srs/docs/proxy/proxy-load-balancer.md

Load Balancer

Overview

The proxy load balancer distributes client streams across multiple backend origin servers. It provides a pluggable interface with two implementations:

1. Memory Load Balancer - For single proxy deployments
2. Redis Load Balancer - For multi-proxy deployments with shared state

Both implementations maintain stream-to-server mappings to ensure stream consistency - once a stream is assigned to a backend server, all subsequent requests for that stream route to the same server.

Core Responsibilities

1. Server Management

Backend Server Registration:

• Origin servers register themselves with the proxy via System API
• Servers provide their endpoints for each protocol (RTMP, HTTP, WebRTC, SRT)
• Registration includes server identity (ServerID, ServiceID, PID)
• Heartbeat mechanism maintains server health status

Server Selection:

• Pick appropriate backend server for new streams
• Consider server health (last heartbeat time)
• Random selection from healthy servers for load distribution
• Maintain stream-to-server mapping for consistency

2. Stream State Management

Protocol-Specific State:

• HLS Streams: Dual-index storage for M3U8 playlists and TS segments

  • Index by stream URL for initial playlist requests
  • Index by SPBHID (SRS Proxy Backend HLS ID) for segment requests

• WebRTC Connections: Dual-index for session management

  • Index by stream URL for initial connection setup
  • Index by ufrag (ICE username) for STUN binding requests

3. Load Balancing Strategy

Stream-Level Stickiness:

• First request for a stream selects a backend server
• All subsequent requests for that stream use the same server
• Ensures session continuity and state consistency on backend

Health-Based Selection:

• Only consider servers with recent heartbeats (within 300 seconds)
• Fallback to any registered server if no healthy servers available
• Random selection among healthy servers for even distribution

Architecture

The load balancer uses a clean interface-based architecture:

Core Interface: SRSLoadBalancer

• Initialization and lifecycle management
• Server registration and updates
• Stream routing (Pick operation)
• Protocol-specific state management (HLS, WebRTC)

Data Models:

• SRSServer: Backend origin server representation
• HLSPlayStream: Interface for HLS streaming sessions
• RTCConnection: Interface for WebRTC connections

Memory Load Balancer

1. Design

Storage: In-memory maps for fast access

• Server registry with thread-safe concurrent access
• Stream-to-server mappings
• Protocol-specific session state (HLS, WebRTC)

Use Case: Single proxy instance handling moderate stream counts

Characteristics:

• Lowest latency (no network operations)
• Simple deployment (no external dependencies)
• State limited to single proxy instance
• Best for deployments where proxy isn't the bottleneck

2. Configuration

PROXY_LOAD_BALANCER_TYPE=memory

Redis Load Balancer

1. Design

Storage: Shared Redis instance for distributed state

• All proxies read/write to same Redis
• TTL-based expiration for automatic cleanup
• JSON serialization for cross-process communication

Use Case: Multiple proxy instances sharing load

Characteristics:

• Enables horizontal scaling of proxies
• Higher latency (network + serialization overhead)
• Requires Redis infrastructure
• Best for large deployments with many streams

2. Configuration

PROXY_LOAD_BALANCER_TYPE=redis
PROXY_REDIS_HOST=127.0.0.1
PROXY_REDIS_PORT=6379
PROXY_REDIS_PASSWORD=
PROXY_REDIS_DB=0

3. Redis Key Design

Server Keys:

• srs-proxy-server:{serverID} - Server registration (300s TTL)
• srs-proxy-all-servers - Server list index (no expiration)

Stream Mapping Keys:

• srs-proxy-url:{streamURL} - Stream-to-server mapping (no expiration)

Session State Keys:

• srs-proxy-hls:{streamURL} - HLS by URL (120s TTL)
• srs-proxy-spbhid:{spbhid} - HLS by SPBHID (120s TTL)
• srs-proxy-rtc:{streamURL} - WebRTC by URL (120s TTL)
• srs-proxy-ufrag:{ufrag} - WebRTC by ufrag (120s TTL)

Expiration and Cleanup

Server Heartbeat: 300 seconds

• Servers must send updates every 30 seconds (recommended)
• Considered dead if no update within 300 seconds
• Memory LB: filtered during selection
• Redis LB: automatic TTL expiration

Session State: 120 seconds

• HLS and WebRTC sessions expire after 120 seconds of inactivity
• Automatic cleanup via TTL (Redis) or garbage collection (Memory)
• Sessions renewed on each request

Stream Mappings: No expiration

• Stream-to-server mappings persist indefinitely
• Ensures consistent routing for long-running streams
• Only reset when backend server dies or mapping explicitly cleared

Comparison: Memory vs Redis

Aspect	Memory Load Balancer	Redis Load Balancer
Deployment	Single proxy	Multiple proxies
State Storage	Local memory	Shared Redis
Latency	Lowest (in-process)	Network + serialization
Scalability	Single instance	Horizontal scaling
Dependencies	None	Redis required
Complexity	Simple	Moderate
Fault Tolerance	Single point of failure	Multiple proxies
Best For	Moderate traffic	High traffic, high availability