ADR-0025: Add Granian as Alternative HTTP Server¶

• Status: Accepted
• Date: 2025-12-21
• Deciders: Core Engineering Team

Context¶

MCP Gateway uses Gunicorn with Uvicorn workers as its production HTTP server stack. This provides good performance with the uvicorn[standard] extras (ADR-0024) and is battle-tested. However, a Rust-based alternative called Granian offers potential benefits:

• Native HTTP/2 support (without requiring a reverse proxy)
• Native WebSocket support
• Native mTLS support
• Lower memory footprint
• Simpler process model

Granian is a Rust-based HTTP server for Python applications that implements ASGI, RSGI, and WSGI interfaces. It's built on: - Hyper: Rust's HTTP library - Tokio: Async runtime - PyO3: Python bindings

Decision¶

We will add Granian as an alternative HTTP server option while keeping Gunicorn + Uvicorn as the default.

Key points: - Gunicorn + Uvicorn remains the default server (stability, maturity) - Granian is available as an alternative for users who need its features - Both servers are production-ready and fully supported - Users can switch via the HTTP_SERVER environment variable

Usage:

# Using Gunicorn + Uvicorn (default)
make serve

# Using Granian (alternative)
make serve-granian

# Granian with HTTP/2 + TLS
make serve-granian-http2

Granian vs Gunicorn+Uvicorn¶

Performance Characteristics¶

Based on community benchmarks:

Note: Actual performance varies by workload. Always benchmark with your specific use case.

Real-World Performance Comparison (Database-Bound Workload)¶

Profiling under load test with 2500 concurrent users against PostgreSQL backend:

Key Insight: When the bottleneck is the database (not HTTP parsing), both servers achieve similar throughput. The difference is in resource usage and overload behavior:

• Gunicorn uses less memory but has no admission control—under extreme load it will queue requests indefinitely, potentially causing OOM or cascading timeouts.
• Granian uses more memory but provides native backpressure—under extreme load it rejects excess requests with immediate 503, protecting system stability.

Why Granian Uses More Memory: - Multi-threaded Rust runtime (Tokio) overhead - Larger HTTP buffers (512KB per connection default) - Backpressure queues holding pending requests - PyO3 Python-Rust bindings overhead

Recommendation by Use Case:

Consequences¶

Positive¶

• Higher throughput potential - Rust-based server can handle more RPS
• Native HTTP/2 - No reverse proxy needed for HTTP/2 support
• Lower memory footprint - Rust's memory efficiency
• Simpler deployment - Single process model, no master/worker complexity
• Choice - Users can pick the server that best fits their needs
• Future-proof - Rust ecosystem continues to improve

Negative¶

• Newer project - Less battle-tested than Gunicorn (though production-ready)
• Binary dependency - Requires Rust compilation or prebuilt wheels
• Different tuning - Different configuration parameters to learn
• Optional dependency - Adds ~20MB to container if installed

Neutral¶

• Same application code - ASGI interface is identical
• Same configuration - Environment variables work the same way
• Coexistence - Both servers can be installed simultaneously

Configuration¶

Environment Variables¶

Backpressure capacity calculation:

Total capacity = GRANIAN_WORKERS × GRANIAN_BACKPRESSURE

Example: 16 workers × 64 backpressure = 1024 concurrent requests
- Requests 1-1024: Processed normally
- Request 1025+: Immediate 503 Service Unavailable (no queuing)

For production with high concurrency, use GRANIAN_BACKPRESSURE=64 with GRANIAN_WORKERS=16 for 1024 total capacity.

Docker Compose¶

# docker-compose.yml - set HTTP_SERVER to switch servers
services:
  gateway:
    environment:
      - HTTP_SERVER=granian     # Rust-based with native backpressure
      # - HTTP_SERVER=gunicorn  # Python-based, stable (default)
      # Granian backpressure configuration (16 × 64 = 1024 concurrent)
      - GRANIAN_WORKERS=16
      - GRANIAN_BACKLOG=4096
      - GRANIAN_BACKPRESSURE=64
      # - GRANIAN_WORKERS=8
      # - GRANIAN_HTTP=2        # Enable HTTP/2

Container Targets¶

# Run container with Gunicorn (default)
make container-run
make container-run-gunicorn
make container-run-gunicorn-ssl

# Run container with Granian (alternative)
make container-run-granian
make container-run-granian-ssl

# Or pass HTTP_SERVER directly
docker run mcpgateway/mcpgateway                         # Gunicorn (default)
docker run -e HTTP_SERVER=granian mcpgateway/mcpgateway  # Granian

Switching Servers¶

To switch from Gunicorn to Granian:

1. Install Granian: pip install "mcpgateway[granian]"
2. Test locally: make serve-granian
3. Benchmark both servers with your workload
4. If Granian performs better, set HTTP_SERVER=granian in your deployment

Files Changed¶

When to Use Each Server¶

Use Gunicorn + Uvicorn when:¶

• You need maximum stability and battle-tested components
• Your team is familiar with Gunicorn configuration
• You're running behind a reverse proxy that handles HTTP/2
• You need gevent/eventlet worker classes

Use Granian when:¶

• You want native HTTP/2 without a reverse proxy
• You're optimizing for memory efficiency
• You want the simplest possible deployment
• You're comfortable with newer technology
• Your benchmarks show better performance

Recommendation¶

For most users: Stick with Gunicorn (default)

Gunicorn + Uvicorn is battle-tested, well-documented, and provides excellent performance for most workloads. The uvicorn[standard] extras (ADR-0024) already provide significant optimizations.

Consider Granian when: - You need native HTTP/2 without a reverse proxy - Memory usage is a primary concern - Your benchmarks show measurable improvement - You're comfortable with newer technology

Note on Python 3.12+: Granian's Rust task implementation is not available on Python 3.12+, which limits some performance benefits. Both servers use uvloop for async I/O, so the main difference is HTTP parsing.

Status¶

This decision has been implemented. Both servers are available: - Gunicorn + Uvicorn: Default (make serve) - Granian: Alternative (make serve-granian or HTTP_SERVER=granian)

References¶

• GitHub Issue: #1695
• Related ADR: ADR-0024 (uvicorn[standard])
• Granian GitHub: emmett-framework/granian
• Granian Documentation: https://granian.dev/
• Hyper (Rust HTTP): https://hyper.rs/
• Tokio (Rust async): https://tokio.rs/