[docs] Add the sglang-runtime-context skill (#30494)

2026-07-09 02:11:36 -07:00
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+---
+name: sglang-runtime-context
+description: How SGLang's runtime configuration and process-global state are organized (RuntimeContext tiers, resolve-at-end ServerArgs, override entry point, resource/stream/buffer leases, per-forward flags), the CI guardrails that enforce the design, and the idioms for developing and testing against it. Load this before touching server_args, model overrides, module-level state, or per-forward state in sglang.
+---
+
+# SGLang runtime-context architecture
+
+One container owns process-static runtime state: `sglang.srt.runtime_context.RuntimeContext`
+(a process singleton reached via `get_context()`). Everything below is a tier on it.
+
+| Tier | Accessor | Holds | Lifecycle |
+|------|----------|-------|-----------|
+| config | `get_server_args()` | the process-wide **resolved** `ServerArgs` | resolved once in `__post_init__`; audited mutations only |
+| runtime flags | `get_flags()` | state that is *not* a pure function of config: `capture` (cuda-graph lifecycle), `moe` (ACTIVE backends, swappable), `dp` (DP-attention runtime flags) | materialized at subsystem init; groups offer `override()` for tests |
+| resources | `get_resources()`, `get_stream(name)`, `get_buffer(name, factory)` | process-level handles: graph pools, EPLB state, EP dispatcher state, named side streams, workspace buffers | lazy; cleared by `reset_context()` |
+| per-forward | `get_forward()` | forward-scoped flags (multi-stream switch, MoE output buffer, attn-TP inputs, extend-in-batch) | contextvar-backed; `scoped(**kw)` restores on exit; new threads see defaults |
+| parallel | `get_parallel()` | read-through wrapper over topology getters (tp/pp/moe/attn sizes, ranks, groups) | stateless; `override()` for tests |
+
+`reset_context()` (unit-test teardown) drops the published server_args and installs fresh
+flags/resources/forward tiers.
+
+## Config: the resolve-at-end contract
+
+**After `ServerArgs.__post_init__` returns, the fields ARE the resolved configuration.**
+Model overrides and normalization passes *declare* values during resolution (into a
+provenance stash); `materialize_declarations()` applies them once at the very end of
+`__post_init__` (gate order, last writer wins). Consequences:
+
+- **Reading config**: read fields directly, in any process, at any time after construction.
+  For global access use `runtime_context.get_server_args()` (the blessed accessor —
+  `get_global_server_args()` is a legacy shim over the same slot and its call-site count is
+  ratcheted; do not add new ones).
+- **Mutating config after resolution**: the ONLY entry point is
+  `ServerArgs.override(source, **fields)`. It records provenance (`_runtime_mutations`),
+  keeps whitelisted resolvable fields consistent with the declaration stash (so a republish
+  resolves the same values), and bypasses the strict guard. Bare `server_args.x = ...`
+  after resolution **raises** under `SGLANG_STRICT_CONFIG_MUTATION=1`, which the test
+  harness (`sglang.test.test_utils`) turns on by default.
+- **Mid-resolution code** (inside `server_args.py` / `arg_groups/` only): fields are
+  read-only during resolution; handlers and hooks read the in-flight state through
+  `resolved_view(server_args)` / `self._resolved()`. This is pipeline-internal — never use
+  `resolved_view` outside the pipeline. (One sanctioned exception: helpers that the pipeline
+  itself invokes mid-resolution, e.g. `adaptive_spec_params`.)
+
+### Adding a model-specific config adjustment
+
+Never assign `server_args` fields from model code. Declare instead
+(`sglang/srt/arg_groups/overrides.py`):
+
+- Constant per-arch values → `MODEL_OVERRIDES["MyArchForCausalLM"] = {...}`.
+- Derived values → `@register_model_override("MyArchForCausalLM")` returning a dict; the
+  callable receives *pristine* `server_args` + `hf_config` and must not write.
+- Normalization that must see earlier declarations → a post-process pass invoked via
+  `run_post_process_pass` at its slot (reads a view, returns a declaration dict).
+- Values only knowable at weight-load time → `declare_load_time_override(source, {...})`
+  (validates the whitelist, routes through `override()`).
+
+Declarable fields form a whitelist: `Arg(..., resolvable=True)` in the `ServerArgs`
+dataclass. A declaration against a non-whitelisted field fails at its slot.
+
+### Load-time vs resolution-time (critical)
+
+`__post_init__` runs in the launcher process before any model/platform import. Logic that
+consults an **extensible registry** (e.g. out-of-tree platforms registering attention
+backends in `init_backend()`, which runs at `model_runner` import) must stay at load time
+(ModelRunner init), writing through `override()`. Before moving any load-time logic into
+resolution, verify everything it reads is already complete at construction time.
+
+## Runtime flags (`get_flags()`)
+
+For state that init-time code *derives* and runtime code reads — parsed enums, platform
+probes, swappable ACTIVE values. Not for config mirrors (those died with resolve-at-end:
+read the field).
+
+- Groups are typed dataclasses on `Flags` (`capture` / `moe` / `dp`): typo-safe writes,
+  transactional test-only `override(**kw)` context manager.
+- `flags.moe` is materialized by `initialize_moe_config(server_args)` at scheduler init;
+  accessors (`get_moe_a2a_backend` etc.) are thin shims with lazy defaults. The speculative
+  contexts (`speculative_moe_backend_context`) swap the ACTIVE leaves around draft forwards.
+- `flags.dp` is materialized by `initialize_dp_attention`; `is_dp_attention_enabled()` is a
+  shim over `flags.dp.enabled`.
+- Adding a leaf: declare the dataclass field with a default equal to the pre-init behavior,
+  materialize it at the owning subsystem's init, keep any public accessor as a shim.
+
+## Resources (`get_resources()`)
+
+Named slots + two keyed-lazy registries:
+
+- `get_stream(name)` — get-or-create a named CUDA side stream; `set_stream(name, stream)`
+  installs explicitly. **Name leases by subsystem ROLE**: all model alternate streams share
+  `"alt"`; the offloader's copy stream is `"offload"`; DP-TBO comm is `"dp_tbo_comm"`; LoRA
+  side stream is `"lora_side"`. Two call sites may share a name only if their work belongs
+  on one stream — sharing across roles serializes intended overlap.
+- `get_buffer(name, factory)` — get-or-create a named persistent buffer. Grow-only or
+  per-device semantics manage their `resources.buffers` entries directly (see tokenspeed /
+  SM120 split / Marlin workspace). Buffer names are per-backend today; do not silently
+  share.
+- Singletons with manager semantics (EP dispatcher buffers, EPLB recorder/metadata, graph
+  memory pool) keep their owning accessors/classes as facades; only the *state* lives in a
+  resources entry. Preserve exact semantics in the shim: lazy defaults (the EPLB recorder
+  defaults to a Noop instance, not None), publish-once asserts, event-reuse contracts.
+- Stream/buffer creation is a driver call — it must happen outside cuda-graph capture;
+  keep lease points at init/warmup time.
+
+## Per-forward flags (`get_forward()`)
+
+Contextvar-backed; a new thread sees the defaults; `scoped(**kw)` is the regular write path
+(transactional, restores on exit and on exception); `set(name, value)` exists for legacy
+sticky setters (`is_extend_in_batch` is intentionally sticky within a thread). Use this
+tier for anything set-per-forward and read-within-forward. Before adding cross-thread
+state here, prove the readers' thread affinity: contextvars do NOT propagate to already-
+running or newly spawned threads. Note TBO ("two-batch overlap") interleaves ubatches on
+ONE thread — do not design for TBO threads that don't exist.
+
+## Testing idioms
+
+- **Force a code path by overriding causes, not effects**: compose
+  `get_context().override_server_args(**fields)` (config tier: publishes a fresh
+  dummy-boundary `ServerArgs` carrying the overrides — `with`-scoped, or
+  `install()`/`restore()` for fixture-lifetime use) + `get_parallel().override(...)` +
+  `get_flags().<group>.override(...)` + `get_forward().scoped(...)` +
+  `get_resources().override(...)`. All are scoped and transactional. Tests control
+  execution through the context — do not hand-build and publish config objects.
+  Note `override_server_args` is itself transitional (to be deprecated): it exists
+  while production still branches on raw `server_args` fields at runtime; prefer the
+  finer-grained tier overrides wherever they already cover the path you need.
+- **Never monkeypatch import bindings** (`module.get_x = lambda: ...`): production code may
+  read a different accessor over the same slot and your patch silently stops intercepting.
+  Publish/inject for real: `get_context().override_server_args(...)` for config;
+  `monkeypatch.setattr(get_resources(), "slot", fake)` for resources.
+- Fixtures standing in for `ServerArgs` need an `override` method if the code under test
+  mutates config (`_fake_server_args`-style: SimpleNamespace + write-through override).
+  MagicMock swallows `override()` calls silently — prefer SimpleNamespace so misses raise.
+- `reset_context()` in teardown; `_IsolatedServerArgs`-style save/restore when a test
+  publishes.
+- `ServerArgs(model_path="dummy")` early-returns `__post_init__` (no materialization, no
+  strict guard) — fine for lightweight fixtures.
+
+## Guardrails (these fail CI; what to do when they fire)
+
+1. **Strict mutation guard** (`SGLANG_STRICT_CONFIG_MUTATION=1`, default-on in tests):
+   bare `server_args.x = ...` after resolution raises. Fix: route through
+   `override(source, ...)`, or move genuinely config-decidable logic into resolution.
+2. **Mutation ratchet** (`test_server_args_mutation_ratchet.py`, exact pin 0 over the whole
+   package minus the pipeline / multimodal_gen / the mock-fixture factory): textual scan
+   for assignment forms. Never raise the baseline.
+3. **Legacy-accessor ratchet** (`test_legacy_global_ratchet.py`): `get_global_server_args`
+   call sites must not grow — new code uses `runtime_context.get_server_args()`.
+4. **Module-state ratchet** (`test_module_state_ratchet.py`): `global` statements in the
+   flag-owning layers are pinned by name. A new module-level runtime global belongs on a
+   flags group / resources slot instead; migrating a pinned survivor must shrink the pin.
+
+## Hard-won pitfalls (check these before/while refactoring)
+
+- **Moving code drops first-line guards**: early returns (`if self.is_draft_worker: return`)
+  are the easiest thing to lose when relocating a method body. Draft workers share the
+  target's `server_args` object — a draft-side write poisons the target.
+- **Registry-completeness timing**: a gate that consults an extensible list is only correct
+  after the registrars ran (platform `init_backend()` at module import). See "load-time vs
+  resolution-time".
+- **Late function-scope imports shadow module names** for the WHOLE function
+  (UnboundLocalError at earlier lines). Audit moves with AST, not grep.
+- **Lease names are per-role**, not per-API-shape (the offloader-vs-"alt" lesson).
+- **Storage matrix for state read inside torch.compile-traced model code**
+  (piecewise cuda graph compiles the whole model forward): contextvars are
+  untraceable (hard error); dict-slot values are guarded per value — for a
+  per-forward int that is one recompile per distinct size, straight into the
+  recompile limit; **class/instance attributes are the only compile-friendly
+  form** (attribute-source ints get automatic-dynamic after the first size
+  change). Bools (≤2 values) are tolerable in any form — see
+  `ForwardFlags._GRAPH_VISIBLE`. Before moving such state, prove its readers
+  sit outside compile coverage; a piecewise-prefill boot of a small model is
+  the fast check (recompile storms show as `torch._dynamo hit
+  config.recompile_limit` during the compile pass).
+- **Engine-booting e2e tests are the only coverage for launcher-path code**; a child crash
+  kills the process tree and pytest dies silently — run with `PYTHONUNBUFFERED=1` and read
+  child logs.
+- CI arms `SGLANG_ENABLE_ASYNC_ASSERT=1` (device-side `torch._assert_async` probes, e.g.
+  KV-cache OOB): a fired device assert kills the tree with no Python traceback, and the
+  same bug is *silent corruption* locally with the flag off. Arm it when reproducing CI
+  crashes.
+- CI startup logs print the full `server_args=ServerArgs(...)`; diffing that dump between
+  runs is the fastest config-divergence check.
+
+## Where to read the code
+
+Key source files: `python/sglang/srt/runtime_context.py` (the container and every tier),
+`python/sglang/srt/arg_groups/overrides.py` (override registry, passes,
+`declare_load_time_override`, `resolved_view`), `python/sglang/srt/server_args.py`
+(`override`, `__setattr__`, `materialize_declarations` call,
+`_handle_model_capability_adjustments`), and the guardrail tests under
+`test/registered/unit/` (`test_server_args_mutation_ratchet.py`,
+`test_legacy_global_ratchet.py`, `test_module_state_ratchet.py`,
+`test_runtime_context.py` — the last one doubles as executable documentation of
+every tier's semantics).