# Mosaic Stack — Federation PRD

**Status:** Draft v1 (locked for implementation)
**Owner:** Jason
**Date:** 2026-04-19
**Scope:** Enables cross-instance data federation between Mosaic Stack gateways with asymmetric trust, multi-tenant scoping, and no cross-boundary data persistence.

---

## 1. Problem Statement

Jarvis operates across 3–4 workstations in two physical locations (home, USC). The user currently reaches back to a single jarvis-brain checkout from every session, and has tried OpenBrain to solve cross-session state — with poor results (cache invalidation, latency, opacity, hard dependency on a remote service).

The goal is a federation model where each user's **home instance** remains the source of truth for their personal data, and **work/shared instances** expose scoped data to that user's home instance on demand — without persisting anything across the boundary.

## 2. Goals

1. A user logged into their **home gateway** (Server A) can query their **work gateway** (Server B) in real time during a session.
2. Data returned from Server B is used in-session only; never written to Server A storage.
3. Server B has multiple users, each with their own Server A. No user's data leaks to another user.
4. Federation works over public HTTPS (no VPN required). Tailscale is a supported optional overlay.
5. Sync latency target: seconds, or at the next data need of the agent.
6. Graceful degradation: if the remote instance is unreachable, the local session continues with local data and a clear "federation offline" signal.
7. Teams exist on both sides. A federation grant can share **team-owned** data without exposing other team members' personal data.
8. Auth and revocation use standard PKI (X.509) so that certificate tooling (Step-CA, rotation, OCSP, CRL) is available out of the box.

## 3. Non-Goals (v1)

- Mesh federation (N-to-N). v1 is strictly A↔B pairs.
- Cross-instance writes. All federation is **read-only** on the remote side.
- Shared agent sessions across instances. Sessions live on one instance; federation is data-plane only.
- Cross-instance SSO. Each instance owns its own BetterAuth identity store; federation is service-to-service, not user-to-user.
- Realtime push from B→A. v1 is pull-only (A pulls from B during a session).
- Global search index. Federation is query-by-query, not index replication.

## 4. User Stories

- **US-1 (Solo user at home):** As the sole user on Server A, I want my agent session on workstation-1 to see the same data it saw on workstation-2, without running OpenBrain.
- **US-2 (Cross-location):** As a user with a home server and a work server, I want a session on my home laptop to transparently pull my USC-owned tasks/notes when I ask for them.
- **US-3 (Work admin):** As the admin of mosaic.uscllc.com, I want to grant each employee's home gateway scoped read access to only their own data plus explicitly-shared team data.
- **US-4 (Privacy boundary):** As employee A on mosaic.uscllc.com, my data must never appear in a session on employee B's home gateway — even if both are federated with uscllc.com.
- **US-5 (Revocation):** As a work admin, when I delete an employee, their home gateway loses access within one request cycle.
- **US-6 (Offline):** As a user in a hotel with flaky wifi, my local session keeps working; federation calls fail fast and are reported as "offline," not hung.

## 5. Architecture Overview

```
┌─────────────────────────────────────┐     mTLS / X.509         ┌─────────────────────────────────────┐
│  Server A — mosaic.woltje.com       │ ───────────────────────► │  Server B — mosaic.uscllc.com        │
│  (home, master for Jason)           │   ◄── JSON over HTTPS    │  (work, multi-tenant)                │
│                                     │                          │                                      │
│  ┌──────────────┐  ┌──────────────┐ │                          │ ┌──────────────┐  ┌──────────────┐  │
│  │  Gateway     │  │  Postgres    │ │                          │ │  Gateway     │  │  Postgres    │  │
│  │  (NestJS)    │──│  (local SSOT)│ │                          │ │  (NestJS)    │──│  (tenant SSOT)│ │
│  └──────┬───────┘  └──────────────┘ │                          │ └──────┬───────┘  └──────────────┘  │
│         │                           │                          │        │                             │
│         │  FederationClient         │                          │        │  FederationServer           │
│         │  (outbound, scoped query) │                          │        │  (inbound, RBAC-gated)      │
│         └───────────────────────────┼──────────────────────────┼────────┘                             │
│                                     │                          │                                      │
│  Step-CA (issues A's client cert)   │                          │  Step-CA (issues B's server cert,    │
│                                     │                          │           trusts A's CA root on grant)│
└─────────────────────────────────────┘                          └──────────────────────────────────────┘
```

- Federation is a **transport-layer** concern between two gateways, implemented as a new internal module on each gateway.
- Both sides run the same code. Direction (client vs. server role) is per-request.
- Nothing in the agent runtime changes — agents query the gateway; the gateway decides local vs. remote.

## 6. Transport & Authentication

**Transport:** HTTPS with mutual TLS (mTLS).

**Identity:** X.509 client certificates issued by Step-CA. Each federation grant materializes as a client cert on the requesting side and a trust-anchor entry (CA root or explicit cert) on the serving side.

**Why mTLS over HMAC bearer tokens:**

- Standard rotation/revocation semantics (renew, CRL, OCSP).
- The cert subject carries identity claims (user, grant_id) that don't need a separate DB lookup to verify authenticity.
- Client certs never transit request bodies, so they can't be logged by accident.
- Transport is pinned at the TLS layer, not re-validated per-handler.

**Cert contents (SAN + subject):**

- `CN=grant-<uuid>`
- `O=<requesting-server-hostname>` (e.g., `mosaic.woltje.com`)
- Custom OIDs embedded in SAN otherName:
  - `mosaic.federation.grantId` (UUID)
  - `mosaic.federation.subjectUserId` (user on the **serving** side that this grant acts-as)
- Default lifetime: **30 days**, with auto-renewal at T-7 days if the grant is still active.

**Step-CA topology (v1):** Each server runs its own Step-CA instance. During onboarding, the serving side imports the requesting side's CA root. A central/shared Step-CA is out of scope for v1.

**Handshake:**

1. Client (A) opens HTTPS to B with its grant cert.
2. B validates cert chain against trusted CA roots for that grant.
3. B extracts `grantId` and `subjectUserId` from the cert.
4. B loads the grant record, checks it is `active`, not revoked, and not expired.
5. B enforces scope and rate-limit for this grant.
6. Request proceeds; response returned.

## 7. Data Model

All tables live on **each instance's own Postgres**. Federation grants are bilateral — each side has a record of the grant.

### 7.1 `federation_grants` (on serving side, Server B)

| Field                       | Type        | Notes                                             |
| --------------------------- | ----------- | ------------------------------------------------- |
| `id`                        | uuid PK     |                                                   |
| `subject_user_id`           | uuid FK     | Which local user this grant acts-as               |
| `requesting_server`         | text        | Hostname of requesting gateway (e.g., woltje.com) |
| `requesting_ca_fingerprint` | text        | SHA-256 of trusted CA root                        |
| `active_cert_fingerprint`   | text        | SHA-256 of currently valid client cert            |
| `scope`                     | jsonb       | See §8                                            |
| `rate_limit_rpm`            | int         | Default 60                                        |
| `status`                    | enum        | `pending`, `active`, `suspended`, `revoked`       |
| `created_at`                | timestamptz |                                                   |
| `activated_at`              | timestamptz |                                                   |
| `revoked_at`                | timestamptz |                                                   |
| `last_used_at`              | timestamptz |                                                   |
| `notes`                     | text        | Admin-visible description                         |

### 7.2 `federation_peers` (on requesting side, Server A)

| Field                 | Type        | Notes                                            |
| --------------------- | ----------- | ------------------------------------------------ |
| `id`                  | uuid PK     |                                                  |
| `peer_hostname`       | text        | e.g., `mosaic.uscllc.com`                        |
| `peer_ca_fingerprint` | text        | SHA-256 of peer's CA root                        |
| `grant_id`            | uuid        | The grant ID assigned by the peer                |
| `local_user_id`       | uuid FK     | Who on Server A this federation belongs to       |
| `client_cert_pem`     | text (enc)  | Current client cert (PEM); rotated automatically |
| `client_key_pem`      | text (enc)  | Private key (encrypted at rest)                  |
| `cert_expires_at`     | timestamptz |                                                  |
| `status`              | enum        | `pending`, `active`, `degraded`, `revoked`       |
| `last_success_at`     | timestamptz |                                                  |
| `last_failure_at`     | timestamptz |                                                  |
| `notes`               | text        |                                                  |

### 7.3 `federation_audit_log` (on serving side, Server B)

| Field         | Type        | Notes                                            |
| ------------- | ----------- | ------------------------------------------------ |
| `id`          | uuid PK     |                                                  |
| `grant_id`    | uuid FK     |                                                  |
| `occurred_at` | timestamptz | indexed                                          |
| `verb`        | text        | `query`, `handshake`, `rejected`, `rate_limited` |
| `resource`    | text        | e.g., `tasks`, `notes`, `credentials`            |
| `query_hash`  | text        | SHA-256 of normalized query (no payload stored)  |
| `outcome`     | text        | `ok`, `denied`, `error`                          |
| `bytes_out`   | int         |                                                  |
| `latency_ms`  | int         |                                                  |

**Audit policy:** Every federation request is logged on the serving side. Read-only requests only — no body capture. Retention: 90 days hot, then roll to cold storage.

## 8. RBAC & Scope

Every federation grant has a scope object that answers three questions for every inbound request:

1. **Who is acting?** — `subject_user_id` from the cert.
2. **What resources?** — an allowlist of resource types (`tasks`, `notes`, `credentials`, `memory`, `teams/:id/tasks`, …).
3. **Filter expression** — predicates applied on top of the subject's normal RBAC (see below).

### 8.1 Scope schema

```json
{
  "resources": ["tasks", "notes", "memory"],
  "filters": {
    "tasks": { "include_teams": ["team_uuid_1", "team_uuid_2"], "include_personal": true },
    "notes": { "include_personal": true, "include_teams": [] },
    "memory": { "include_personal": true }
  },
  "excluded_resources": ["credentials", "api_keys"],
  "max_rows_per_query": 500
}
```

### 8.2 Access rule (enforced on serving side)

For every inbound federated query on resource R:

1. Resolve effective identity → `subject_user_id`.
2. Check R is in `scope.resources` and NOT in `scope.excluded_resources`. Otherwise 403.
3. Evaluate the user's **normal RBAC** (what would they see if they logged into Server B directly)?
4. Intersect with the scope filter (e.g., only team X, only personal).
5. Apply `max_rows_per_query`.
6. Return; log to audit.

### 8.3 Team boundary guarantees

- Scope filters are additive, never subtractive of the native RBAC. A grant cannot grant access the user would not have had themselves.
- `include_teams` means "only these teams," not "these teams in addition to all teams."
- `include_personal: false` hides the user's personal data entirely from federation, even if they own it — useful for work-only accounts.

### 8.4 No cross-user leakage

When Server B has multiple users (employees) all federating back to their own Server A:

- Each employee has their own grant with their own `subject_user_id`.
- The cert is bound to a specific grant; there is no mechanism by which one grant's cert can be used to impersonate another.
- Audit log is per-grant.

## 9. Query Model

Federation exposes a **narrow read API**, not arbitrary SQL.

### 9.1 Supported verbs (v1)

| Verb           | Purpose                                    | Returns                         |
| -------------- | ------------------------------------------ | ------------------------------- |
| `list`         | Paginated list of a resource type          | Array of resources              |
| `get`          | Fetch a single resource by id              | One resource or 404             |
| `search`       | Keyword search within allowed resources    | Ranked list of hits             |
| `capabilities` | What this grant is allowed to do right now | Scope object + rate-limit state |

### 9.2 Not in v1

- Write verbs.
- Aggregations / analytics.
- Streaming / subscriptions (future: see §13).

### 9.3 Agent-facing integration

Agents never call federation directly. Instead:

- The gateway query layer accepts `source: "local" | "federated:<peer_hostname>" | "all"`.
- `"all"` fans out in parallel, merges results, tags each with `_source`.
- Federation results are in-memory only; the gateway does not persist them.

## 10. Caching

- **In-memory response cache** with short TTL (default 30s) for `list` and `get`. `search` is not cached.
- Cache is keyed by `(grant_id, verb, resource, query_hash)`.
- Cache is flushed on cert rotation and on grant revocation.
- No disk cache. No cross-session cache.

## 11. Bootstrap & Onboarding

### 11.1 Instance capability tiers

| Tier         | Storage  | Queue   | Memory   | Can federate?         |
| ------------ | -------- | ------- | -------- | --------------------- |
| `local`      | PGlite   | in-proc | keyword  | No                    |
| `standalone` | Postgres | Valkey  | keyword  | No (can be client)    |
| `federated`  | Postgres | Valkey  | pgvector | Yes (server + client) |

Federation requires `federated` tier on **both** sides.

### 11.2 Onboarding flow (admin-driven)

1. Admin on Server B runs `mosaic federation grant create --user <user-id> --peer <peer-hostname> --scope-file scope.json`.
2. Server B generates a `grant_id`, prints a one-time enrollment URL containing the grant ID + B's CA root fingerprint.
3. Admin on Server A (or the user themselves, if allowed) runs `mosaic federation peer add <enrollment-url>`.
4. Server A's Step-CA generates a CSR for the new grant. A submits the CSR to B over a short-lived enrollment endpoint (single-use token in the enrollment URL).
5. B's Step-CA signs the cert (with grant ID embedded in SAN OIDs), returns it.
6. A stores the signed cert + private key (encrypted) in `federation_peers`.
7. Grant status flips from `pending` to `active` on both sides.
8. Cert auto-renews at T-7 days using the standard Step-CA renewal flow as long as the grant remains active.

### 11.3 Revocation

- **Admin-initiated:** `mosaic federation grant revoke <grant-id>` on B flips status to `revoked`, adds the cert to B's CRL, and writes an audit entry.
- **Revoke-on-delete:** Deleting a user on B automatically revokes all grants where that user is the subject.
- Server A learns of revocation on the next request (TLS handshake fails) and flips the peer to `revoked`.

### 11.4 Rate limit

Default `60 req/min` per grant. Configurable per grant. Enforced at the serving side. A rate-limited request returns `429` with `Retry-After`.

## 12. Operational Concerns

- **Observability:** Each federation request emits an OTEL span with `grant_id`, `peer`, `verb`, `resource`, `outcome`, `latency_ms`. Traces correlate across both servers via W3C traceparent.
- **Health check:** `mosaic federation status` on each side shows active grants, last-success times, cert expirations, and any CRL mismatches.
- **Backpressure:** If the serving side is overloaded, it returns `503` with a structured body; the client marks the peer `degraded` and falls back to local-only until the next successful handshake.
- **Secrets:** `client_key_pem` in `federation_peers` is encrypted with the gateway's key (sealed with the instance's master key — same mechanism as `provider_credentials`).
- **Credentials never cross:** The `credentials` resource type is in the default excluded list. It must be explicitly added to scope (admin action, logged) and even then is per-grant and per-user.

## 13. Future (post-v1)

- B→A push (e.g., "notify A when a task assigned to subject changes") via Socket.IO over mTLS.
- Mesh (N-to-N) federation.
- Write verbs with conflict resolution.
- Shared Step-CA (a "root of roots") so that onboarding doesn't require exchanging CA roots.
- Federated memory search over vector indexes with homomorphic filtering.

## 14. Locked Decisions (was "Open Questions")

| #   | Question                                                                  | Decision                                                                                                                                       |
| --- | ------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------- |
| 1   | What happens to a grant when its subject user is deleted?                 | **Revoke-on-delete.** All grants where the user is subject are auto-revoked and CRL'd.                                                         |
| 2   | Do we audit read-only requests?                                           | **Yes.** All federated reads are audited on the serving side. Bodies are not captured; query hash + metadata only.                             |
| 3   | Default rate limit?                                                       | **60 requests per minute per grant,** override-able per grant.                                                                                 |
| 4   | How do we verify the requesting-server's identity beyond the grant token? | **X.509 client cert tied to the user,** issued by Step-CA (per-server) or locally generated. Cert subject carries `grantId` + `subjectUserId`. |

### M1 decisions

- **Postgres deployment:** **Containerized** alongside the gateway in M1 (Docker Compose profile). Moving to a dedicated host is a M5+ operational concern, not a v1 feature.
- **Instance signing key:** **Separate** from the Step-CA key. Step-CA signs federation certs; the instance master key seals at-rest secrets (client keys, provider credentials). Different blast-radius, different rotation cadences.

## 15. Acceptance Criteria

- [ ] Two Mosaic Stack gateways on different hosts can establish a federation grant via the CLI-driven onboarding flow.
- [ ] Server A can query Server B for `tasks`, `notes`, `memory` respecting scope filters.
- [ ] A user on B with no grant cannot be queried by A, even if A has a valid grant for another user.
- [ ] Revoking a grant on B causes A's next request to fail with a clear error within one request cycle.
- [ ] Cert rotation happens automatically at T-7 days; an in-progress session survives rotation without user action.
- [ ] Rate-limit enforcement returns 429 with `Retry-After`; client backs off.
- [ ] With B unreachable, a session on A completes using local data and surfaces a "federation offline for `<peer>`" signal once.
- [ ] Every federated request appears in B's `federation_audit_log` within 1 second.
- [ ] A scope excluding `credentials` means credentials are not returnable even via `search` with matching keywords.
- [ ] `mosaic federation status` shows cert expiry, grant status, and last success/failure per peer.

## 16. Implementation Milestones (reference)

Milestones live in `docs/federation/MILESTONES.md` (to be authored next). High-level:

- **M1:** Server A runs `federated` tier standalone (Postgres + Valkey + pgvector, containerized). No peer yet.
- **M2:** Step-CA embedded; `federation_grants` / `federation_peers` schema + admin CLI.
- **M3:** Handshake + `list`/`get` verbs with scope enforcement.
- **M4:** `search` verb, audit log, rate limits.
- **M5:** Cache layer, offline-degradation UX, observability surfaces.
- **M6:** Revocation flows (admin + revoke-on-delete), cert auto-renewal.
- **M7:** Multi-user RBAC hardening on B, team-scoped grants end-to-end, acceptance suite green.

---

**Next step after PRD sign-off:** author `docs/federation/MILESTONES.md` with per-milestone acceptance tests and estimated token budget, then file tracking issues on `git.mosaicstack.dev/mosaicstack/stack`.
