Update README: haproxy proxy layer, /health probe fix, current state

README.md

@@ -8,22 +8,51 @@ The vLLM production stack handles model lifecycle, scaling, and routing — but
 
 ## How It Works
 
-Two interception paths:
+### Invocation interception
+
+Two interception paths catch however the vLLM stack tries to start the server:
 
 | What the stack calls | What happens |
 |---|---|
-| `vllm serve <model> [flags]` | Shell shim (`vllm-shim.sh`) parses args, execs `python -m sglang.launch_server` |
-| `python -m vllm.entrypoints.openai.api_server` | Python shim (shadow module on `PYTHONPATH`) does the same |
+| `vllm serve <model> [flags]` | Shell shim (`vllm-shim.sh`) replaces the `vllm` binary |
+| `python -m vllm.entrypoints.openai.api_server` | Python shim (shadow module on `PYTHONPATH`) intercepts the import |
 
-Both extract `--host` and `--port` from whatever the stack sends and forward them to SGLang. Everything else is currently hardcoded for the target model.
+Both extract `--host` and `--port` from whatever the stack sends.
+
+### haproxy proxy layer
+
+Rather than launching SGLang directly on the vLLM port, the shim runs **haproxy** on the original port and **SGLang on port+1**. This solves two critical problems:
+
+1. **`/metrics` stub** — The vLLM stack expects a Prometheus metrics endpoint at `/metrics`. SGLang doesn't serve one. haproxy intercepts `/metrics` and returns an empty 200 response instantly.
+
+2. **`/health` probe timing** — SGLang's `/health` endpoint takes ~1.001s to respond, which races the 1s k8s probe timeout and causes repeated `Startup probe failed: context deadline exceeded`. haproxy health-checks SGLang in the background (every 5s, with a 3s timeout) and responds to `/health` probes **instantly** — 200 if the backend is up, 503 if it's not. No more timeout roulette.
+
+```
+┌─────────────────────────────────────────────┐
+│  k8s probes / vLLM stack                    │
+│         │                                   │
+│         ▼                                   │
+│  haproxy (port 8000)                        │
+│    /metrics ──► 200 empty (stub)            │
+│    /health  ──► 200/503 instant (backend    │
+│                 health-checked in bg)        │
+│    /*       ──► proxy to SGLang             │
+│                       │                     │
+│                       ▼                     │
+│              SGLang (port 8001)             │
+└─────────────────────────────────────────────┘
+```
+
+haproxy 2.4 compat: uses `errorfile` + `http-request deny deny_status` for stub responses (the `http-request return` payload syntax requires haproxy 2.8+).
 
 ## Current State
 
-**PoC — hardcoded for `mistralai/Devstral-2-123B-Instruct-2512` on 8× MI300X.**
+**Running in production — `mistralai/Devstral-2-123B-Instruct-2512` on 8× MI300X.**
 
 - Model path, `--tp 8`, and `--tool-call-parser mistral` are baked into both shims
 - The Dockerfile builds on `lmsysorg/sglang-rocm` and patches a broken `aiter` build from the base image
 - MI300X tuning env vars are set (`HIP_FORCE_DEV_KERNARG`, `NCCL_MIN_NCHANNELS`, etc.)
+- All received args are logged to `/tmp/vllm-shim.log` (configurable via `VLLM_SHIM_LOG` env var)
 
 ## Building
 
@@ -31,14 +60,23 @@ Both extract `--host` and `--port` from whatever the stack sends and forward the
 docker build -t vllm-to-sglang .
 ```
 
+Or use the Jenkins pipeline:
+
+```bash
+curl -X POST "https://jenkins.sweetapi.com/job/vllm-to-sglang/buildWithParameters" \
+  -u "${JENKINS_USER}:${JENKINS_PASS}" \
+  -d "BRANCH=metrics" \
+  -d "TAG=nightly3"
+```
+
 Then use this image anywhere the vLLM stack expects its server image.
 
 ## Making It Work For Other Models
 
 Right now the model config is hardcoded in three places:
 
-- `vllm-shim.sh` — the `exec python -m sglang.launch_server` line
-- `vllm_shim_module.py` — the `os.execvp()` call
+- `vllm-shim.sh` — the `python -m sglang.launch_server` line
+- `vllm_shim_module.py` — the `subprocess.Popen()` call
 - `Dockerfile` — base image and ROCm-specific patches
 
 To adapt for a different model, change `--model-path`, `--tp`, and `--tool-call-parser` in both shim files. A future pass will make this configurable via env vars or args so you don't have to edit source.
@@ -47,6 +85,6 @@ To adapt for a different model, change `--model-path`, `--tp`, and `--tool-call-
 
 | File | Purpose |
 |---|---|
-| `Dockerfile` | Builds the image: ROCm SGLang base + aiter fix + shims + MI300X env |
-| `vllm-shim.sh` | Shell shim — replaces the `vllm` binary |
-| `vllm_shim_module.py` | Python shim — shadows `vllm.*` module imports |
+| `Dockerfile` | Builds the image: ROCm SGLang base + haproxy + shims + MI300X env |
+| `vllm-shim.sh` | Shell shim — replaces the `vllm` binary, launches SGLang + haproxy |
+| `vllm_shim_module.py` | Python shim — shadows `vllm.*` module imports, launches SGLang + haproxy |