feat(router): opt-in HTTP/2 prior-knowledge connections to workers

[slin1237]

Description

Problem

The router speaks HTTP/1.1 to every HTTP worker. Under streaming traffic that costs one TCP connection per in-flight request plus keep-alive idles: on a large fleet a single router holds on the order of 150k upstream sockets. Beyond file-descriptor pressure, every socket that drains slowly — a stalled relay, a slow peer — pins kernel memory for its lifetime, and connection churn (RST/FIN storms on rollouts) scales with the socket count.

Engines are growing cleartext HTTP/2 support (SGLang ships --enable-http2, serving HTTP/1.1 + h2c by auto-negotiation per connection), but the router has no way to use it: with default-features = false the http2 feature isn't even compiled into reqwest, and plaintext connections never ALPN-negotiate.

Solution

--upstream-http2 (config: upstream_http2, default off). When set, the shared upstream client is built with HTTP/2 prior knowledge — h2c on cleartext — so every request to a worker multiplexes over one connection: roughly one connection per worker per router instead of one per in-flight request.

Tuning that makes it viable for streaming, applied only under the flag:

• Flow-control windows start at 2MB (stream) / 16MB (connection) with the adaptive window enabled. The h2 defaults (64KB) would let a few thousand concurrent token streams throttle each other into lockstep.
• h2 PING keepalives (30s interval / 20s timeout, active while idle) replace per-connection TCP keepalive churn and detect dead peers under long-lived streams.

The flag is deliberately a whole-client switch, not per-worker plumbing: the router already builds exactly one upstream client for all workers (single security domain, documented at the with_client FIXME), and prior knowledge composes with that design. Mixed fleets — some workers h2-capable, some not — should not set the flag; because h2c-capable servers auto-negotiate per connection, backends can enable HTTP/2 fleet-wide before the router opts in, and the router can roll back independently, in either order, with no coordination.

Interaction with worker selection

None. Connection protocol is transport-level; routing policies, health checks, and load polling are unchanged. Load polling (/v1/loads) rides the same multiplexed connection, so a poll tick over N workers stops being N cold TCP round-trips.

Changes

• model_gateway/Cargo.toml — add reqwest http2 feature (workspace pins default-features = false, so it was previously not compiled in).
• model_gateway/src/config/types.rs — RouterConfig.upstream_http2 (serde-defaulted) + Default.
• model_gateway/src/config/builder.rs — builder method.
• model_gateway/src/main.rs — --upstream-http2 CLI flag (Worker Configuration) wired through to_router_config.
• model_gateway/src/app_context.rs — client construction applies prior knowledge + window/keepalive tuning under the flag.
• bindings/python — upstream_http2 threaded through _Router kwargs, RouterArgs dataclass, --upstream-http2 CLI arg, and the Router docstring (from_cli_args auto-maps dataclass fields, so the launcher path picks it up with no further wiring).

Test Plan

Two tests in app_context, using a loopback axum::serve listener — which accepts HTTP/1.1 and prior-knowledge h2c on the same port via hyper-util's auto builder, i.e. exactly the dual-protocol behavior of an h2-enabled engine:

Test	Asserts
upstream_http2_client_speaks_h2c_prior_knowledge	client built with the flag negotiates HTTP_2 on a cleartext connection and round-trips a body
default_client_stays_http1	flag off → HTTP_11, byte-identical behavior to today

$ cargo test -p smg          # lib + every integration binary
2126 passed; 0 failed

$ cargo build -p smg-python && cargo build -p smg-golang   # both green
$ cargo +nightly fmt --all                                  # clean
$ cargo clippy --all-targets -- -D warnings                 # clean

Clippy was run without --all-features: the full feature set pulls an opencv dependency that does not build in my local environment. CI covers the complete matrix.

Checklist

• cargo +nightly fmt passes
• cargo clippy --all-targets --all-features -- -D warnings passes
• (Optional) Documentation updated
• (Optional) Please join us on Slack #sig-smg to discuss, review, and merge PRs

[claude]

Clean, well-tested PR. Config wiring is complete across all necessary paths (types.rs, builder.rs, main.rs CLI, Python bindings). The HTTP/2 client tuning (flow-control windows, adaptive window, h2 PING keepalives) is appropriate for multiplexing streaming traffic. Tests cover both the h2c and HTTP/1.1 paths. No issues found.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📥 Commits

Reviewing files that changed from the base of the PR and between 9f4e377 and 719ab96.

📒 Files selected for processing (2)

• bindings/python/src/lib.rs
• bindings/python/src/smg/router_args.py

🚧 Files skipped from review as they are similar to previous changes (2)

• bindings/python/src/lib.rs
• bindings/python/src/smg/router_args.py

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📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Added an optional upstream_http2 setting for Python, CLI, and router configuration.
  • Added support for cleartext HTTP/2 prior-knowledge connections to upstream workers.
  • HTTP/2 upstream communication is disabled by default and requires compatible workers.
  • Added the --upstream-http2 command-line option.

• Tests

  • Added coverage confirming HTTP/2 communication when enabled and HTTP/1.1 behavior by default.

Walkthrough

Changes

Upstream HTTP/2 support

Layer / File(s)	Summary
Configuration and CLI wiring model_gateway/src/config/types.rs, model_gateway/src/config/builder.rs, model_gateway/src/main.rs, bindings/python/src/smg/router_args.py	Adds the disabled-by-default upstream_http2 configuration field and exposes it through Rust and Python CLI options.
HTTP/2 upstream client behavior model_gateway/Cargo.toml, model_gateway/src/app_context.rs	Enables reqwest HTTP/2 support and conditionally configures h2c prior-knowledge connections. Tests cover enabled HTTP/2 and default HTTP/1.1 behavior.
Python Router integration bindings/python/src/lib.rs, bindings/python/src/smg/router.py	Adds the Python constructor option, stores it, forwards it to RouterConfig, and documents its worker requirements.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 719ab

The HTTP/2 upstream behavior is opt-in and preserves existing HTTP/1.1 behavior by default; no actionable merge-blocking risk remains after normal checks and review.

Sequence Diagram(s)

sequenceDiagram
  participant RouterConfig
  participant AppContext
  participant reqwest
  participant UpstreamWorker
  RouterConfig->>AppContext: provide upstream_http2
  AppContext->>reqwest: configure h2c prior knowledge when enabled
  reqwest->>UpstreamWorker: send HTTP/2 or HTTP/1.1 request
  UpstreamWorker-->>reqwest: return protocol response

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Possibly related PRs

• smg-project/smg#2038: Propagates a new Router configuration option through the Python bindings, CLI, RouterConfig, and builder.

Suggested reviewers: catherinesue, key4ng

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: opt-in HTTP/2 prior-knowledge connections from the router to workers.
Description check	✅ Passed	The description directly explains the HTTP/2 feature, configuration changes, implementation details, and test coverage.
Docstring Coverage	✅ Passed	Docstring coverage is 85.71% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

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• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/upstream-http2

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@bindings/python/src/smg/router_args.py`:
- Line 79: Preserve positional constructor compatibility by moving
upstream_http2 after prefix_hash_balance_abs_threshold in RouterArgs, the PyO3
constructor signature in bindings/python/src/lib.rs:910, and Router::new in
bindings/python/src/lib.rs:1045; add regression tests covering existing
positional calls and confirming later arguments retain their original bindings.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 9cf0c66e-764a-4e56-9a86-a50dac35566f

📥 Commits

Reviewing files that changed from the base of the PR and between e40d010 and 9f4e377.

📒 Files selected for processing (8)

• bindings/python/src/lib.rs
• bindings/python/src/smg/router.py
• bindings/python/src/smg/router_args.py
• model_gateway/Cargo.toml
• model_gateway/src/app_context.rs
• model_gateway/src/config/builder.rs
• model_gateway/src/config/types.rs
• model_gateway/src/main.rs