feat(stargate): add warmup stabilization detection for early readiness promotion

[navyabijoy]

TL;DR

Stargate replicas configured with a readiness warmup window now promote to ready as soon as their backend count is nonzero and stable, rather than always waiting for the full window. The fixed window remains the upper bound and fallback.

Additional Details

Issue #1320 noted that the fixed warmup window added in #1272 always runs to completion even when Pylons register and stabilize within seconds, slowing scale-up and rolling restarts in the common case.

This adds a background sampler task that polls StargateState::total_active_backend_count every sample_interval (default 1 s) and cancels the readiness token early once the count is nonzero and unchanged for stabilization_window consecutive samples (default 5). A tokio::select! with the fixed deadline as the first arm preserves the original timed-warmup behavior as the fallback.

Two new CLI flags and matching Helm values expose the knobs. Defaults were chosen to match the values described in the issue.

For the Reviewer

• runtime.rs: WarmupConfig struct and run_warmup_stabilization task
• http_proxy.rs: ReadinessState constructors (already_ready, warming_up)
• main.rs: new CLI flag definitions and unit tests
• main/startup.rs: config wiring
• deploy/helm/llm-request-router/llm-request-router/: Helm values and template

For QA

• cargo test -p stargate --lib: 344 passed, 0 failed
• cargo test -p stargate --test stargate_integration health_lifecycle: 5 passed, 0 failed
• cargo clippy -p stargate --all-targets -- -D warnings: clean
• helm lint deploy/helm/llm-request-router/llm-request-router: 0 failed

QA needed: no, covered by unit and integration tests above.

Issues

Closes #1320

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.

Summary by CodeRabbit

• New Features

  • Added configurable readiness stabilization using sampling intervals and consecutive stable backend counts.
  • Readiness can be promoted early after stable backend activity or automatically after the configured warmup period.
  • Added support for disabling warmup and omitting unconfigured readiness settings in Helm deployments.
  • Updated the router image to version 0.14.2.

• Bug Fixes

  • Improved readiness behavior during shutdown and when TLS identity information is unavailable.

• Tests

  • Expanded coverage for warmup, stabilization, timeout, shutdown, and custom configurations.

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• deploy/helm/llm-request-router/llm-request-router/templates/deployment.yaml
• src/libraries/rust/stargate/crates/stargate/src/runtime.rs

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📝 Walkthrough

Walkthrough

Readiness warmup now supports configurable sampling and stabilization. Runtime readiness uses a cancellable token, aggregate active-backend counts, a fixed warmup timeout, and shutdown promotion. Helm values, CLI parsing, startup wiring, proxy checks, and lifecycle tests were updated.

Changes

Readiness warmup

Layer / File(s)	Summary
Readiness configuration and propagation deploy/helm/llm-request-router/..., src/.../main.rs, src/.../main/startup.rs, src/.../runtime.rs	Helm and CLI settings define warmup duration, sampling interval, and stabilization window. Startup propagates these values into WarmupConfig.
Backend count and readiness state src/.../routing_state/*, src/.../runtime.rs, src/.../http_proxy.rs	Runtime sampling reads total active backend counts. Readiness uses a cancellable token and is promoted after stable nonzero samples, timeout, or shutdown.
Readiness lifecycle validation src/.../http_proxy.rs, src/.../runtime.rs, src/.../tests/*	Tests cover configuration validation, warming-up and ready states, timeout promotion, shutdown promotion, and /readyz behavior.

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

Merge Risk: ⚪ Minimal · up to f1715

The change can promote replicas once backend capacity is stable while retaining the fixed warmup as a fallback; no actionable merge-blocking risk remains after normal checks and review.

Sequence Diagram(s)

sequenceDiagram
  participant StargateRuntime
  participant StargateState
  participant ReadinessState
  participant readyz
  StargateRuntime->>StargateState: Sample total_active_backend_count
  StargateState-->>StargateRuntime: Return active backend count
  StargateRuntime->>ReadinessState: Cancel token after stabilization or timeout
  readyz->>ReadinessState: Check readiness
  ReadinessState-->>readyz: Return readiness status

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Possibly related PRs

• NVIDIA/nvcf#1272: Adds the fixed-duration readiness warmup that this change extends.

Suggested reviewers: harshm98

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 52.27% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 44 functions across 8 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title uses the required Conventional Commits format, includes the required scope for a feature, and accurately describes the readiness warmup stabilization change.
Linked Issues check	✅ Passed	The changes satisfy issue #1320. They add configurable sampling and stabilization settings, expose them through CLI, environment, and Helm configuration, promote readiness after stable nonzero backend…
Out of Scope Changes check	✅ Passed	The changes remain within the readiness warmup and stabilization scope of issue #1320. The Helm image update, startup wiring, runtime state changes, and tests support delivery and validation of the fe…

Full details: Linked Issues check

Explanation

The changes satisfy issue #1320. They add configurable sampling and stabilization settings, expose them through CLI, environment, and Helm configuration, promote readiness after stable nonzero backend counts, preserve the warmup timeout fallback, and use success-oriented readiness terminology.

Full details: Out of Scope Changes check

Explanation

The changes remain within the readiness warmup and stabilization scope of issue #1320. The Helm image update, startup wiring, runtime state changes, and tests support delivery and validation of the feature.

Full details: Docstring Coverage

Explanation

Docstring coverage is 52.27% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 44 functions across 8 files. (1 skipped: 1 unsupported.)

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Inline comments:
In `@deploy/helm/llm-request-router/llm-request-router/templates/deployment.yaml`:
- Around line 138-143: Update the readiness argument rendering around the
stabilizationSampleIntervalMs and stabilizationWindow lookups so explicitly
configured numeric zero values are preserved instead of filtered out by with’s
emptiness semantics. Use a presence-aware Helm lookup while retaining omission
when the keys are genuinely absent, allowing the CLI parser to reject zero
values.

In `@src/libraries/rust/stargate/crates/stargate/src/runtime.rs`:
- Line 551: Update the sampling interval created in the runtime sampling flow
around sample_interval to configure Tokio’s missed-tick behavior as
MissedTickBehavior::Delay, preserving the existing interval duration and
preventing delayed ticks from being delivered in a burst.

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📥 Commits

Reviewing files that changed from the base of the PR and between 79b0dc3 and 665efe8.

📒 Files selected for processing (10)

• deploy/helm/llm-request-router/llm-request-router/templates/deployment.yaml
• deploy/helm/llm-request-router/llm-request-router/values.yaml
• src/libraries/rust/stargate/crates/stargate/src/http_proxy.rs
• src/libraries/rust/stargate/crates/stargate/src/main.rs
• src/libraries/rust/stargate/crates/stargate/src/main/startup.rs
• src/libraries/rust/stargate/crates/stargate/src/routing_state/clusters.rs
• src/libraries/rust/stargate/crates/stargate/src/routing_state/mod.rs
• src/libraries/rust/stargate/crates/stargate/src/runtime.rs
• src/libraries/rust/stargate/crates/stargate/tests/common/mod.rs
• src/libraries/rust/stargate/crates/stargate/tests/suite/health_lifecycle.rs

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