xpyd 1.1.0

Lightweight Prefill-Decode proxy for disaggregated LLM serving

xPyD Proxy

A lightweight Prefill-Decode (PD) proxy for disaggregated LLM serving.

Architecture

xPyD Proxy supports two operating modes:

Prefill-Decode (P/D) Disaggregated Mode

Requests are routed through two phases with KV cache transfer:

1. Prefill — KV cache preparation on prefill nodes (max_tokens=1)
2. Decode — autoregressive token generation on decode nodes (receives KV cache from prefill)

Dual-Role Mode

A single instance handles both prefill and decode in one pass — no KV transfer needed. This simplifies deployment when disaggregation is not required or for smaller-scale setups.

The proxy handles scheduling (load-balanced, round-robin, consistent hash, power-of-two, cache-aware), health monitoring, circuit breaking, and dynamic instance management. Multi-model routing allows serving multiple models through a single proxy with per-model scheduler configuration.

See docs/architecture.md for details.

Installation

pip install .

# Verify
xpyd --version

Quick Start

# Generate a config template
xpyd proxy --init-config

# Edit xpyd.yaml with your model and node addresses, then:
xpyd proxy -c xpyd.yaml

Configuration

All configuration is done via YAML. Three config formats are supported.

Format 1: Legacy (Single Model)

Simple prefill/decode address lists for a single model:

model: /path/to/model
prefill:
  - "10.0.0.3:8100"
decode:
  - "10.0.0.1:8200"
  - "10.0.0.2:8200"
port: 8000
scheduling: loadbalanced

Topology-style config is also supported in Format 1:

model: /path/to/model
port: 8868

prefill:
  nodes:
    - "10.0.0.1:8100"
  tp_size: 8
  dp_size: 1
  world_size_per_node: 8

decode:
  nodes:
    - "10.0.0.2:8200"
    - "10.0.0.3:8200"
  tp_size: 1
  dp_size: 16
  world_size_per_node: 8

Format 2: Instances (Multi-Model, Per-Instance Role)

Explicit per-instance configuration with role and model assignment. Supports dual role:

instances:
  - address: "10.0.0.1:8000"
    role: prefill
    model: llama-3
  - address: "10.0.0.2:8000"
    role: decode
    model: llama-3
  - address: "10.0.0.3:8000"
    role: dual
    model: qwen-2
port: 8000
scheduling: loadbalanced

Format 3: Models Shorthand (Multi-Model, Per-Model Scheduler)

Compact format with per-model scheduler override and dual shorthand:

models:
  - name: llama-3
    prefill:
      - "10.0.0.1:8000"
    decode:
      - "10.0.0.2:8000"
    scheduler: round_robin
  - name: qwen-2
    dual:
      - "10.0.0.3:8000"
      - "10.0.0.4:8000"
    scheduler: loadbalanced
port: 8000

Note: instances and models cannot be combined. Legacy prefill/decode lists cannot be used with instances or models.

See examples/proxy.yaml for a fully-commented example.

CLI Reference

xpyd proxy [OPTIONS]

Options:
  --config, -c PATH         Path to YAML config (default: ./xpyd.yaml or XPYD_CONFIG env)
  --init-config [PATH]      Generate a config template and exit
  --validate-config PATH    Validate a config file and exit
  --port PORT               Override port from config
  --log-level LEVEL         Override log level: debug|info|warning|error
  --version, -V             Show version and exit

xpyd fix-config CONFIG_PATH [OPTIONS]

Auto-fix common config mistakes (typos, missing ports, whitespace).

Arguments:
  CONFIG_PATH               Path to YAML config file to fix

Options:
  --write                   Write fixes back to file (creates timestamped .bak backup).
                            Note: does not preserve YAML comments or formatting.
  --interactive             Prompt for confirmation on ambiguous suggestions

Config resolution order

1. --config / -c CLI argument
2. XPYD_CONFIG environment variable
3. ./xpyd.yaml in the current directory

YAML Config

# Required
model: /path/to/model
decode:
  - "10.0.0.1:8200"
  - "10.0.0.2:8200"

# Optional
prefill:
  - "10.0.0.3:8100"
port: 8000
log_level: warning
scheduling: loadbalanced   # roundrobin | loadbalanced | consistent_hash | power_of_two | cache_aware
generator_on_p_node: false

See examples/proxy.yaml for a fully-commented example.

YAML Fields Reference

Field	Type	Default	Description
model	string	—	Model name / path (required in Format 1)
port	int	8000	Proxy listen port
log_level	string	warning	Log level: debug, info, warning, error
prefill	list or topology	[]	Prefill node config (Format 1)
decode	list or topology	—	Decode node config (Format 1, required)
instances	list	—	Per-instance config (Format 2): {address, role, model}
models	list	—	Per-model shorthand (Format 3): {name, prefill, decode, dual, scheduler}
scheduling	string	loadbalanced	Global scheduling policy
scheduling_config	dict	{}	Policy-specific options
generator_on_p_node	bool	false	Generate first token on prefill node
admin_api_key	string	—	Admin API key (env ADMIN_API_KEY overrides)
openai_api_key	string	—	OpenAI API key (env OPENAI_API_KEY overrides)
startup.wait_timeout_seconds	int	600	Max wait for nodes at startup
startup.probe_interval_seconds	int	10	Health probe interval

Valid role values: prefill, decode, dual

Valid scheduling values: loadbalanced, roundrobin (alias: round_robin), load_balanced, consistent_hash, power_of_two, cache_aware

API

The proxy exposes an OpenAI-compatible API:

• POST /v1/chat/completions — Chat completions (streaming and non-streaming)
• POST /v1/completions — Text completions (streaming and non-streaming)
• GET /v1/models — List all registered models in OpenAI-compatible format

Startup Node Discovery

The proxy returns 503 on business endpoints until the minimum instance requirement is met: at least 1 prefill + 1 decode node, or 1 dual node per model must respond healthy. Health/status/metrics endpoints are always available.

Docker

# Full local topology (prefill + decode + proxy)
docker compose up --build

# Proxy only, connecting to existing GPU nodes
docker build -t xpyd .
docker run -p 8868:8868 -v ./config.yaml:/app/xpyd.yaml xpyd

See docs/deployment.md for production deployment.

Benchmark

python -m vllm bench serve \
  --base-url http://localhost:8868 \
  --model DeepSeek-R1 \
  --dataset-name sonnet \
  --sonnet-input-len 1024 \
  --sonnet-output-len 128 \
  --num-prompts 100 \
  --request-rate 10

Development

# Install in dev mode
pip install -e ".[dev]"

# Run tests
python -m pytest tests/unit/ tests/integration/ -v

# Lint
pre-commit run --all-files

Environment Variables

Variable	Description
XPYD_CONFIG	Default config file path
ADMIN_API_KEY	Admin API key (overrides YAML)
OPENAI_API_KEY	Bearer token for backend nodes (overrides YAML)
PREFILL_DELAY_PER_TOKEN	Simulated prefill latency for dummy nodes (default: 0.001s)
DECODE_DELAY_PER_TOKEN	Simulated decode latency for dummy nodes (default: 0.01s)

Documentation

Document	Description
Architecture	System architecture overview
API Reference	HTTP API endpoints
Configuration	YAML config reference
CLI	xpyd command-line tool
Scheduling	Load balancing strategies
Resilience	Health checks, circuit breakers, retry
Metrics	Prometheus metrics endpoint
Deployment	Deployment and Docker guide
Contributing	Contribution guidelines

License

Apache-2.0