local player immediately.
+  2. Renders the predicted position.
+  3. Sends the input to the server (batched, with `seq`).
+
+### 5.2 Reconciliation
+
+- The server sends authoritative `snapshot_state` at 30 Hz containing the
+  server's tick number and authoritative player positions.
+- The client keeps a **ring buffer of predicted states** keyed by input `seq`.
+- On receiving a snapshot:
+  1. Find the last predicted state that matches the server's tick.
+  2. Compute the **error** between predicted and authoritative position.
+  3. If error exceeds a threshold, **snap or smoothly interpolate** the local
+     player to the authoritative position (error correction).
+  4. Re-simulate any inputs that occurred after the server's tick (rollback).
+
+### 5.3 Interpolation of Remote Entities
+
+- **Local player:** predicted + reconciled (see above).
+- **Remote players / entities:** rendered with **interpolation delay** of
+  `2 × TICK_DURATION` (≈ 66 ms) to smooth out snapshot jitter.
+- The client buffers the last N snapshots and interpolates between them.
+
+### 5.4 Prediction Buffer Sizing
+
+- Buffer size is adaptive based on measured RTT.
+- Default: hold up to `ceil(RTT / TICK_DURATION) + 2` predicted states.
+
+---
+
+## 6. Entity & World State Replication
+
+### 6.1 Entity Model
+
+Every networked entity has:
+
+- A globally unique `entity_id` (server-assigned).
+- An `EntityType` (player, enemy, item, projectile, door, ...).
+- A `Transform` (position, rotation).
+- A typed `props` dictionary of gameplay state (health, state machine, etc.).
+
+### 6.2 Snapshot Format
+
+```
+snapshot_state:
+  tick: uint32
+  entities:
+    - entity_id: uint32
+      type: uint8
+      transform: { x, y, rot }
+      props: { ... }        # delta-encoded where possible
+  player_states:
+    - peer_id: int
+      entity_id: uint32
+      health: int
+      state: uint8
+```
+
+### 6.3 Delta Compression
+
+- Only **changed** entity fields are sent per snapshot (dirty-flag per entity).
+- Static or unchanged entities are omitted from the snapshot.
+- This keeps bandwidth proportional to *activity*, not world size.
+
+---
+
+## 7. Connection & Session Lifecycle
+
+1. **Connect:** Client opens an ENet connection to the server.
+2. **Authenticate / Join:** Client sends `request_join`; server validates and
+   assigns a `peer_id` and spawns the player entity.
+3. **Spawn:** Server broadcasts `spawn_entity` for the new player to all clients.
+4. **Play:** Input → prediction → server simulation → snapshot → reconciliation.
+5. **Disconnect:** Server removes the player entity, broadcasts `despawn_entity`,
+   and cleans up session state.
+
+---
+
+## 8. Security & Anti-Cheat
+
+- **No client-authoritative gameplay values.** Clients only send *inputs* and
+  *requests*; the server computes all outcomes.
+- **Input validation:** Server clamps movement speed, rejects out-of-bounds
+  positions, and validates interaction targets.
+- **Rate limiting:** Per-peer input and RPC rate limits prevent flooding.
+- **Sequence validation:** Gaps or replays in input `seq` are detected and
+  rejected.
+
+---
+
+## 9. Testing & Determinism
+
+- The simulation is a **pure function of (state, inputs, tick)** so it can be
+  unit-tested headlessly.
+- A **replay system** records inputs and can re-simulate to verify server and
+  client produce identical results.
+- Network code is tested with a **mock transport** that can inject latency,
+  jitter, and packet loss.
+
+---
+
+## 10. Open Questions / Future Work
+
+- **Interest management / spatial partitioning:** For large worlds, restrict
+  snapshots to entities near each client (AOI culling).
+- **Server-side physics:** Decide whether to use Godot's physics server on the
+  host or a custom deterministic physics step.
+- **Dedicated server vs. listen server:** The architecture supports both; a
+  dedicated headless server is the target for production.
+- **WebSocket fallback:** Implement the transport abstraction for browser builds.
+
+---
+
+## 11. Summary
+
+| Concern                | Decision                                        |
+|------------------------|-------------------------------------------------|
+| Transport              | ENet (primary), WebSocket (secondary)           |
+| Tick rate              | 30 Hz fixed timestep                            |
+| Authority              | Server-authoritative; client sends input only   |
+| Prediction             | Client-side prediction + reconciliation         |
+| Remote interpolation   | 2-tick interpolation delay                      |
+| Replication            | Delta-compressed snapshots at 30 Hz             |
+| Interaction RPCs       | Reliable channel                                |
+| Movement input         | Unreliable channel, batched with sequence       |