feat: add Prometheus metrics, fix GPU resource rewriting, and harden deployment by shenxianpeng · Pull Request #10 · castops/cast-slice

shenxianpeng · 2026-03-24T21:56:49Z

Summary

Prometheus metrics: Add castslice_requests_total, castslice_mutations_total, castslice_noop_total, castslice_errors_total counters scraped via :8080/metrics
Grafana FinOps dashboard: 5-panel ConfigMap with request rate, mutation rate, no-op rate, error rate, and mutation efficiency ratio (with division-by-zero guard)
Critical bug fix: GPU resource rewriting now mirrors nvidia.com/gpu → nvidia.com/gpu-shared in both Limits and Requests — previously Requests was not updated, causing pod scheduling failures (Kubernetes device plugin contract violation)
Bug fix: Proportional scaling corrected — 2 GPUs × ratio 4 = 8 shared (was previously always = ratio, ignoring original GPU count)
Deployment hardening: replicas 1→2, liveness probe, PodDisruptionBudget (minAvailable: 1)
slice-ratio upper bound: Values > 64 now rejected with a clear error message
Safe metric registration: mustRegister() helper replaces promauto to prevent panic on duplicate registration (go test -count=2)
Production logger: Development: false in zap options

Test Coverage

CODE PATH COVERAGE
===========================
[+] internal/webhook/pod_webhook.go
    ├── resolveSliceRatio()
    │   ├── [★★★ TESTED] slice-ratio valid — TestHandle_SliceRatioAnnotation_ExplicitOverride
    │   ├── [★★★ TESTED] slice-ratio zero/negative/non-integer — TestHandle_InvalidSliceRatio
    │   ├── [★★★ TESTED] slice-ratio > 64 rejected — TestHandle_SliceRatioOverLimit (NEW)
    │   ├── [★★★ TESTED] workload-type training/inference → ratio — TestHandle_WorkloadType_*
    │   └── [★★★ TESTED] workload-type unknown rejected — TestHandle_InvalidWorkloadType
    ├── rewriteGPUResources()
    │   ├── [★★★ TESTED] Requests mirrored when set — TestHandle_RequestsMirrored (NEW)
    │   └── [★★★ TESTED] Proportional scaling 2 GPUs × ratio 4 = 8 — TestHandle_ProportionalScaling (NEW)
    └── Handle()
        ├── [★★★ TESTED] No annotation → no-op
        ├── [★★★ TESTED] Init containers
        └── [★★  TESTED] Multiple containers

Tests: 11 → 14 (+3 new)

Pre-Landing Review

Full /review pass completed (structured + adversarial, large-tier, 4 passes including Codex). 8 issues fixed, 0 remaining. Logged status: clean.

Design Review

No frontend UI files changed — design review skipped.

Eval Results

No prompt-related files changed — evals skipped.

TODOS

No TODO items completed in this PR.

Test plan

All Go tests pass: 14 tests, 0 failures (go test ./internal/webhook/... -count=1)
Build clean: go build ./...
Metrics registration safe: go test -count=2 (no panic on duplicate registration)

🤖 Generated with Claude Code

- Add castslice_requests_total, mutations_total, noop_total, errors_total counters registered against controller-runtime metrics registry - Use mustRegister() helper (AlreadyRegisteredError) instead of promauto to prevent panic on duplicate registration (e.g. go test -count=2) - Fix rewriteGPUResources to mirror Limits change into Requests (required by Kubernetes device plugin contract for extended resources) - Fix proportional scaling: shared = originalGPUCount × sliceRatio - Add maxSliceRatio=64 upper-bound validation on slice-ratio annotation - Set zap.Options{Development: false} for production logging

- Increase replicas from 1 to 2 for high availability - Add liveness probe (httpGet /healthz :8081) to detect deadlocked replicas - Add PodDisruptionBudget (minAvailable: 1) to prevent total webhook outage during voluntary node drains - Add cast-slice-metrics Service exposing :8080 with Prometheus scrape annotations - Add Grafana FinOps dashboard ConfigMap with 5 panels (request rate, mutation rate, no-op rate, error rate, mutation efficiency ratio with division-by-zero guard)

…slice-ratio upper bound

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Copilot

Pull request overview

This PR adds operational observability (Prometheus + Grafana) to the CastSlice mutating webhook, fixes GPU resource rewrite behavior to satisfy Kubernetes extended-resource rules, and hardens the in-cluster deployment for higher availability.

Changes:

Add Prometheus counters (requests/mutations/noop/errors) registered in the controller-runtime metrics registry and scraped via :8080/metrics.
Fix GPU rewrite logic to (a) scale gpu-shared proportionally by original GPU count and (b) mirror rewrites into Requests when explicitly set.
Harden deployment (replicas 2, liveness probe, metrics Service, PodDisruptionBudget) and update docs/versioning.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
main.go	Switch zap to production-mode logging (`Development: false`).
internal/webhook/pod_webhook.go	Add slice-ratio upper bound; rewrite GPU logic + increment metrics counters.
internal/webhook/metrics.go	Define/register Prometheus counters with safe registration helper.
internal/webhook/pod_webhook_test.go	Add/extend tests for metrics deltas and GPU rewrite correctness.
config/deploy/deployment.yaml	Increase replicas, add liveness probe, add metrics Service, add PDB.
config/monitoring/grafana-dashboard.yaml	Add Grafana ConfigMap dashboard for the new counters.
go.mod	Add `prometheus/client_golang` as a direct dependency.
README.md	Document metrics/dashboard and update project structure/roadmap.
CHANGELOG.md	Introduce changelog for v0.2.1.0.
VERSION	Add version file (0.2.1.0).
TODOS.md	Collapse TODO list to “no open items”.

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Copilot · 2026-03-24T22:05:08Z

CHANGELOG.md

+- Added liveness probe to Deployment to detect deadlocked webhook replicas
+- Grafana mutation efficiency panel division-by-zero when idle — fixed with `or vector(0)`
+- `actions/checkout` pinned to `v4` (was `v6`, which does not exist)


This changelog entry claims actions/checkout was pinned to v4, but the current workflows still reference actions/checkout@v6 (e.g. .github/workflows/ci.yaml and release.yaml). Either update those workflows in this PR or remove/adjust this changelog line so it matches what was actually changed.

Copilot · 2026-03-24T22:05:09Z

CHANGELOG.md

+- Multi-GPU containers received wrong shared count (e.g., 2 GPUs with ratio 4 now yields 8 shared, not 4)
+- `promauto` replaced with safe `mustRegister` helper to prevent panic on duplicate metric registration during `go test -count=2`
+- Added liveness probe to Deployment to detect deadlocked webhook replicas
+- Grafana mutation efficiency panel division-by-zero when idle — fixed with `or vector(0)`


The changelog says the mutation-efficiency panel avoids division-by-zero with or vector(0), but the current dashboard query will still produce NaN when both rates are 0 (since or doesn’t replace an existing NaN series). After adjusting the dashboard query (e.g., using clamp_min on the denominator), update this line to accurately describe the fix.

Suggested change

- Grafana mutation efficiency panel division-by-zero when idle — fixed with `or vector(0)`

- Grafana mutation efficiency panel division-by-zero/NaN when idle — fixed by clamping the denominator in the dashboard query (e.g., using `clamp_min`)

Copilot · 2026-03-24T22:05:09Z

internal/webhook/pod_webhook.go

+// rewriteGPUResources replaces nvidia.com/gpu with nvidia.com/gpu-shared in both
+// Limits and Requests of the given ResourceRequirements. The shared-GPU quantity
+// equals the original GPU count multiplied by sliceRatio, preserving per-container
+// resource proportions. Returns true if the resources were modified.
+func rewriteGPUResources(resources *corev1.ResourceRequirements, sliceRatio int64) bool {
+	if resources == nil {
 		return false
 	}
-	if _, ok := limits[gpuResource]; !ok {
+	gpuQty, ok := resources.Limits[gpuResource]
+	if !ok {
 		return false
 	}


The doc comment says this rewrites GPU resources in both Limits and Requests, but the function only triggers when nvidia.com/gpu is present in resources.Limits. If a Pod specifies the GPU resource only in Requests (or if Limits is omitted for other reasons), it will be treated as a no-op. Either clarify the comment to reflect the actual behavior (Limits-driven rewrite that optionally mirrors Requests), or extend the logic to also handle a Requests-only GPU specification.

Copilot · 2026-03-24T22:05:09Z

config/deploy/deployment.yaml

          readinessProbe:
            httpGet:
              path: /healthz
              port: 8081
            initialDelaySeconds: 5
            periodSeconds: 10


The readinessProbe is hitting /healthz, but main.go registers a distinct ready check via mgr.AddReadyzCheck(...) (served at /readyz). Using /healthz for readiness means the readyz check is never exercised and can report the pod ready even if readiness semantics change later. Recommend switching the readinessProbe path to /readyz (and keep /healthz for liveness).

Copilot · 2026-03-24T22:05:09Z

config/monitoring/grafana-dashboard.yaml

+          "gridPos": { "x": 0, "y": 16, "w": 24, "h": 8 },
+          "targets": [
+            {
+              "expr": "(rate(castslice_mutations_total[5m]) / rate(castslice_requests_total[5m])) or vector(0)",


The PromQL division-by-zero guard here won’t work when both rates are 0: 0/0 yields NaN, and ... or vector(0) does not override an existing time series with a NaN value. This panel will still show NaN when idle. Consider using rate(mutations_total[5m]) / clamp_min(rate(requests_total[5m]), 1) (or a small epsilon) so the ratio evaluates to 0 when there are no requests.

Suggested change

"expr": "(rate(castslice_mutations_total[5m]) / rate(castslice_requests_total[5m])) or vector(0)",

"expr": "rate(castslice_mutations_total[5m]) / clamp_min(rate(castslice_requests_total[5m]), 1)",

Copilot · 2026-03-24T22:05:10Z

README.md

 - [x] **v0.1.0**: Basic Mutating Webhook (Static Slicing).
 - [x] **v0.2.0**: Smart Slicing (Dynamic ratios based on workload type).
- [ ] **v0.3.0**: FinOps Dashboard (Live GPU utilization metrics).
+- [x] **v0.3.0**: FinOps Dashboard (Live GPU utilization metrics).


The roadmap marks v0.3.0 as shipped, but this PR introduces VERSION/changelog content for 0.2.1.0. This is a documentation/versioning inconsistency for readers. Either bump the release/version references to 0.3.0, or keep the roadmap item unchecked / renumber it to match the current release versioning scheme.

Suggested change

- [x] **v0.3.0**: FinOps Dashboard (Live GPU utilization metrics).

- [ ] **v0.3.0**: FinOps Dashboard (Live GPU utilization metrics).

shenxianpeng and others added 5 commits March 24, 2026 23:55

test: add coverage for Requests mirroring, proportional scaling, and …

0e2fcdd

…slice-ratio upper bound

chore: bump version and changelog (v0.2.1.0)

45c5004

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs: update metrics section version tag to v0.2.1.0

dd0cb5d

shenxianpeng added the enhancement New feature or request label Mar 24, 2026

shenxianpeng requested a review from Copilot March 24, 2026 22:00

Copilot started reviewing on behalf of shenxianpeng March 24, 2026 22:01 View session

chore: remove VERSION file (release managed via release-drafter)

0ec28ba

Copilot AI reviewed Mar 24, 2026

View reviewed changes

shenxianpeng changed the title ~~feat: add Prometheus metrics, fix GPU resource rewriting, and harden deployment (v0.2.1.0)~~ feat: add Prometheus metrics, fix GPU resource rewriting, and harden deployment (v0.3.0) Mar 24, 2026

shenxianpeng merged commit 2bb6d4e into main Mar 24, 2026
1 check passed

shenxianpeng deleted the feature/my-new-feature branch March 24, 2026 22:13

shenxianpeng added major-rfe Major enhancement. Will be highlighted on the top minor A minor version bump and removed major-rfe Major enhancement. Will be highlighted on the top labels Mar 24, 2026

shenxianpeng changed the title ~~feat: add Prometheus metrics, fix GPU resource rewriting, and harden deployment (v0.3.0)~~ feat: add Prometheus metrics, fix GPU resource rewriting, and harden deployment Mar 24, 2026

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shenxianpeng merged 6 commits intomainfrom
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	- Grafana mutation efficiency panel division-by-zero when idle — fixed with `or vector(0)`
	- Grafana mutation efficiency panel division-by-zero/NaN when idle — fixed by clamping the denominator in the dashboard query (e.g., using `clamp_min`)

	"expr": "(rate(castslice_mutations_total[5m]) / rate(castslice_requests_total[5m])) or vector(0)",
	"expr": "rate(castslice_mutations_total[5m]) / clamp_min(rate(castslice_requests_total[5m]), 1)",

	- [x] v0.3.0: FinOps Dashboard (Live GPU utilization metrics).
	- [ ] v0.3.0: FinOps Dashboard (Live GPU utilization metrics).

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shenxianpeng commented Mar 24, 2026

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