# SGLang Component

Dynamo's SGLang backend wraps SGLang's inference engine (`sgl.Engine`) and diffusion
generator (`DiffGenerator`) behind Dynamo's distributed runtime. It handles model
registration, request routing, metrics, and disaggregated serving.

## SGLang Backwards Compatibility

SGLang is pre-1.0 and regularly moves/renames internal APIs between releases. We
support the current version plus 1 version back (N and N-1). The pattern:

1. **Only SGLang imports that have actually broken across a version upgrade go through
   `_compat.py`.** Do not preemptively route every `sglang.*` import through the shim --
   import directly until a real breakage is observed. When an upgrade breaks an import,
   move that specific symbol into `_compat.py` and replace direct imports in component
   code with the shim.
2. `_compat.py` uses try/except ImportError: new path first, old path fallback.
3. When SGLang introduces a new class/function that doesn't exist in older versions
   (e.g., `NetworkAddress`), add a minimal polyfill in the except branch -- just
   enough surface area to cover what Dynamo actually calls.
4. Each fallback branch in `_compat.py` MUST have a comment noting which SGLang
   version it supports and when it can be removed, e.g.:
   `# Fallback for sglang <= 0.5.9. Remove when min supported version is 0.6.0+`
5. When a new SGLang version is released and the old N-1 falls outside the support
   window, delete the corresponding fallback branches and polyfills from `_compat.py`.
   If `_compat.py` becomes trivial re-exports, inline the imports and delete the file.

**When you encounter a new SGLang API breakage**: add the affected imports to
`_compat.py` following the existing pattern. Do not scatter try/except blocks across
component files. Do not version-check with `sglang.__version__` -- import probing is
more reliable since SGLang's internal layout doesn't always match the version string.

## Entry Point

`__main__.py` -> `main.py:main()` -> `main.py:worker()`

`worker()` parses args, creates the distributed runtime, installs graceful shutdown,
then dispatches to one of 10 init functions based on CLI flags:

```
args.py:parse_args() -> Config(server_args, dynamo_args)

Worker dispatch (main.py:60-132):
  --image-diffusion-worker    -> init_diffusion.init_image_diffusion()
  --video-generation-worker   -> init_diffusion.init_video_diffusion()
  --embedding-worker          -> init_embedding.init_embedding()
  --multimodal-encode-worker  -> init_multimodal.init_multimodal_encode_worker()
  --multimodal-worker         -> init_multimodal.init_multimodal_worker() or _prefill_worker()
  --dllm-algorithm <algo>     -> init_diffusion.init_llm_diffusion()
  (default, prefill mode)     -> init_llm.init_prefill()
  (default, decode/agg mode)  -> init_llm.init_decode()
```

## Config / Args

`args.py:parse_args()` is the main parsing function. It returns `Config(server_args, dynamo_args)`.

**Two config paths:**

1. **LLM workers** (decode, prefill, embedding, multimodal-worker, dllm): Creates full
   `sglang.srt.server_args.ServerArgs` via `ServerArgs.from_cli_args()`. This triggers
   model config loading, tokenizer detection, etc.

2. **Diffusion workers** (image, video): Creates a minimal `types.SimpleNamespace` stub
   (args.py:350-366) with only the fields needed for `DiffGenerator`. The stub does NOT
   have `max_running_requests`, `dllm_algorithm_config`, or other LLM-specific fields.
   Use `getattr()` when accessing fields that may not exist on the stub.

**DynamoConfig** combines `DynamoRuntimeConfig` (common flags like `--namespace`,
`--output-modalities`, `--media-output-fs-url`) with `DynamoSGLangConfig` (sglang-specific
flags like `--multimodal-encode-worker`, `--embedding-worker`).

Key gotcha: `--output-modalities` defaults to `["text"]` globally. Image/video diffusion
workers override this in their init functions to `["image"]`/`["video"]` to ensure correct
registration with the Rust side.

## Handler Hierarchy

```
BaseGenerativeHandler (handler_base.py)
  Abstract base. Has config, publisher, tracing. No engine.
  Subclasses: ImageDiffusionWorkerHandler, VideoGenerationWorkerHandler

  BaseWorkerHandler (handler_base.py)
    Adds sgl.Engine, tokenizer, priority support, engine routes,
    cancellation, bootstrap (disagg), weight update APIs.
    Constructor accepts engine=None for encode-only workers.

    DecodeWorkerHandler (llm/decode_handler.py)
      Aggregated + disaggregated decode. Token/text streaming.
      Logprob passthrough via _build_logprob_kwargs() + _extract_logprobs().

      DiffusionWorkerHandler (llm/diffusion_handler.py)
        LLM diffusion (DLLM). Simplified decode without disagg.

    PrefillWorkerHandler (llm/prefill_handler.py)
      Disaggregated prefill. Yields bootstrap info first, then consumes.

    EmbeddingWorkerHandler (embedding/embedding_handler.py)
      Uses engine.async_encode() instead of async_generate().

    MultimodalWorkerHandler (multimodal/worker_handler.py)
      Multimodal inference. Aggregated or disaggregated paths.
      Has EmbeddingsProcessor for NIXL-transferred image embeddings.

    MultimodalPrefillWorkerHandler (multimodal/worker_handler.py)
      Multimodal prefill phase. Yields bootstrap info.

    MultimodalEncodeWorkerHandler (multimodal/encode_worker_handler.py)
      Front-facing. No engine. Uses MMEncoder from SGLang. Receives
      pre-tokenized requests (ModelInput.Tokens) from Rust frontend,
      encodes images, NIXL for embeddings transfer.
```

## Engine Types by Worker

| Worker | Engine | Notes |
|--------|--------|-------|
| decode, prefill, dllm, embedding | `sgl.Engine` | Full SGLang inference engine |
| multimodal-worker, multimodal-prefill | `sgl.Engine` | Plus EmbeddingsProcessor |
| multimodal-encode-worker | None | `MMEncoder` from SGLang, pre-tokenized input |
| image-diffusion-worker | `DiffGenerator` | From `sglang.multimodal_gen` |
| video-generation-worker | `DiffGenerator` | From `sglang.multimodal_gen` |

`DiffGenerator.generate()` returns `GenerationResult | list[GenerationResult] | None`
(dataclass, NOT a dict). Access `result.frames` for images/video frames,
`result.samples` for raw tensors.

## Registration

`register.py` has three paths:

1. **LLM** (`register_model_with_readiness_gate`): Builds `ModelRuntimeConfig` with
   bootstrap info, scheduler stats, parser configs. Calls Rust `register_model()` which
   downloads `config.json` + tokenizer from HuggingFace.

2. **Image diffusion** (`register_image_diffusion_model`): Calls `register_model()` with
   `ModelType.Images`. The Rust side skips HF downloads for Images/Videos/Tensor types
   (lib/bindings/python/rust/lib.rs:314) and uses `ModelDeploymentCard::with_name_only()`.

3. **Video generation** (`register_video_generation_model`): Same fast path with
   `ModelType.Videos`.

## Init Flow (typical LLM decode)

```
init_decode():
  engine = sgl.Engine(server_args)
  handler = DecodeWorkerHandler(engine, config, publisher, endpoint, shutdown_event)
  handler.register_engine_routes(runtime)  # profiling, weight updates, memory mgmt
  setup_sgl_metrics(engine, config, endpoint)  # Prometheus + KV events via ZMQ
  asyncio.gather(
    endpoint.serve_endpoint(handler.generate, ...),
    register_model_with_readiness_gate(engine, endpoint, ...),
  )
```

## Disaggregated Serving

Prefill and decode workers coordinate via a bootstrap mechanism:

1. **Prefill handler** generates a `bootstrap_room` (random 63-bit ID)
2. Prefill yields bootstrap info (host, port, room) as its first response
3. **Decode handler** receives bootstrap info, passes it to `engine.async_generate()`
4. SGLang transfers KV cache via NIXL/RDMA between workers

Key functions: `BaseWorkerHandler._get_bootstrap_info()`,
`BaseWorkerHandler._generate_bootstrap_room()`.

## Metrics & Publishing

`publisher.py:DynamoSglangPublisher` manages:
- **Scheduler metrics**: Received via ZMQ from SGLang's scheduler, published to Prometheus
- **KV events**: ZMQ subscribers per DP rank, forwarded via `KvEventPublisher`

Only leader nodes (node_rank==0) run the metrics loop. Non-leader nodes just wait.

`setup_sgl_metrics()` returns `(publisher, metrics_task, metrics_labels)`.

## Graceful Shutdown

`shutdown.py:install_graceful_shutdown()` monkey-patches `loop.add_signal_handler()` to
capture SGLang's internal signal registrations and defer them. On SIGTERM/SIGINT:
1. Unregisters from discovery (stops new requests)
2. Waits grace period for in-flight requests
3. Runs deferred SGLang signal handlers

## Request Flow

```
Frontend (Rust, lib/llm/)
  -> Preprocessor (tokenizes, builds PreprocessedRequest with token_ids + sampling + stop + output_options)
  -> Dynamo RPC to endpoint (dyn://{namespace}.{component}.{endpoint})
  -> Python handler.generate(request_dict, context)
       handler._build_sampling_params(request) -> SGLang-native params
       handler._build_logprob_kwargs(request) -> {return_logprob, top_logprobs_num, logprob_start_len}
       engine.async_generate(**params, **logprob_kwargs) -> async iterator of dicts
       handler yields {token_ids, text, finish_reason, log_probs, top_logprobs, ...} back to frontend
  -> Frontend postprocesses into OpenAI-compatible response
```

Two request formats depending on `--skip-tokenizer-init`:
- **Token-based** (skip_tokenizer_init=True): Frontend tokenizes. Request has `token_ids`,
  `sampling_options`, `stop_conditions`. Handler maps to SGLang params.
- **Text-based** (skip_tokenizer_init=False): SGLang tokenizes. Request is an OpenAI
  `ChatCompletionRequest`. Only `/v1/chat/completions` available.

Image/video diffusion handlers receive the full OpenAI-format request dict directly
(not preprocessed), since the frontend passes through diffusion requests without
tokenization.

## Logprobs

`DecodeWorkerHandler` supports logprob passthrough, matching the vLLM and TRT-LLM backends.
Controlled by `output_options` in the preprocessed request (from Rust `OutputOptions` struct
in `lib/llm/src/protocols/common.rs`).

**Mapping from OutputOptions to SGLang kwargs** (`_build_logprob_kwargs`):

| OutputOptions field | SGLang kwarg | Notes |
|---------------------|-------------|-------|
| `logprobs: N` | `return_logprob=True, top_logprobs_num=N` | N top logprobs per output token |
| `prompt_logprobs: M` | `return_logprob=True, logprob_start_len=0` | Compute from prompt start |
| Both set | `top_logprobs_num=max(N, M)` | SGLang has a single top_logprobs_num for both |

`logprob_start_len` is SGLang-internal, not exposed in OutputOptions. It controls the
absolute sequence position where logprob computation starts: `-1` (default) = output tokens
only (`len(prompt) - 1`), `0` = from prompt start. We set it to 0 when `prompt_logprobs`
is requested.

**Streaming behavior** (`_extract_logprobs`):

Dynamo forces `stream_output=True` (args.py:374), making `output_ids` disjoint per chunk.
However, SGLang's `meta_info["output_token_logprobs"]` and `meta_info["output_top_logprobs"]`
are always **cumulative** — they grow with each chunk. The handler tracks
`num_output_logprobs_so_far` to slice out only new entries per chunk.

SGLang logprob format: `(logprob, token_id, text_or_None)` tuples.
Dynamo output format: `log_probs` = list of floats, `top_logprobs` = list of lists of
`{rank, token_id, token, logprob}` dicts (same as vLLM/TRT-LLM).

## Health Checks

Each worker type has a custom health check payload (`health_check.py`):
- **Decode/Aggregated**: `SglangHealthCheckPayload` -- sends BOS token, expects 1 token back
- **Prefill (disagg)**: `SglangPrefillHealthCheckPayload` -- wrapped `{request, sampling_params}`
- **Image diffusion**: `ImageDiffusionHealthCheckPayload` -- 512x512, 1 inference step, b64_json
- **Video generation**: `VideoGenerationHealthCheckPayload` -- 256x256, 8 frames, 1 step, b64_json

Health check payloads can be overridden via `DYNAMO_HEALTH_CHECK_PAYLOAD` env var (JSON).

## Launch Scripts

Examples in `examples/backends/sglang/launch/`. Each script starts a frontend + worker(s)
in one terminal. GPU requirements are documented in script headers.

```
agg.sh              # 1 GPU  - Single aggregated worker
agg_embed.sh        # 1 GPU  - Embedding model
agg_vision.sh       # 1 GPU  - Multimodal (vision + LLM)
agg_router.sh       # 2 GPUs - Two workers behind KV-aware router
disagg.sh           # 2 GPUs - Prefill + decode on separate GPUs
disagg_router.sh    # 4 GPUs - 2 prefill + 2 decode with KV routing
disagg_same_gpu.sh  # 1 GPU  - Both workers on single GPU (16+ GB VRAM)
multimodal_epd.sh   # 2 GPUs - Encoder + PD worker
multimodal_disagg.sh # 3 GPUs - Encoder + prefill + decode
diffusion_llada.sh  # 1 GPU  - Diffusion language model
image_diffusion.sh  # 1 GPU  - Text-to-image (~38 GB VRAM for FLUX.1-dev)
text-to-video-diffusion.sh  # 1-2 GPUs - Text-to-video (Wan2.1)
```

## Common Pitfalls

- **SimpleNamespace vs ServerArgs**: Image/video diffusion workers use SimpleNamespace
  stubs. Always use `getattr(server_args, field, default)` for fields that may not exist.
- **engine=None**: Multimodal encode worker passes `engine=None` to
  BaseWorkerHandler. Any code in the base class that touches engine must guard with
  `if engine is not None`.
- **GenerationResult is a dataclass**: SGLang `DiffGenerator.generate()`
  returns `GenerationResult` (not a dict). Use `result.frames`, not `result["frames"]`.
- **output_modalities default**: Global default is `["text"]`. Image/video diffusion
  workers must override to `["image"]`/`["video"]` or the Rust registration path tries
  to load `config.json` (which doesn't exist for diffusers models).
- **Cumulative logprobs in streaming**: SGLang's `output_token_logprobs`/`output_top_logprobs`
  in `meta_info` are cumulative even though `output_ids` are disjoint (stream_output=True).
  Always slice with an offset, don't assume per-chunk logprobs.
- **Zombie GPU processes**: `sgl_diffusion::scheduler` spawns a child process that
  survives parent kill. Always check `nvidia-smi` after teardown.
- **Session control graceful degradation**: Session control is request-driven --
  the router's `AgentController` and `StickySessionRouter` are always created but
  activate lazily. If no worker has `--enable-streaming-session`, the router warns
  once and ignores `session_control` in requests. On the handler side,
  `_session_kwargs()` checks `enable_streaming_session` before injecting
  `session_params` into SGLang calls. Both layers must agree: the router skips
  lifecycle RPCs, and the handler skips session params. Without both guards,
  SGLang errors with "session id does not exist".

For troubleshooting (CuDNN, config.json errors, OOM, disagg connectivity), see
`docs/backends/sglang/sglang-examples.md#troubleshooting`.

## Adding a New Worker Type

Checklist for adding a new worker (e.g., a new modality or serving mode):

1. **CLI flag**: Add to `backend_args.py` (DynamoSGLangConfig) and parse in `args.py`
2. **Init function**: Create `init_<type>.py` with `init_<type>(config, runtime)` that:
   - Creates the engine (sgl.Engine, DiffGenerator, or None for encode-only)
   - Creates the handler
   - Sets up metrics (`setup_sgl_metrics` if applicable)
   - Calls `endpoint.serve_endpoint(handler.generate, ...)`
   - Registers the model
3. **Handler**: Subclass `BaseWorkerHandler` (if engine-backed) or `BaseGenerativeHandler`
   (if no engine). Implement `async generate(request, context) -> AsyncGenerator`
4. **Registration**: Add a function in `register.py`. Choose the right `ModelType`:
   - `Chat | Completions` for LLM (Rust downloads config.json + tokenizer)
   - `Images`, `Videos`, `Tensor` for non-LLM (Rust skips HF downloads)
5. **Health check**: Add a payload class in `health_check.py`
6. **Dispatch**: Add the flag check in `main.py:worker()` dispatch block
7. **output_modalities**: If not text, override in the init function (default is `["text"]`)
8. **Launch script**: Add to `examples/backends/sglang/launch/` with GPU count in header

## Tips for AI Assistants

- **Read before editing**: Always read handler_base.py and the relevant init_*.py before
  modifying handler or registration code. The inheritance chain matters.
- **Test with launch scripts**: The fastest way to validate changes is to run the
  corresponding launch script in `examples/backends/sglang/launch/`.
- **Kill zombies between tests**: `pkill -9 -f sglang; sleep 3` before relaunching.
  Diffusion workers spawn child processes (`sgl_diffusion::scheduler`) that survive kills.
- **Check nvidia-smi**: If a launch OOMs, check for orphaned GPU processes from prior runs.
- **SimpleNamespace stubs**: When touching args.py or code that reads server_args, always
  use `getattr(server_args, field, default)` -- image/video workers don't have full ServerArgs.
- **engine can be None**: Encode-only workers (multimodal-encode-worker)
  pass engine=None. Guard any engine access in shared base class code.
- **Rebuild after Rust changes**: If changing registration (register.py interacts with Rust
  bindings), rebuild: `cd lib/bindings/python && maturin develop --uv && cd <root> && uv pip install -e .`
- **Troubleshooting**: See `docs/backends/sglang/sglang-examples.md#troubleshooting`
  for CuDNN, config.json, OOM, and disagg connectivity issues.

## File Index

```
sglang/
  _compat.py               # SGLang version compat shim (network imports + NetworkAddress polyfill)
  __main__.py              # Entry point
  main.py                  # Worker dispatch
  args.py                  # Config parsing (ServerArgs vs SimpleNamespace)
  backend_args.py          # Dynamo-specific SGLang CLI flags
  init_llm.py              # init_decode(), init_prefill()
  init_diffusion.py        # init_llm_diffusion(), init_image_diffusion(), init_video_diffusion()
  init_multimodal.py       # init_multimodal_{encode_worker,worker,prefill_worker}()
  init_embedding.py        # init_embedding()
  register.py              # Model registration (LLM, image, video)
  publisher.py             # Metrics + KV event publishing
  protocol.py              # Request/response Pydantic models
  health_check.py          # Health check payloads per worker type
  shutdown.py              # Graceful shutdown with deferred signal handling
  request_handlers/
    handler_base.py        # BaseGenerativeHandler, BaseWorkerHandler
    llm/
      decode_handler.py    # DecodeWorkerHandler (agg + disagg)
      prefill_handler.py   # PrefillWorkerHandler (disagg prefill)
      diffusion_handler.py # DiffusionWorkerHandler (DLLM)
    embedding/
      embedding_handler.py # EmbeddingWorkerHandler
    image_diffusion/
      image_diffusion_handler.py  # ImageDiffusionWorkerHandler (DiffGenerator)
    video_generation/
      video_generation_handler.py # VideoGenerationWorkerHandler (DiffGenerator)
    multimodal/
      encode_worker_handler.py   # MultimodalEncodeWorkerHandler (MMEncoder, front-facing)
      worker_handler.py          # MultimodalWorkerHandler + PrefillWorkerHandler
```
