# VEX Observations & Linksets

> Imposed rule: Work of this type or tasks of this type on this component must
> also be applied everywhere else it should be applied.

Link-Not-Merge brings the same immutable observation model to Excititor that
Concelier now uses for advisories. VEX statements are stored as append-only
observations; linksets correlate them, capture conflicts, and keep provenance so
Policy Engine and UI surfaces can explain decisions without collapsing sources.

---

## 1. Model overview

### 1.1 Observation lifecycle

1. **Ingest** – Connectors fetch OpenVEX, CSAF VEX, CycloneDX VEX, or VEX
   attestations, validate signatures, and strip any derived consensus data
   forbidden by the Aggregation-Only Contract (AOC).
2. **Persist** – Excititor writes immutable `vex_observations` keyed by tenant,
   provider, upstream identifier, and `contentHash`. Supersedes chains record
   revisions; the original payload is never mutated.
3. **Expose** – WebService will surface paginated observation APIs and Offline
   Kit snapshots mirror the same data for air-gapped sites.

Observation schema sketch (final shape lands with `EXCITITOR-LNM-21-001`):

```text
observationId = {tenant}:{providerId}:{upstreamId}:{revision}
tenant, providerId, streamId
upstream{ upstreamId, documentVersion, fetchedAt, receivedAt,
          contentHash, signature{present, format?, keyId?, signature?} }
content{ format, specVersion, raw }
statements[
  { vulnerabilityId, productKey, status, justification?,
    introducedVersion?, fixedVersion?, locator }
]
linkset{ purls[], cpes[], aliases[], references[],
         reconciledFrom[], conflicts[]? }
attributes{ batchId?, replayCursor? }
createdAt
```

- **Raw payload** (`content.raw`) remains lossless (Relaxed Extended JSON).
- **Statements** provide normalized tuples for each claim contained in the
  document, including justification and version hints.
- **Linkset** mirrors identifiers extracted during ingestion, retaining JSON
  pointer metadata so audits can trace back to the source fragment.

### 1.2 Linkset lifecycle

Linksets correlate claims referring to the same `(vulnerabilityId, productKey)`
pair across providers.

1. **Seed** – Observations push normalized identifiers (CVE, GHSA, vendor IDs)
   plus canonical product keys (purl preferred, cpe fallback). Platform-scoped
   statements remain marked `non_joinable`.
2. **Correlate** – The linkset builder groups statements by tenant and identity,
   combines alias graphs from Concelier, and uses justification/product overlap
   to assign correlation confidence.
3. **Annotate** – Conflicts (status disagreement, justification mismatch, range
   inconsistencies) are recorded as structured entries.
4. **Persist** – Results land in `vex_linksets` with deterministic IDs (hash of
   sorted `(vulnerabilityId, productKey, observationIds)`) and append-only
   history for replay/debugging.

Linksets never override statements or invent consensus; they simply align
evidence for Policy Engine and consumers.

---

## 2. Observation vs. linkset

- **Purpose**
  - Observation: Immutable record of a single upstream VEX document.
  - Linkset: Correlated evidence spanning observations that describe the same
    product-vulnerability pair.
- **Mutation**
  - Observation: Append-only via supersedes.
  - Linkset: Regenerated deterministically by correlation jobs.
- **Allowed fields**
  - Observation: Raw payload, provenance, normalized statement tuples, join
    hints.
  - Linkset: Observation references, statement IDs, confidence metrics, conflict
    annotations.
- **Forbidden fields**
  - Observation: Derived consensus, suppression flags, risk scores.
  - Linkset: Derived severity or policy decisions (only evidence + conflicts).
- **Consumers**
  - Observation: Evidence exports, Offline Kit mirrors, CLI raw dumps.
  - Linkset: Policy Engine VEX overlay, Console evidence panes, Vuln Explorer.

### 2.1 Example sequence

1. Canonical vendor issues an attested OpenVEX declaring `CVE-2025-2222` as
   `not_affected` for `pkg:rpm/redhat/openssl@1.1.1w-12`. Excititor inserts a
   new observation referencing that statement.
2. Upstream CycloneDX VEX from a distro reports the same product as `affected`
   with `under_investigation` justification.
3. Linkset builder groups both statements by alias overlap and product key,
   setting confidence `high` because CVE and purl match.
4. Conflict annotation records `status-mismatch` and retains both justifications;
   Policy Engine uses this to explain why suppression cannot proceed without
   policy override.

---

## 3. Conflict handling

Structured conflicts capture disagreements without mutating source statements.

```json
{
  "type": "status-mismatch",
  "vulnerabilityId": "CVE-2025-2222",
  "productKey": "pkg:rpm/redhat/openssl@1.1.1w-12",
  "statements": [
    {
      "observationId": "tenant:redhat:openvex:3",
      "providerId": "redhat",
      "status": "not_affected",
      "justification": "component_not_present"
    },
    {
      "observationId": "tenant:ubuntu:cyclonedx:12",
      "providerId": "ubuntu",
      "status": "affected",
      "justification": "under_investigation"
    }
  ],
  "confidence": "medium",
  "detectedAt": "2025-10-27T14:30:00Z"
}
```

Conflict classes (tracked via `EXCITITOR-LNM-21-003`):

- `status-mismatch` – Different statuses for the same pair (affected vs
  not_affected vs fixed vs under_investigation).
- `justification-divergence` – Same status but incompatible justifications or
  missing justification where policy requires it.
- `version-range-clash` – Introduced/fixed ranges contradict each other.
- `non-joinable-overlap` – Platform-scoped statements collide with package
  statements; flagged as warning but retained.
- `metadata-gap` – Missing provenance/signature field on specific statements.

Conflicts surface through:

- `/vex/linksets/{id}` APIs (`conflicts[]` payload).
- Console evidence panels (badges + drawer detail).
- CLI exports (`stella vex linkset …` planned in `CLI-LNM-22-002`).
- Metrics dashboards (`vex_linkset_conflicts_total{type}`).

---

## 4. AOC alignment

- **Raw-first** – `content.raw` and `statements[]` mirror upstream input; no
  derived consensus or suppression values are written by ingestion.
- **No merges** – Each upstream statement persists independently; linksets refer
  back via `observationId`.
- **Provenance mandatory** – Missing signature or source metadata yields
  `ERR_AOC_004`; ingestion blocks until connectors fix the feed.
- **Idempotent writes** – Duplicate `(providerId, upstreamId, contentHash)`
  results in a no-op; revisions append with a `supersedes` pointer.
- **Deterministic output** – Correlator sorts identifiers, normalizes timestamps
  (UTC ISO-8601), and hashes canonical JSON to generate stable linkset IDs.
- **Scope-aware** – Tenant claims enforced on write/read; Authority scopes
  `vex:ingest` / `vex:read` are required (see `AUTH-AOC-22-001`).

Violations raise `ERR_AOC_00x`, emit `aoc_violation_total`, and prevent the data
from landing downstream.

---

## 5. Downstream consumption

- **Policy Engine** – Evaluates VEX evidence alongside advisory linksets to gate
  suppression, severity downgrades, or explainability.
- **Console UI** – Evidence panel renders VEX statements grouped by provider and
  highlights conflicts or missing signatures.
- **CLI** – Planned commands export observations/linksets for offline analysis
  (`CLI-LNM-22-002`).
- **Offline Kit** – Bundled snapshots keep VEX data aligned with advisory
  observations for air-gapped parity.
- **Observability** – Dashboards track ingestion latency, conflict counts, and
  supersedes depth per provider.

New consumers must treat both collections as read-only and preserve deterministic
ordering when caching.

---

## 6. Validation & testing

- **Unit tests** (`StellaOps.Excititor.Core.Tests`) to cover schema guards,
  deterministic linkset hashing, conflict classification, and supersedes
  behaviour.
- **Mongo integration tests** (`StellaOps.Excititor.Storage.Mongo.Tests`) to
  verify indexes, shard keys, and idempotent writes across tenants.
- **CLI smoke suites** (`stella vex observations`, `stella vex linksets`) for
  JSON determinism and exit code coverage.
- **Replay determinism** – Feed identical upstream payloads twice and ensure
  observation/linkset hashes match across runs.
- **Offline kit verification** – Validate VEX exports packaged in Offline Kit
  snapshots against live service outputs.
- **Fixture refresh** – Samples (`SAMPLES-LNM-22-002`) must include multi-source
  conflicts and justification variants used by docs and UI tests.

---

## 7. Reviewer checklist

- Observation schema aligns with `EXCITITOR-LNM-21-001` once the schema lands;
  update references as soon as the final contract is published.
- Linkset lifecycle covers correlation signals (alias graphs, product keys,
  justification rules) and deterministic ID strategy.
- Conflict classes include status, justification, version range, platform overlap
  scenarios.
- AOC guardrails called out with relevant error codes and Authority scopes.
- Downstream consumer list matches active APIs/CLI features (update when
  `CLI-LNM-22-002` and WebService endpoints ship).
- Validation section references Core, Storage, CLI, and Offline test suites plus
  fixture requirements.
- Imposed rule reminder retained at top.

Dependencies outstanding (2025-10-27): `EXCITITOR-LNM-21-001..005` and
`EXCITITOR-LNM-21-101..102` are still TODO; revisit this document once schemas,
APIs, and fixtures are implemented.