feat(stargate): add affinity waits and honor engine concurrency

[barrygreengus]

Why

WaitAndWiden needs an explicit affinity hold before opening global TTFT buckets. Global fallback must retain the original affinity candidates because full-cost scoring can make a candidate selectable even when discounted affinity buckets cannot select it. Affinity discounts must also remain in final TTFT ranking when bucket ignore flags are enabled. Timed routing must stop when every candidate is permanently excluded and preserve known bucket wake-up times so eligible affine capacity is reconsidered promptly.

Queue estimates also treated active prefill work as waiting time even when the engine had free request slots. A large request already processing could exclude a backend with spare concurrency. Stargate routing and Pylon admission must apply the same capacity rule to prevent avoidable waits and queue-mismatch rejections. Engines without a stats endpoint also need a configured concurrency fallback so operators can supply their known capacity.

What changed

• Return zero effective queue delay when a positive engine concurrency limit exceeds the active request count. Count prefill, decode, and pending routing assignments. Preserve the request's own prefill cost.
• Apply the capacity rule to Stargate queue-based comparators, including PowerOfN, and the expected queue header. Pylon applies it to local admission while excluding the incoming request's own reservation.
• Retain raw token backlog and priority work estimates so filling the last slot restores the existing queue estimate. Full capacity and unknown limits retain existing estimation behavior.
• Carry optional model concurrency limits from engine stats pings into Pylon publication through one update type. Attach and check model generation before applying queued updates; an explicit zero withdraws the engine override.
• Add pylon --max-engine-concurrency N as an optional positive fallback for each model. Publish it from the first registration and use it for local admission. Engine reports take precedence; withdrawal restores the fallback. New model generations inherit the configured fallback.
• Add cache_affinity_wait_ms as X, measured from request arrival. Its default is zero.
• Use the existing TTFT buckets, queue admission, capacity checks, sampling, and comparator within the affinity group.
• Keep global buckets closed before X. After X, global buckets include all backends, including the original affinity group, at full prefill cost. Continue trying affinity selection first on each routing attempt.
• Preserve the original affinity group across retry exclusions. Cold successors remain public candidates with full prefill cost and fallback metrics. When no unexcluded candidates remain, use the existing failure classification and diagnostics without waiting out X.
• Reject non-default affinity wait and prefill-scale settings for PulsarWaitAndWiden.
• Document the fixed-X policy, retry behavior, defaults, and metric impact in the README, configuration reference, and NVCF gateway guide.
• Open the first global TTFT bucket at X and widen later global buckets using only elapsed time after X. Preserve queue admission, capacity checks, and retry exclusions in global selection.
• Return concrete selected, wait, or unavailable decisions to the proxy, and attach algorithm metadata only after selection. Use 25 ms capacity polls, shortened at affine/global bucket unlocks and request deadlines, honor shorter explicit routing deadlines, and retain the existing immediate-selection APIs. Carry affine wait hints through indexed selection and use the earliest known wake-up even after X; selectable global candidates still dispatch immediately.
• Keep cache_affinity_input_tokens_scale in the inclusive range 0 through 1, with default 1. Only the current request prefill estimate during affinity selection is discounted. All global candidates use full prefill cost. Queued work, routing reservations, and the expected queue header sent to Pylon are not discounted.
• Compute bucket and final TTFT scores from the same scaled prefill estimate. Ignore flags apply only to bucket formation; final ranking retains the queue and scaled prefill components. Keep the candidate iterator limited to the exact-length inputs used by current callers.
• Preserve invalid-throughput rejection at scale zero and return configuration errors from constructors instead of panicking.

Existing optimistic routing reservations continue accounting for newly assigned work.

Customer Release Notes

Requests no longer accumulate estimated queue delay solely from active prefill work while the backend reports a free execution slot. Pylon uses the same capacity check for local admission and can use a configured concurrency fallback when the engine does not report a limit.

WaitAndWiden can keep requests within their cache-affinity group for a configured duration before opening global buckets that include every backend. Global bucket widening begins when that duration expires. Affinity prefill discounts remain effective when bucket ignore flags are enabled.

Plan Summary

Not applicable.

Usage

For an engine without a stats endpoint, configure Pylon with its actual request capacity:

pylon --upstream-http-base-url=http://127.0.0.1:8090 --model-name=llama --max-engine-concurrency=25

The fallback is available in every engine stats mode and has no default. Positive engine reports override it per model. A zero report restores the fallback; omitting the field or disconnecting the stream retains the last report. This option informs routing and queue estimates; it does not configure the engine scheduler.

Example model configuration with X = 200 ms:

{
  "algorithm": "wait-and-widen",
  "cache_affinity_backend_selection_count": 2,
  "cache_affinity_wait_ms": 200,
  "cache_affinity_input_tokens_scale": 0.1
}

Send an affinity key to enable the affinity group. Omitting cache_affinity_wait_ms, or setting it to zero, opens global fallback immediately when no affine candidate can be selected.

Neither an SLO header nor x-max-wait-ms is required for a configured affinity wait. x-request-slo-ms continues controlling queue-admission interpolation; it does not shorten X. An explicit x-max-wait-ms shorter than X can expire before global routing becomes eligible.

Each routing attempt
        |
        v
Check affine set A, B -------- selectable ------> dispatch
        |
   no selection
        |
        +--- elapsed < X ----------------------> wait, then retry from top
        |
        +--- elapsed >= X ---> check global set A, B, C, D
                                      |
                                      +--- selectable ---> dispatch
                                      +--- no selection -> retry if budget remains

Affine candidates remain preferred whenever affinity selection can choose them. Global fallback includes the same affine candidates at full prefill cost.

Testing

• cargo test -p stargate: 589 tests passed, including all 142 proxy integration tests.
• New regressions exercise the actual proxy failure path with permanent exclusions and affine bucket wake-ups before and after X. Existing tests cover unexcluded public candidates, shorter request deadlines, global selection, and Pulsar compatibility.
• Affinity-discount regressions verify final ranking with either bucket ignore flag.
• cargo clippy -p stargate --all-targets -- -D warnings: passed.
• Scoped Bazel stargate:stargate_test: passed.
• Focused rustfmt --check and git diff --check: passed.
• Prior validation at 592101755: 511 Pylon library tests passed, along with Pylon/Pylon-library Clippy and Bazel checks. Earlier validation covered 72 Pylon Cargo tests and 55 protocol tests.
• Documentation validation at 7ec0e9883 reported zero Fern errors; redirect checks were skipped because Fern authentication is unavailable. The routing-wait corrections change no documentation files.

The CLI fallback, final TTFT ranking correction, and routing-wait corrections were validated locally; they have not been deployed in the mock-workload environment.

Prior benchmark: deployed both Stargate and Pylon from f9058ea9f and repeated the same 768-request high-context WAW workload with 32 workers, four backends, empty caches, a 200 ms affinity hold, a 60-second request SLO/routing wait, and a 90-second client timeout. All 768 requests succeeded with zero client retries in 250.565 seconds (3.065 requests/second); TTFT p99 was 17.039 seconds and cache-hit requests were 42.32%. All 768 Pylon admissions reported zero expected and local queue delay. Post-run regional verification passed.

This is one mock-workload run. Relative to the previous WAW benchmark, the Pylon binary and startup calibration also changed: calibration was enabled with a concurrency maximum of 25 while periodic canaries stayed off. The newer Pylon includes different throughput accounting, so this comparison does not isolate the free-slot change alone. PowerOf2 was not rerun.

Notes

X is a fixed configuration value. It is independent of the cache-prefill discount and backend estimates.

Existing routing duration, retry count, trace fields, and selection metrics cover this behavior. Metric names and label sets are unchanged. Affinity-phase choices use rank depth 1; global fallback uses a higher rank, including when global selection chooses an affine backend. The existing fallback selection series records global escalation.

Issues

Relates to #1639

Relates to #1659

References

• Cached prefill estimates
• Affinity fallback behavior

Related Pull Requests

None. The Pylon flag does not change invocation APIs; src/clis/nvcf-cli needs no matching change.

Dependencies

None. No third-party dependency, license, or NOTICE changes.

Summary by CodeRabbit

• New Features

  • Added wait-and-widen routing that briefly waits for eligible capacity.
  • Added configurable cache-affinity wait periods and input-token scaling.
  • Routing now falls back to broader capacity after the affinity wait expires.
  • Requests receive clearer outcomes when capacity is selected, temporarily unavailable, or requires waiting.

• Bug Fixes

  • Improved deadline, expiration, unavailable-capacity, and invalid-throughput handling.
  • Corrected latency estimates when input throughput is unavailable.

• Documentation

  • Documented cache-affinity settings, fallback timing, request headers, and routing deadlines.

• Tests

  • Expanded coverage for affinity routing, waiting, fallback behavior, deadlines, validation, and latency calculations.

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📝 Walkthrough

Walkthrough

The load balancer now returns explicit Selected, Wait, and Unavailable decisions. Wait-and-Widen supports cache-affinity prefill scaling, timed fallback, unified affinity caching, and bounded HTTP retry delays.

Changes

Wait-and-Widen affinity routing

Layer / File(s)	Summary
Routing decision contract src/libraries/rust/stargate/crates/stargate/src/load_balancer/{request.rs,algorithm.rs,router.rs,mod.rs}, src/libraries/rust/stargate/crates/stargate/src/http_proxy/run.rs	Adds explicit load-balancer decisions and resolves selected choices through routing metadata before execution.
Affinity configuration and TTFT scaling src/libraries/rust/stargate/crates/stargate/src/load_balancer/{config.rs,cluster_comparator.rs,wait_and_widen.rs,wait_and_widen/estimates.rs,factory.rs,pulsar_wait_and_widen.rs}	Validates affinity scaling and wait settings. Applies scaled prefill estimates and propagates fallible configuration construction.
Affinity wait-and-widen decisions and caching src/libraries/rust/stargate/crates/stargate/src/load_balancer/{wait_and_widen.rs,wait_and_widen/cache_affinity.rs,tests.rs}	Evaluates affinity candidates, returns timed waits for eligible buckets, applies unified key-based FIFO caching, and falls back to public candidates after the affinity hold.
HTTP retry timing and documentation src/libraries/rust/stargate/{crates/stargate/src/http_proxy/{routing.rs,run.rs},crates/stargate/tests/suite/load_balancing.rs,README.md,docs/load-balancer-configuration.md}, docs/user/llm-request-router-load-balancing.md	Bounds retry delays at 25 ms, applies request deadlines, tests affinity fallback timing, and documents configuration and header behavior.

Estimated code review effort: 4 (Complex) | ~45 minutes

Suggested reviewers: famousdirector, along-2017, mikeyrcamp

Merge Risk: 🟡 Moderate · up to 32b11

Requests without a request SLO may remain in affinity routing longer than intended, increasing latency or causing avoidable failures. This behavior should be reconciled before merge.

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 42.47% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 73 functions across 15 files. (1 skipped:…	Write docstrings for the functions missing them to satisfy the coverage threshold.
Title check	⚠️ Warning	The title uses valid Conventional Commits syntax and correctly identifies a feature. However, "honor engine concurrency" is not supported by the described changes and makes the title misleading.	Remove the unsupported engine-concurrency claim or revise the title to accurately describe the primary change, for example: "feat(stargate): add cache-affinity waits to WaitAndWiden".

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Check name	Status	Explanation
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Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

Full details: Docstring Coverage

Explanation

Docstring coverage is 42.47% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 73 functions across 15 files. (1 skipped: 1 unsupported.)

✨ Finishing Touches 💡 1

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[FamousDirector]

adding readme changes would be good to have

[FamousDirector]

Review summary. 146 load-balancer unit tests pass locally on this branch; inline items marked "reproduced locally" were confirmed with scratch unit tests that were not committed.

Two behavior regressions stand out: the new None while the affinity wait budget is locked becomes an immediate 503 for requests that send x-request-slo-ms without x-max-wait-ms, and the ordered primary check ignores TTFT entirely under the default config or when backends report max_engine_concurrency: 0. Details inline.

Items outside the diff:

Docs. src/libraries/rust/stargate/docs/load-balancer-configuration.md L164-167 still say TTFT selection runs within the affinity subset and that selection falls back to the complete candidate set; both are no longer true. The cache-affinity field table (L272-276) lacks cache_affinity_input_tokens_scale, its default, and its 0.0..=1.0 constraint. L277-278 should list it among fields accepted but ignored by pulsar-wait-and-widen (config.rs validates it for that algorithm while pulsar_wait_and_widen.rs hardcodes 1.0; comparator is rejected outright, so consider matching that). The x-request-slo-ms row (L367) and docs/user/llm-request-router-load-balancing.md L94 should say the header now also defers WaitAndWiden global fallback and, without x-max-wait-ms, can produce a 503 with healthy candidates. CONTRIBUTING.md L74 asks for docs updates with user-facing behavior changes.

DCO. Neither commit carries a Signed-off-by trailer. CONTRIBUTING.md L146-148 and .github/dco.yml require it, and recent commits on main all have it. git rebase --signoff or git commit -s --amend before merge.

Direction suggestion: fold the discounted affinity candidates into the existing bucket-unlock pass with a sticky-primary tie-break and the SLO cap applied to sleep_for_at_least_ms, instead of a parallel wait mechanism that expresses waiting as Option::None.

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Actionable comments posted: 3

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Inline comments:
In `@src/libraries/rust/stargate/crates/stargate/src/http_proxy/run.rs`:
- Around line 153-165: Update the affinity decision flow around
WaitAndWidenLoadBalancer::decide_at so requests with
LoadBalancerRequest::request_slo set to None return the public-candidate result
immediately instead of Wait or sleeping in the routing retry branch. Preserve
waiting behavior for requests with an SLO, and add an HTTP routing regression
test covering the no-SLO immediate global fallback path.

In
`@src/libraries/rust/stargate/crates/stargate/src/load_balancer/pulsar_wait_and_widen.rs`:
- Line 55: Update create_load_balancer_with_config to reject PulsarWaitAndWiden
configurations whose wait_and_widen_settings specify a
cache_affinity_input_tokens_scale other than 1.0, returning a clear validation
error while allowing absent or default values. Update the related validation
test to cover this rejection.

In
`@src/libraries/rust/stargate/crates/stargate/src/load_balancer/wait_and_widen.rs`:
- Around line 176-235: The affinity fallback in decide_at must use a
request-specific hold rather than always waiting cache_affinity_wait. Derive the
hold from request.request_slo, reserving the fastest eligible non-affinity
candidate’s TTFT, and cap it at cache_affinity_wait; use that same hold for the
elapsed subtraction before decide_from_candidate_iter while preserving immediate
selection and existing affinity filtering.

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• src/libraries/rust/stargate/crates/stargate/src/http_proxy/routing.rs
• src/libraries/rust/stargate/crates/stargate/src/http_proxy/run.rs
• src/libraries/rust/stargate/crates/stargate/src/load_balancer/algorithm.rs
• src/libraries/rust/stargate/crates/stargate/src/load_balancer/cluster_comparator.rs
• src/libraries/rust/stargate/crates/stargate/src/load_balancer/config.rs
• src/libraries/rust/stargate/crates/stargate/src/load_balancer/factory.rs
• src/libraries/rust/stargate/crates/stargate/src/load_balancer/mod.rs
• src/libraries/rust/stargate/crates/stargate/src/load_balancer/pulsar_wait_and_widen.rs
• src/libraries/rust/stargate/crates/stargate/src/load_balancer/request.rs
• src/libraries/rust/stargate/crates/stargate/src/load_balancer/router.rs
• src/libraries/rust/stargate/crates/stargate/src/load_balancer/tests.rs
• src/libraries/rust/stargate/crates/stargate/src/load_balancer/wait_and_widen.rs
• src/libraries/rust/stargate/crates/stargate/src/load_balancer/wait_and_widen/estimates.rs
• src/libraries/rust/stargate/crates/stargate/tests/suite/load_balancing.rs

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🛡️ CodeQL Analysis

🚨 Found 5 issue(s)

Severity Breakdown:

• 🔴 Errors: 0
• 🟡 Warnings: 0
• 🔵 Notes: 0

📋 Top Issues

🔗 View full details in Security tab

_{🕐 Last updated: 2026-09-09 19:51:47 UTC | Commit: 6cdd97f}

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🌿 Preview your docs: https://nvidia-preview-codex-waw-sticky-slo-fallback.docs.buildwithfern.com/nvcf

[barrygreengus]

Review follow-up for the general comments:

• README request: updated the Stargate README, configuration reference, and NVCF gateway guide. They now cover X, defaults, prefill scaling, retry membership, unsupported Pulsar settings, headers, and metric impact.
• DCO: all five PR commits now include the configured author's Signed-off-by trailer. I verified that the history rewrite preserved the tested source tree and pushed it with an exact lease.
• CodeQL summary: that message refers to 6cdd97f. The later CodeQL check at ff4f6ef reported no new alerts in changed code and zero annotations. The summary contains no actionable locations.
• Docstring coverage: documented the public routing decision contracts and operator behavior. I retained concise private helpers and tests under the repository's instruction to comment on non-obvious logic; the coverage advisory does not justify redundant docstrings.

All 15 inline threads have replies and are resolved. The two SLO-policy requests were not adopted because they would open public candidates before the configured X.

Validation at the new source tree: 580 Cargo tests, Clippy, Bazel, and formatting passed. The docs check found zero Fern errors. Catalog sync was advisory because this checkout has no imports.yaml; redirect checking was unavailable without Fern authentication.

[balajinvda]

🎉 This PR is included in version stargate-v0.18.0 🎉

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