backend.ai-appproxy-worker 26.4.4

Backend.AI AppProxy Worker

Backend.AI App Proxy Worker

Purpose

The App Proxy Worker is a high-performance reverse proxy that routes user traffic to compute session services (Jupyter, SSH, TensorBoard, etc.) running on agents. It receives routing information from the Coordinator and handles SSL/TLS termination, load balancing, and traffic forwarding.

Key Responsibilities

1. Traffic Proxying

• Proxy HTTP/HTTPS requests to session services
• Proxy WebSocket connections for interactive services
• Handle SSL/TLS termination
• Stream responses efficiently

2. Route Resolution

• Receive routing tables from Coordinator
• Resolve session services from URLs
• Cache routing information locally
• Update routes dynamically

3. Health Checking

• Monitor backend service health
• Detect failed services
• Report health status to Coordinator
• Handle service failover

Architecture

1. Traffic Proxy (Main)

Framework: aiohttp + custom reverse proxy

Port: 5050 (default, HTTPS)

Protocol: HTTP/HTTPS, WebSocket

Key Features:

HTTP/HTTPS Proxy

• Route user requests to session services
• URL Pattern: https://<worker-domain>/<session-id>/<service-name>/...

WebSocket Proxy

• Interactive service communication (Jupyter Kernel, SSH, etc.)
• Real-time log streaming

Key Characteristics:

• SSL/TLS termination (Let's Encrypt auto-certificate)
• High-performance async proxy
• Connection pooling and reuse
• Streaming support (large file downloads)
• Sticky session support
• Auto-retry and failover

Processing Flow:

HTTP Proxy Flow

User → HTTPS Request → Worker (SSL termination)
                           ↓
                       Parse URL (extract session_id, service_name)
                           ↓
                       Lookup route from local cache
                           ↓
                       Resolve backend address (agent:port)
                           ↓
                       Proxy request to agent
                           ↓
                       Stream response back to user

WebSocket Proxy Flow

User → WS Upgrade Request → Worker
                               ↓
                           Establish WS connection to agent
                               ↓
                           Bidirectional message forwarding

2. REST API (Management)

Framework: aiohttp (async HTTP server)

Port: 6040 (default, separate management port)

Key Features:

• Communication with Coordinator
• Health check endpoints
• Metrics exposure (Prometheus)
• Internal management (no external access)

Component Interaction

Traffic Proxy Flow:

User (Browser) → Worker (Port 5050) → Kernel (on Agent)
                    │
                    ├─ SSL/TLS termination
                    ├─ Route resolution
                    └─ Traffic proxying

Management Flow:

Coordinator → Worker REST API (Port 6040) → Route updates