feat(stargate): make pulsar-wait-and-widen wait for affinity and widen gradually

[barrygreengus]

TL;DR

pulsar-wait-and-widen becomes an iteration of wait-and-widen. It waits for its Pulsar affinity group, widens through the Pulsar ranking one band per interval, and by default sends overflow only to clusters with a free engine slot. This removes an overload collapse where overflow queued on other keys' primaries.

Additional Details (optional for docs, build, test, refactor, ci, chore, style, and revert PRs)

Why: today a single routing decision walks ranking bands of 1, then 2, 4, 8 and so on, taking the first band with capacity. When a key's primary is busy, overflow goes immediately to its rank-2 and rank-3 clusters, and it may queue there up to max_queued. Those clusters are other keys' primaries. Under load the queued overflow slows their warm traffic, those keys overflow in turn, and cache reuse collapses.

Simulation evidence, from the discrete-event routing simulator in #2223. It runs these load balancers in virtual time against a five-region, 20-backend topology using the older MockDynamo engine model, so the results are relative:

• The current algorithm's token reuse fell from about 95% to under 20% between 240 and 280 RPS, with goodput dropping to cold-prefill capacity (about 200 RPS).
• Waiting for affinity alone only delayed the collapse. Time-gated widening without the guard still collapsed.
• With the overflow guard, the algorithm held 245-251 RPS through 320 RPS offered, and had the highest goodput of every policy tested above capacity, including wait-and-widen. Below capacity it kept about 3x lower TTFT p99 than wait-and-widen, because overflow lands on clusters that already hold the key.

What changed in pulsar_wait_and_widen.rs:

• Affinity group: the top cache_affinity_backend_selection_count clusters of the Pulsar ranking (default 1, the primary). wait-and-widen selection runs within it, with cache_affinity_input_tokens_scale applied. Until cache_affinity_wait_ms elapses, the decision is a timed wait. These two fields were previously rejected for this algorithm.
• Widening: after the wait, the open set grows by one ranking band per band_widen_interval_ms: ranks 1..k+2, then 1..k+6, and so on, until it covers every cluster. wait-and-widen selection runs over the open set at full prefill cost, with bucket unlocks counted from the end of the wait. If nothing in the open set is selectable, it waits for the next band or bucket. The interval defaults to cache_affinity_wait_ms; 0 opens every cluster at once.
• Overflow guard: fallback_max_queued replaces max_queued for open-set selection after the wait. It defaults to 0, so overflow needs a cluster that reports a free engine slot. The affinity group keeps max_queued and is still checked first on every attempt.
• Waits instead of unavailability: when a band or bucket will open later, the decision is a timed wait rather than unavailable, matching wait-and-widen.
• Unchanged: without queue-SLO fields, an eligible primary is still selected immediately.
• Startup validation: band_widen_interval_ms and fallback_max_queued are rejected for wait-and-widen. Both are read only by the Pulsar wait-and-widen constructor; the shared wait-and-widen runtime config does not carry them.
• Routing expressions: pulsar-wait-and-widen expressions accept the affinity group size, wait, and prefill scale, plus the two new fields. wait-and-widen expressions reject the new fields.
• Docs: docs/load-balancer-configuration.md describes the algorithm, the new fields and an example.

Behavior changes for existing pulsar-wait-and-widen configs that set none of the new fields:

• The wait and the interval both default to 0, so every cluster opens at once when the affinity group cannot serve. That is wait-and-widen global selection, instead of the instant ranked-band walk.
• Overflow candidates without a free engine slot are skipped.
• Requests may wait for a band or bucket to open instead of failing over immediately.

For the Reviewer

• decide_at in pulsar_wait_and_widen.rs: the affinity check, the wait, band opening and wait aggregation.
• SelectionPhase selects the wait-and-widen instance and prefill scale for affinity versus fallback. Fallback uses a second WaitAndWidenLoadBalancer whose max_queued is fallback_max_queued.
• keeps_primary_until_queue_slo_requires_first_band_fallback and kv_skip_uses_ranked_fallback_band_and_composes_with_slo encoded the instant walk. They now set an explicit widen interval and elapsed times.
• Suggested deployment values from the simulation: cache_affinity_wait_ms: 300 and band_widen_interval_ms: 300, with the default fallback_max_queued: 0. A deeper primary max_queued (16 in simulation) raised capacity further with the guard on.

For QA (optional for docs, build, test, refactor, ci, chore, style, and revert PRs)

• New tests:
  • affinity_wait_holds_full_primary_before_widening
  • affinity_group_size_widens_the_group_before_fallback
  • locked_fallback_bucket_returns_timed_wait
  • widen_interval_opens_bands_progressively
  • widen_interval_defaults_to_the_affinity_wait
  • fallback_max_queued_defaults_to_free_slot_overflow
  • pulsar_only_settings_are_rejected_for_wait_and_widen
  • pulsar_wait_and_widen_accepts_affinity_settings
• cargo test --locked -p stargate: all pass (405 unit and 150 integration tests).
• cargo clippy --locked -p stargate --all-targets -- -D warnings and cargo fmt --all -- --check: clean.
• bazel test //src/libraries/rust/stargate/crates/stargate/...: 5 of 5 targets pass.
• QA: validate on stargate-dev before changing a production default. These results come from simulation, not from deployed traffic.

No new third-party dependencies.

Issues

Closes #2287

Related Pull Requests

• test(stargate): add discrete-event routing simulator #2223: the routing simulator used for the evidence above.
• feat(stargate): use maximum input TPS for Pulsar weighting #1002: makes maximum input TPS the Pulsar default. Independent of this change.

Checklist

• I am familiar with the Contributing Guidelines.
• I have signed off my commits for Developer Certificate of Origin (DCO) compliance.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features
  • Pulsar wait-and-widen load balancing checks affinity candidates during an initial wait, then progressively opens larger sets of ranked candidates.
  • Configure the interval between ranking-band expansions and the fallback queue limit. Omitted values use defaults, and a zero interval opens all candidates immediately.
  • Affinity settings, including prefill scaling at either end of its valid range, are supported for this algorithm.
• Documentation
  • Updated configuration guidance with the algorithm’s selection behavior, settings, and defaults.

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📝 Walkthrough

Walkthrough

pulsar-wait-and-widen checks a Pulsar-ranked affinity group before opening progressively wider ranking bands. Configuration adds a widening interval and a fallback queue limit. Validation, tests, and documentation cover the settings and selection behavior.

Changes

Pulsar wait-and-widen

Layer / File(s)	Summary
Configure algorithm settings 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/expression.rs, src/libraries/rust/stargate/crates/stargate/src/load_balancer/tests.rs, src/libraries/rust/stargate/docs/load-balancer-configuration.md	The configuration adds band_widen_interval_ms and fallback_max_queued. PulsarWaitAndWiden accepts affinity settings and the new Pulsar-specific settings. WaitAndWiden rejects the new settings. Tests and documentation describe the accepted settings and defaults.
Wait and widen the Pulsar ranking src/libraries/rust/stargate/crates/stargate/src/load_balancer/pulsar_wait_and_widen.rs, src/libraries/rust/stargate/docs/load-balancer-configuration.md	The load balancer checks the affinity group first, then opens wider ranking prefixes after the affinity wait. It returns timed waits for affinity or fallback bucket unlocks and unopened bands. Tests cover widening intervals, selection, and fallback queue limits.

Priority: ➖ Normal

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

Change: Feature · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant LoadBalancer as PulsarWaitAndWidenLoadBalancer
  participant Decision as decide_at
  participant Prefix as decide_from_ranking_prefix
  LoadBalancer->>Decision: evaluate candidates at request elapsed time
  Decision->>Prefix: evaluate affinity ranking prefix
  Prefix-->>Decision: selection, wait, or unavailable
  alt affinity wait has not expired
    Decision-->>LoadBalancer: wait for affinity deadline or bucket
  else affinity wait has expired
    Decision->>Prefix: evaluate opened fallback prefix
    Prefix-->>Decision: selection, wait, or unavailable
    Decision-->>LoadBalancer: timed decision
  end

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Suggested reviewers: famousdirector

Merge Risk: ⚪ Minimal · up to 6f494

No actionable merge-blocking risk remains from the reviewed changes. The default behavior for clusters that do not report concurrency is documented.

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	#2287's affinity group, timed affinity wait, progressive ranking bands, timed bucket and band waits, tests, and configuration documentation are implemented. The fallback selector uses `fallback_max_qu…	Ensure that fallback selection with fallback_max_queued: 0 does not treat max_engine_concurrency == 0 as proof of a free engine slot. Add a test for a candidate with zero reported concurrency.
Docstring Coverage	⚠️ Warning	Docstring coverage is 53.85% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 26 functions across 6 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title follows Conventional Commits with the required scope for feat, and it accurately describes the main change: affinity waiting and gradual band widening for pulsar-wait-and-widen.
Out of Scope Changes check	✅ Passed	The configuration, routing-expression, factory, load-balancer, test, and documentation changes support the objectives in #2287. No unrelated changes are evident.

Full details: Linked Issues check

Explanation

#2287's affinity group, timed affinity wait, progressive ranking bands, timed bucket and band waits, tests, and configuration documentation are implemented. The fallback selector uses fallback_max_queued, which defaults to 0. However, wait_and_widen/estimates.rs::has_capacity returns true when max_engine_concurrency == 0. Therefore, the default fallback limit can admit a candidate without a confirmed free engine slot. The current PR does not change that behavior.

Full details: Docstring Coverage

Explanation

Docstring coverage is 53.85% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 26 functions across 6 files. (1 skipped: 1 unsupported.)

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🛡️ CodeQL Analysis

🚨 Found 5 issue(s)

Severity Breakdown:

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

📋 Top Issues

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_{🕐 Last updated: 2026-10-05 17:16:56 UTC | Commit: dae5545}

[FamousDirector]

Reviewed the routing changes in pulsar_wait_and_widen.rs, the factory validation, config plumbing, and docs. Band math, next-band wait timing, wait aggregation, and the Selected/Wait/Unavailable handoff to the proxy wait loop all check out. cargo test -p stargate --lib load_balancer passes locally (169 tests).

Non-blocking notes:

• has_capacity returns true when max_engine_concurrency == 0, so fallback_max_queued: 0 does not guard engines that don't report concurrency. The docs say overflow "never queues behind another key's primary work"; consider qualifying that.
• Without queue-SLO fields, an eligible primary still wins immediately, so the affinity wait, widening, and overflow guard only engage when the primary is ineligible. The suggested deployment values only help if SLO fields are set too; worth saying in the rollout notes.
• A retry-excluded primary still holds the request for the full cache_affinity_wait_ms even though it can never become eligible. This matches wait-and-widen, so fine to leave as is.

[barrygreengus]

Follow-up on the review-body items for this PR. Changes are in 002ee39, 5405d87, and ad00a1e.

@FamousDirector, on your non-blocking notes:

1. has_capacity treats max_engine_concurrency == 0 as free, so fallback_max_queued: 0 does not guard engines that report no concurrency. The fallback_max_queued row already says this (96faa8d): "A cluster that reports no max_engine_concurrency counts as free." ad00a1e also states that the open set re-checks the affinity group under fallback_max_queued, so a value above max_queued can admit a group member that its own check rejected.
2. Without queue-SLO fields, an eligible primary wins immediately. 002ee39 adds this to the algorithm section of docs/load-balancer-configuration.md: the affinity wait, band widening, and fallback_max_queued apply only when queue-SLO fields are set or the primary is ineligible. ad00a1e drops "capacity" from the overview row for the same reason.
3. A retry-excluded primary waits out cache_affinity_wait_ms. No code change, because this matches wait-and-widen, which fixes group membership before it applies exclusions. ad00a1e documents it, along with the same hold for a group member that consider_kv_free_tokens skips.

CodeRabbit summary comment:

• Docstring Coverage warning (52% of touched functions): no change. The undocumented functions are private helpers and tests, and the struct doc comment explains the algorithm. Doc comments that restate function names would add noise without helping readers.
• No actionable or nitpick comments were posted.

GitHub CodeQL comment: it reports 5 issues with 0 errors, 0 warnings, and 0 notes. The code-scanning API returns no alerts for this PR's ref, so there was nothing to fix.

Other changes from the follow-up review:

• Routing expressions now accept cache_affinity_backend_selection_count, cache_affinity_input_tokens_scale, cache_affinity_wait_ms, band_widen_interval_ms, and fallback_max_queued for pulsar-wait-and-widen. Before this, static config accepted these fields but routing expressions rejected them as not applicable. wait-and-widen expressions reject the two Pulsar-only fields (5405d87).
• The band_widen_interval_ms default now comes from the parsed affinity wait. A redundant empty-candidate check is removed, and two overlong doc lines are rewrapped (002ee39).
• The group-first test is now deterministic instead of relying on 32 random draws. The next-band wait assertion checks the exact value (ad00a1e).

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Review comments at
@src/libraries/rust/stargate/crates/stargate/src/load_balancer/pulsar_wait_and_widen.rs:
- Line 65: Update the fallback admission logic configured by max_queued in
pulsar_wait_and_widen.rs so the default free-slot policy excludes fallback
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