Universal Runtime Skills

Best practices and code review checklists for the Universal Runtime - LlamaFarm's local ML inference server.

Overview

The Universal Runtime provides OpenAI-compatible endpoints for HuggingFace models:

• Text generation (Causal LMs: GPT, Llama, Mistral, Qwen)
• Text embeddings (BERT, sentence-transformers, ModernBERT)
• Classification, NER, and reranking
• OCR and document understanding
• Anomaly detection

Directory: runtimes/universal/ Python: 3.11+ Key Dependencies: PyTorch, Transformers, FastAPI, llama-cpp-python

Links to Shared Skills

This skill extends the shared Python practices. Always apply these first:

Topic	File	Priority
Patterns	python-skills/patterns.md	Medium
Async	python-skills/async.md	High
Typing	python-skills/typing.md	Medium
Testing	python-skills/testing.md	Medium
Errors	python-skills/error-handling.md	High
Security	python-skills/security.md	Critical

Runtime-Specific Checklists

Topic	File	Key Points
PyTorch	pytorch.md	Device management, dtype, memory cleanup
Transformers	transformers.md	Model loading, tokenization, inference
FastAPI	fastapi.md	API design, streaming, lifespan
Performance	performance.md	Batching, caching, optimizations

Architecture

runtimes/universal/
├── server.py              # FastAPI app, model caching, endpoints
├── core/
│   └── logging.py         # UniversalRuntimeLogger (structlog)
├── models/
│   ├── base.py            # BaseModel ABC with device management
│   ├── language_model.py  # Transformers text generation
│   ├── gguf_language_model.py  # llama-cpp-python for GGUF
│   ├── encoder_model.py   # Embeddings, classification, NER, reranking
│   └── ...                # OCR, anomaly, document models
├── routers/
│   └── chat_completions/  # Chat completions with streaming
├── utils/
│   ├── device.py          # Device detection (CUDA/MPS/CPU)
│   ├── model_cache.py     # TTL-based model caching
│   ├── model_format.py    # GGUF vs transformers detection
│   └── context_calculator.py  # GGUF context size computation
└── tests/

Key Patterns

1. Model Loading with Double-Checked Locking

_model_load_lock = asyncio.Lock()

async def load_encoder(model_id: str, task: str = "embedding"):
    cache_key = f"encoder:{task}:{model_id}"
    if cache_key not in _models:
        async with _model_load_lock:
            # Double-check after acquiring lock
            if cache_key not in _models:
                model = EncoderModel(model_id, device, task=task)
                await model.load()
                _models[cache_key] = model
    return _models.get(cache_key)

2. Device-Aware Tensor Operations

class BaseModel(ABC):
    def get_dtype(self, force_float32: bool = False):
        if force_float32:
            return torch.float32
        if self.device in ("cuda", "mps"):
            return torch.float16
        return torch.float32

    def to_device(self, tensor: torch.Tensor, dtype=None):
        # Don't change dtype for integer tensors
        if tensor.dtype in (torch.int32, torch.int64, torch.long):
            return tensor.to(device=self.device)
        dtype = dtype or self.get_dtype()
        return tensor.to(device=self.device, dtype=dtype)

3. TTL-Based Model Caching

_models: ModelCache[BaseModel] = ModelCache(ttl=300)  # 5 min TTL

async def _cleanup_idle_models():
    while True:
        await asyncio.sleep(CLEANUP_CHECK_INTERVAL)
        for cache_key, model in _models.pop_expired():
            await model.unload()

4. Async Generation with Thread Pools

# GGUF models use blocking llama-cpp, run in executor
self._executor = ThreadPoolExecutor(max_workers=1)

async def generate(self, messages, max_tokens=512, ...):
    loop = asyncio.get_running_loop()
    return await loop.run_in_executor(self._executor, self._generate_sync)

Review Priority

When reviewing Universal Runtime code:

1. Critical - Security

   • Path traversal prevention in file endpoints
   • Input sanitization for model IDs

2. High - Memory & Device

   • Proper CUDA/MPS cache clearing on unload
   • torch.no_grad() for inference
   • Correct dtype for device

3. Medium - Performance

   • Model caching patterns
   • Batch processing where applicable
   • Streaming implementation

4. Low - Code Style

   • Consistent with patterns.md
   • Proper type hints