# Shared Mesh Replication Protocol (v1)

[← Docs index](./README.md)

This document is the canonical contract for Fusion multi-leader mesh replication.

## 1. Goals and non-goals

### Goals
- Preserve one shared durable project state across multiple nodes.
- Keep task and planning state strongly coordinated by default.
- Allow local progress during peer outages via durable queues.
- Support deterministic replay/reconciliation after recovery.
- Expose read staleness so clients can decide whether to trust last-known global state.

### Runtime scope and non-goals (v1, updated for FN-4772/FN-4813)
- `HybridExecutor` is the canonical multi-project/multi-node runtime orchestration path.
- Scheduler failover (a peer node taking over another node's live scheduler tick loop) is an explicit non-goal.
- Live-process state migration (moving in-memory executor/session state between nodes mid-task) is an explicit non-goal.
- Supported alternative: lease handoff under `OwningNodeHandoffPolicy` (`park`, `reassign-to-local`, `reassign-any-healthy`) so tasks resume from durable state on the picking node.
- Immediate global consistency for every data class remains a non-goal.

## 2. Terms

- **Node**: A Fusion runtime instance participating in mesh sync.
- **Coordinator**: Node currently responsible for committing a write intent.
- **Intent**: Durable write proposal before global ack quorum completes.
- **Envelope**: Wire record carrying replication metadata + payload.
- **Epoch**: Monotonic lease/fencing generation for coordinator authority.
- **Fence token**: `epoch + coordinatorNodeId + sequence` token that invalidates stale coordinators.
- **Queue entry**: Durable locally-accepted write waiting for replay.

## 3. Versioning

- Protocol id: `fusion.shared-mesh`
- Initial version: `1.0`
- All envelopes must include `{ protocol, version }`.
- Minor versions (`1.x`) are backward-compatible additive.
- Major versions (`2.0+`) may change semantics and require explicit compatibility checks.

## 4. Data-class coordination matrix

| Data class | Mode | Notes |
|---|---|---|
| Tasks (core fields, deps, steps, column transitions) | Strongly coordinated | Quorum-acked intent/commit path; replayable with fencing |
| Task metadata (priority, model overrides, docs metadata refs) | Strongly coordinated | Same write path as tasks |
| Missions/milestones/slices/features | Strongly coordinated | Ordered writes preserve hierarchy invariants |
| Agent definitions/configuration | Strongly coordinated | Durable config replicated; runtime process handles excluded |
| Agent runtime state (heartbeat ticks, local process internals, worktree paths) | Node-local only | Exposed as local telemetry, not global truth |
| Project settings | Strongly coordinated | Existing settings payloads remain canonical payload shape |
| Auth material / provider credentials | Queued-for-later (secured transport only) | Explicit auth snapshot channel (`sharedState.authMaterial`); never merged as ordinary settings payload |
| Execution runs / live activity streams | Node-local + queued summary | Live events local; durable run outcomes appended later |
| Audit / event streams (`activityLog`, `runAuditEvents`) | Append-only replicated | Immutable event replication with origin metadata |
| Filesystem blobs (`.fusion/tasks/*` prompts/logs/attachments) | Queued-for-later | Metadata in replicated records, blob transfer out-of-band |

## 5. Write classes

- **`strong`**: Requires coordinator fence + quorum ack before `committed`.
- **`append-only`**: Event-style immutable replication; dedupe by event id.
- **`queued`**: Accept locally when peers unavailable; replay later.
- **`local`**: Never replicated globally.

## 6. Replication envelope

Every replicated record uses:
- `protocol`, `version`
- `recordId`, `entityType`, `entityId`
- `originNodeId`, `originSeq`
- `writeClass`
- `leaseEpoch`, `fenceToken`
- `intentId` and `state` (`intent` | `committed` | `rejected` | `queued` | `reconciled`)
- `createdAt`, `committedAt?`
- `payload`
- `precondition?` (base revision / expected epoch)

`PeerSyncRequest` / `PeerSyncResponse` remain mesh exchange carriers. v1 envelopes are payloads exchanged through current mesh sync infrastructure and follow-on sync endpoints.

### Auth snapshot contract (v1)

Auth replication uses `AuthMaterialSnapshot` (`version`, `exportedAt`, `checksum`, `payload`) with:
- `payload.providerAuth: Record<string, ProviderAuthEntry>`
- `ProviderAuthEntry.type`: `api_key | oauth`
- `api_key` fields: `key`
- `oauth` fields: `accessToken`, `refreshToken`, `expires`, optional `accountId`

Transport paths:
- Mesh shared-state channel: `POST /api/mesh/sync` (`sharedState.authMaterial`)
- Explicit node auth channel: `POST /api/nodes/:id/auth/sync` and inbound `POST /api/settings/auth-receive` / `GET /api/settings/auth-export`

Security/redaction rules:
- Auth snapshots are only exchanged over API-key-authenticated node links.
- Raw secrets (`key`, `accessToken`, `refreshToken`, bearer headers) MUST NOT be logged.
- Route diagnostics may emit provider names/counts only.

## 7. Quorum and acknowledgements

For `strong` writes:
1. Coordinator accepts intent locally.
2. Coordinator requests acknowledgements from peers in current membership view.
3. Commit requires `quorum = floor(eligibleVoters / 2) + 1` including coordinator.
4. If quorum fails before timeout, intent becomes `queued` with retry metadata.

`append-only` writes can be accepted locally and replicated asynchronously, but must preserve origin ordering `(originNodeId, originSeq)`.

## 8. Lease epochs and fencing

- Coordinator authority is leased with a monotonic `leaseEpoch`.
- Any write with stale epoch/fence must be rejected (`fenced`).
- Restarted nodes must reacquire lease and increment epoch before coordinating strong writes.
- Replay workers must carry original fence metadata; reconciler can reject stale queued entries after epoch advancement.

## 9. Offline queueing and replay

When a strong/queued write cannot reach quorum:
- Persist queue entry durably in `meshWriteQueue` (`status`: `pending | replaying | applied | failed`).
- Retryable queueing is limited to transport/outage failures: HTTP `502/503/504`, timeout/abort, and transport errors (`TypeError` fetch/network rejections, or Node-style `ECONNREFUSED`, `ENOTFOUND`, `ETIMEDOUT`, `ECONNRESET`).
- Non-retryable HTTP failures (`400/401/403/404/409/422`) are recorded as immediate failures and are not queued for replay.
- `applied` and `failed` rows are retained as durable reconciliation history (no auto-cleanup in v1).

Replay ordering:
1. Sort by `(createdAt asc, id asc)`.
2. Transition idempotently on the same row (`pending → replaying → applied|failed`) while incrementing `attemptCount` on each attempt.
3. Re-validate preconditions/fencing and record deterministic outcome.

## 10. Reconciliation

Reconciliation outcomes are explicit:
- `applied` — replayed successfully.
- `noop_already_applied` — idempotent duplicate.
- `superseded` — newer committed revision already exists.
- `conflict_requires_merge` — semantic conflict; requires policy/agent/manual resolution.
- `rejected_fenced` — stale epoch/fence.

Conflict policy must never silently downgrade strong writes to local-only updates.

## 11. Restart recovery hooks

On node startup:
1. Load durable queue.
2. Rebuild last known lease epoch / origin sequence.
3. Mark in-flight intents without terminal state as `queued` recovery candidates.
4. Start replay loop only after mesh membership snapshot and lease status are known.

## 12. Degraded reads and staleness

Mesh state reads expose a canonical `MeshDegradedReadState` contract:

```ts
type MeshDegradedReadState = {
  mode: "fresh" | "degraded";
  asOf: string;
  sourceNodeId: string | null;
  snapshotVersion: string | null;
  stalenessMs: number;
  queueDepth: number;
  pendingWriteCount: number;
  failedWriteCount: number;
};
```

Rules:
- `mode="fresh"` only when data came from the live mesh read path.
- `mode="degraded"` when read falls back to `meshSharedSnapshots`.
- `asOf` comes from snapshot `capturedAt`; `stalenessMs = Date.now() - new Date(asOf).getTime()`.
- `snapshotVersion` is the stored 64-char SHA-256 hex digest of snapshot payload.
- `queueDepth` is computed from queue rows where `status IN ('pending','replaying','failed')`.

API behavior must never hide fallback mode: degraded reads are explicit so clients can distinguish stale last-known state from fresh cluster state.

Concrete exported/runtime surfaces:
- Core types are exported from `@fusion/core`: `MeshSnapshotQuery`, `MeshSnapshotRecord`, `MeshSnapshotRecordInput`, `MeshWriteQueueStatus`, `MeshWriteQueueEntry`, `MeshWriteQueueInput`, `MeshWriteQueueFilter`, `MeshWriteApplyResult`, `MeshWriteFailureResult`, `MeshWriteReplaySummary`, and `MeshDegradedReadState`.
- `CentralCore` persistence/assertion methods: `recordMeshSnapshot`, `getLatestMeshSnapshot`, `enqueueMeshWrite`, `listPendingMeshWrites`, `markMeshWriteReplayStarted`, `markMeshWriteApplied`, `markMeshWriteFailed`, and `getMeshDegradedReadState`.
- Runtime replay/assertion methods: `PeerExchangeService.replayPendingWritesForNode(targetNodeId)` and `NodeHealthMonitor` recovery callback `onNodeRecovered(nodeId, previousStatus)`.

## 13. End-to-end v1 write path

1. **Intent creation**: Node creates write intent + envelope.
2. **Coordinator selection**: Node routes to current coordinator lease holder for the entity scope.
3. **Commit/ack**:
   - strong: quorum commit
   - append-only: local append + async replication
4. **Fallback**: if unreachable/quorum-fail, persist queue entry (`queued`).
5. **Replay**: on recovery, replay durable queue in canonical order with fencing checks.
6. **Reconciliation**: produce explicit outcome and update entity revision state.

## 14. Contract for FN-3449 through FN-3456

Follow-on tasks must implement against this contract and not redefine it:
- **FN-3449**: distributed ids/origin sequence allocation + monotonic ordering.
- **FN-3450**: coordinator selection and lease management runtime.
- **FN-3451**: strong-write commit path + quorum ack handling.
- **FN-3452**: durable offline queue persistence and replay engine.
- **FN-3453**: reconciliation executor + conflict outcome handling.
- **FN-3454**: restart recovery bootstrap and in-flight intent recovery.
- **FN-3455**: degraded-read APIs exposing staleness metadata.
- **FN-3456**: partition behavior policy, observability, and operator controls.

## 15. Security boundary

- Mesh transport authentication (node API keys / trust) is mandatory for replication traffic.
- Auth credential replication is explicit and separately controlled from ordinary settings replication.
- Sensitive payloads must be redacted from non-secure logs and diagnostics.
