# ADR-0033: Workflow Engine Hardening — Persistence, Saga Compensation, Cedar Authorization

**Status**: Implemented
**Date**: 2026-02-21
**Deciders**: VAPORA Team
**Technical Story**: `vapora-workflow-engine` lost all state on restart, had no rollback mechanism on failure, and applied no per-stage access control.

---

## Decision

Harden `vapora-workflow-engine` with three independent layers inspired by the stratum-orchestrator project:

1. **SurrealDB persistence** (`SurrealWorkflowStore`) — crash-recoverable `WorkflowInstance` state
2. **Saga compensation** (`SagaCompensator`) — reverse-order rollback dispatch via `SwarmCoordinator`
3. **Cedar authorization** (`CedarAuthorizer`) — per-stage policy enforcement before task dispatch

All three are implemented natively inside `vapora-workflow-engine` — stratum-orchestrator is **not** a direct dependency.

---

## Context

### Gaps Before This ADR

| Gap | Consequence |
|-----|-------------|
| In-memory `DashMap` only | All running workflows lost on server restart |
| No compensation on failure | Stage 3 failure left Stage 1 and 2 side effects live |
| No authorization check per stage | Any caller could trigger any stage in any workflow |

### Why Not Import stratum-orchestrator Directly

The plan initially included:

```toml
stratum-orchestrator = { path = "../../../stratumiops/crates/stratum-orchestrator" }
```

This fails because `stratum-orchestrator → platform-nats → nkeys = { workspace = true }`. The `nkeys` dependency is resolved only inside the `stratumiops` workspace; it is not published to crates.io and has no path resolvable from vapora's workspace root. Cargo errors with `failed to select a version for nkeys`.

The `CedarAuthorizer` inside stratum-orchestrator is 88 self-contained lines using only `cedar-policy`. Implementing it locally is zero duplication risk and avoids a circular workspace dependency.

---

## Implementation

### New Modules

```text
crates/vapora-workflow-engine/src/
├── auth.rs        — CedarAuthorizer: loads .cedar policy files, authorize()
├── persistence.rs — SurrealWorkflowStore: save/load/load_active/delete
└── saga.rs        — SagaCompensator: compensate(workflow_id, stages, ctx)
```

### New Migration

```text
migrations/009_workflow_state.surql — SCHEMAFULL workflow_instances table
```

### Config Changes

```toml
[engine]
cedar_policy_dir = "/etc/vapora/cedar"   # optional; Cedar disabled if absent

[[workflows.stages]]
name = "deploy"
agents = ["devops"]
compensation_agents = ["devops"]         # receives rollback task if Saga fires
```

### Dependency Addition

```toml
# crates/vapora-workflow-engine/Cargo.toml
surrealdb    = { workspace = true }
cedar-policy = "4.9"
```

`cedar-policy` enters directly; it was previously only transitive via `secretumvault` (4.8). Cargo resolves the workspace to 4.9 (semver compatible, same major).

### WorkflowOrchestrator Constructor Change

```rust
// Before
WorkflowOrchestrator::new(config_path, swarm, kg, nats)

// After
WorkflowOrchestrator::new(config_path, swarm, kg, nats, db: Surreal<Client>)
```

`db` is the existing backend connection — the store does not open its own connection.

---

## Data Flow

```text
start_workflow()
  → WorkflowInstance::new()
  → store.save()                    ← persistence
  → execute_current_stage()
      → cedar.authorize()           ← auth (if configured)
      → swarm.assign_task()

on_task_completed()
  → task.mark_completed()
  → store.save()                    ← persistence

on_task_failed(can_retry=false)
  → mark_current_task_failed()      ← stage transition
  → saga.compensate(stages, ctx)    ← saga (reverse-order dispatch)
  → instance.fail()
  → store.save()                    ← persistence

startup crash recovery
  → store.load_active()             ← restores active_workflows DashMap
```

---

## Saga Compensation Protocol

Compensation is **best-effort**: errors are logged, never propagated. Stage order is reversed: the last executed stage receives a rollback task first.

Only stages with `compensation_agents` defined in their `StageConfig` receive a compensation task. Stages without the field are silently skipped.

Compensation task payload sent to `SwarmCoordinator`:

```json
{
  "type": "compensation",
  "stage_name": "deploy",
  "workflow_id": "abc-123",
  "original_context": { "…" : "…" },
  "artifacts_to_undo": ["artifact-id-1"]
}
```

---

## Cedar Authorization

`CedarAuthorizer::load_from_dir(path)` reads all `*.cedar` files from the directory and compiles them into a single `PolicySet`. Before each stage dispatch:

```rust
cedar.authorize(
    "vapora-orchestrator",      // principal
    "execute-stage",            // action
    "Stage::\"architecture\"",  // resource
)?;
```

A `Deny` decision returns `WorkflowError::Unauthorized`, halting the workflow without dispatching the stage. If `cedar_policy_dir` is not set in `EngineConfig`, Cedar is disabled and all stages proceed without policy checks.

---

## Rationale

### Why SurrealDB (not Redis / SQLite)

SurrealDB is already the persistence layer for every other stateful component in vapora. Adding `workflow_instances` as one more table keeps the operational footprint at zero (no new service, no new connection pool). `WorkflowInstance` already implements `Serialize/Deserialize`; the store serializes via `serde_json::Value` to satisfy the `SurrealValue` trait requirement introduced in surrealdb v3.

### Why Saga Over Two-Phase Commit

Workflows already span multiple async agent executions over NATS. Two-phase commit across these boundaries would require protocol changes in every agent. Saga achieves eventual consistency via compensating transactions that each agent already understands (a task with `type: "compensation"`).

### Why Cedar Over RBAC / Custom Middleware

Cedar policies are already used by the rest of the VAPORA platform (see ADR-0010). Per-stage rules expressed in `.cedar` files are reviewable outside the codebase and hot-swappable without redeployment (restart required to reload, by current design). A custom middleware table would require schema migrations for every policy change.

---

## Consequences

### Positive

- Workflows survive server restarts (crash recovery via `load_active()`)
- Non-retryable stage failure triggers best-effort rollback of completed stages
- Per-stage access control via auditable policy files
- Zero new infrastructure (uses existing SurrealDB connection)
- 31/31 existing tests continue to pass; 5 new tests added (auth × 3, saga × 2)

### Negative

- `WorkflowOrchestrator::new()` signature change requires callers to pass `Surreal<Client>`
- Cedar requires `.cedar` files on disk; missing `cedar_policy_dir` disables auth silently
- Compensation is best-effort — no guarantee of full rollback if compensation agent also fails

### Mitigations

| Risk | Mitigation |
|------|------------|
| Saga partial rollback | Metrics track compensation dispatch; dead-letter queue via NATS for retry |
| Cedar files missing | `cedar_policy_dir = None` → no-auth mode; documented explicitly |
| Signature change | Backend already owns `db: Arc<Surreal<Client>>`; passed at construction |

---

## Verification

```bash
cargo test -p vapora-workflow-engine        # 31/31 pass
cargo clippy -p vapora-workflow-engine -- -D warnings  # 0 warnings
```

New tests:

- `auth::tests::test_permit_allows`
- `auth::tests::test_deny_returns_unauthorized`
- `auth::tests::test_empty_dir_fails`
- `saga::tests::test_stages_with_compensation_agents_are_included`
- `saga::tests::test_stages_with_no_compensation_agents_are_skipped`

---

## Related ADRs

- [ADR-0028](./0028-workflow-orchestrator.md) — Workflow Orchestrator (original implementation)
- [ADR-0010](./0010-cedar-authorization.md) — Cedar Authorization
- [ADR-0004](./0004-surrealdb-database.md) — SurrealDB as single persistence layer
- [ADR-0018](./0018-swarm-load-balancing.md) — SwarmCoordinator (Saga dispatch target)