Module access model Future

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Access Model Module - Future Enhancements

Scope

• refinement of promotion/demotion policy expressiveness beyond age-based rules.
• hardening of multi-tier coordinator behavior under concurrent load and degraded tier availability.
• expansion of observability and trace integration for distributed deployments.
• operator-grade diagnostics for tier transition incidents and policy conflicts.

Design Constraints

• access_model contracts remain backward compatible within major release line.
• AccessTier interface must not change in a breaking way without a migration note and CHANGELOG entry.
• coordinator must remain opt-in via feature flag; existing cache/storage APIs must remain unaffected.
• no legacy or compatibility shim paths without explicit human approval.
• trace context propagation must remain non-blocking in hot-path operations.

Required Interfaces

Interface	Requirement
AccessCoordinator	deterministic promotion/demotion under concurrent event streams and tier permutations
AgeBasedPolicy	extensible threshold model allowing per-tier and per-key policy overrides
AccessMetrics	stable Prometheus-compatible surface with p50/p95/p99 histograms per transition type
EvictionListener / PromotionListener	explicit delivery semantics, bounded callback latency, and failure classification
Trace context	non-blocking propagation with correlation-ID flow through entire event chain

Implementation Notes

• extend policy model to support:
  • per-key or per-tag promotion/demotion threshold overrides in age_based_policy.h.
  • frequency-based policy alongside age-based decisions.
• harden coordinator for partial tier degradation:
  • explicit fail-closed behavior when a target tier is unavailable.
  • bounded retry and fallback path with structured diagnostic output.
• expand trace integration:
  • align access_model_trace.h correlation IDs with Wave D distributed tracing spans (see docs/operability/RUNBOOK_ACCESS_MODEL_PROMOTION.md §Wave D D1 cross-links).
  • OpenTelemetry-compatible span emission for promotion/demotion events.
• improve benchmark coverage:
  • per-tier latency benchmarks for all six tier transition types.
  • memory overhead benchmark under sustained high-frequency promotion events.

Test Strategy

• unit and integration suites for policy-override and frequency-based paths.
• degraded-tier and fail-closed determinism tests.
• trace propagation correctness under concurrent event storms.
• release-profile benchmark runs for all GATE-ACM-01..06 target gates.
• minimum acceptance expansion (Q4 2026):
  • policy override matrix (per-key, per-tag, frequency vs age interaction)
  • negative-path validation (unavailable tier, malformed event payload, queue overflow)
  • coordinator shutdown determinism (in-flight promotion completion vs abort)

Performance Targets

• promotion/demotion hot paths remain within GATE-ACM-01..06 release thresholds.
• concurrent coordinator and tier operations remain stable at p95/p99.
• benchmark manifests for all tier transition targets reach no-missing-case status.
• measurable targets:
  • regression threshold: <= 10% vs baseline for mapped access_model benchmark cases
  • L1→L2 promotion p99 <= 50 µs (GATE-ACM-01)
  • cache eviction→storage feedback p99 <= 100 µs (GATE-ACM-02)
  • cold→warm promotion p99 <= 100 ms (GATE-ACM-03)
  • event processing throughput >= 10 K events/sec (GATE-ACM-04)
  • memory overhead <= 50 MB (GATE-ACM-05)
  • policy decision p99 <= 10 µs (GATE-ACM-06)

Security / Reliability

• coordinator must not silently drop or reorder promotion/demotion events.
• all tier transition decisions remain auditable via structured log records with correlation IDs.
• fail-closed on unavailable tier — no silent fallback to an unsafe promotion path.
• enforce bounded resource behavior: bounded thread pool, bounded event queue depth.
• diagnostics must remain actionable for production incidents and policy conflict resolution.
• reliability gate intent:
  • degraded tier states must remain observable, bounded, and explicitly surfaced to operators.
  • partial tier participation must never silently report success for a promotion/demotion.

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