# Bench Prep — PREP-BENCH-POLICY-20-002 (Policy delta benchmark)

Status: **Ready for implementation** (2025-11-20)
Owners: Bench Guild · Policy Guild · Scheduler Guild
Scope: Provide deterministic inputs and harness expectations to measure delta policy evaluation vs full runs.

## Goals
- Compare delta evaluation (incremental changes) against full evaluation over the same dataset.
- Capture throughput, latency (p50/p95/p99), and memory/GC impact under deterministic conditions.

## Dataset
- Baseline snapshot: `docs/samples/policy/policy-delta-baseline.ndjson`
  - 5,000 records of `{ "tenant": "bench", "policyId": "pol-<0001..5000>", "package": "bench.pkg.<n>", "version": "1.0.<n>", "decision": "allow|deny", "factors": { ... } }`
  - Deterministic ordering; SHA256 file saved as `policy-delta-baseline.ndjson.sha256`.
- Delta patch: `docs/samples/policy/policy-delta-changes.ndjson`
  - 500 changes mixing updates/inserts/deletes (encoded with `op`: "upsert"|"delete").
  - Sorted by `policyId` then `op` for deterministic replay.

## Harness plan (to be built under `src/Bench/StellaOps.Bench.Policy`)
- Run 1 (Full): load baseline snapshot, evaluate full policy set; record metrics.
- Run 2 (Delta): apply delta patch to in-memory store, run incremental evaluation; record metrics.
- Metrics captured to NDJSON per run:
  - `{ run: "full"|"delta", startedAtUtc, durationMs, evaluationsPerSec, p50Ms, p95Ms, p99Ms, rssMb, managedMb, gcGen2 }`
- Determinism:
  - Use fixed random seed `2025-01-01` for any shuffling; single-threaded mode flag `--threads 1` when reproducibility needed.
  - All timestamps in UTC ISO-8601; output NDJSON sorted by `run`.

## Acceptance criteria
- Baseline + delta sample files and SHA256 hashes present under `docs/samples/policy/`.
- Harness reads only local files, no network dependencies; replays produce consistent NDJSON for given hardware.
- Delta run shows reduced duration vs full run; metrics captured for both p95/p99 and throughput.

## Next steps
- Add sample files + hashes to `docs/samples/policy/` (can be generated with fixed seed).
- Implement harness CLI wrapper `dotnet run -- policy-delta --baseline <path> --delta <path> [--threads 1]` writing outputs to `out/bench/policy/` with `.sha256`.