rplace/docs/system-architecture.md

System Architecture

Overview

rplace is a collaborative pixel canvas deployed as a single Cloudflare Worker. The frontend (Svelte SPA) is served as static assets, the API (Hono) handles pixel operations, and a Durable Object manages WebSocket broadcasting.

Data Flow

Pixel Placement

1. User draws on canvas → optimistic render into a pending buffer
2. User hits Submit → POST /api/place { pixels: [{x, y, color}] }
3. Worker validates input (bounds, types, batch size ≤ 2048)
4. Worker checks cooldown via SET NX EX (atomic per-user lock)
5. Worker writes pixels via Redis BITFIELD (atomic batch)
6. Worker sends pixels to Durable Object /broadcast
7. Durable Object fans out to all WebSocket clients
8. Response: { ok: true }

Canvas Loading

1. Client fetches GET /api/canvas
2. Worker reads Redis key via GETRANGE → raw binary
3. Client receives ~2.5MB (5-bit packed pixels)
4. Client decodes 5-bit values → color indices → RGBA ImageData
5. Renders onto HTML5 Canvas with OffscreenCanvas

Real-time Updates

1. Client connects WS /api/ws
2. Worker upgrades to Durable Object WebSocket
3. DO stores connection in memory Set
4. On pixel placement → DO broadcasts JSON to all connections
5. Client updates local ImageData + re-renders
6. On disconnect → auto-reconnect with exponential backoff (1s→30s)

Storage

Redis BITFIELD (Canvas)

• Key: canvas
• Encoding: 5 bits per pixel (u5), 32 colors
• Size: 2048 * 2048 * 5 / 8 = 2,621,440 bytes (~2.5MB)
• Offset: y * CANVAS_WIDTH + x
• Atomic batch writes: single BITFIELD command with chained .set() calls

Redis STRING (Cooldown)

• Key pattern: cooldown:{userId}
• Value: "1" (presence is the signal; content is irrelevant)
• TTL: REQUEST_COOLDOWN_SEC (1s) — auto-expires, no explicit cleanup
• Atomic via SET key "1" NX EX 1

Rate Limiting

Fixed-window cooldown, one request per second per user:

On placement request:
1. SET cooldown:{userId} "1" NX EX 1
2. If reply == "OK" → allow (key set, TTL 1s)
3. If reply == null → reject (429, retryAfter = 1)

Batch size is independent of the cooldown; it is validated separately (MAX_BATCH_SIZE = 2048). Anonymous identity via CF-Connecting-IP hash.

Security

• Rate limiting: Atomic SET NX EX prevents race conditions
• Identity: CF-Connecting-IP (set by Cloudflare, unspoofable)
• Input validation: Bounds checking, type checking, integer validation on all pixel data
• Batch cap: Max 2048 pixels per request + request body size guard
• DO isolation: /broadcast route only reachable via DO stub, not externally