shard-wiki/workplans/SHARD-WP-0002-federation-architecture.md

---
id: SHARD-WP-0002
type: workplan
title: "federation architecture design"
domain: whynot
repo: shard-wiki
status: active
owner: tegwick
topic_slug: whynot
created: "2026-06-08"
updated: "2026-06-13"
depends_on:
  - SHARD-WP-0001
state_hub_workstream_id: "2af4c46d-cbfd-40ea-a94b-d9e60b0f9945"
---

# SHARD-WP-0002 — Federation architecture design

## Goal

Produce a **federation architecture specification** for shard-wiki: positioning
against prior art (Federated Wiki, git-backed wikis, ActivityPub), documented
**decisions and tradeoffs**, and ADR-ready design notes that resolve the open
questions raised by `research/260608-federation-concepts/` and UseCaseCatalog
UC-26–UC-33.

Primary deliverable: `spec/FederationArchitecture.md` (created and filled by
this workplan's tasks).

Secondary deliverable (T11–T15, added 2026-06-13): the **shard adapter contract**
constraints distilled from the engine deep dives, feeding
`spec/TechnicalSpecificationDocument.md`. Federation architecture decides *what the
union does*; the adapter contract decides *what a backend must expose to participate*.
The two are co-dependent (the capability matrix T10 consumes the contract vocabulary
T11 defines), so they live in one workplan.

## Context

- Federation research: `research/260608-federation-concepts/findings.md`
- Engine deep dives (adapter-contract inputs, T11–T15):
  - `research/260608-wikiengines-overview/findings.md` (landscape; capability
    heterogeneity, write granularity)
  - `research/260613-xwiki-deep-dive/findings.md` (DB/component app-platform)
  - `research/260613-twiki-deep-dive/findings.md` (file+RCS app-platform)
  - `research/260613-foswiki-deep-dive/findings.md` (`Foswiki::Store` versioned
    store interface = adapter-contract prior art)
- Use cases: `spec/UseCaseCatalog.md` § A (UC-26–UC-43; UC-34–43 are the
  engine-attachment / adapter-contract cases)
- Aspiration: `INTENT.md` (orchestrator, not engine; mechanism over policy;
  capability-aware adapters; Markdown-first, backend-neutral)
- Related workplan: `SHARD-WP-0001` (page resolution, namespaces, overlays,
  provenance — federation architecture must align with, not duplicate, those
  outputs)

**Non-goal:** Implement federation. This workplan produces architecture and
decision records only.

## Decision topics (overview)

| Topic | Key tradeoff | Primary UCs |
|-------|--------------|-------------|
| Orchestrator positioning | Adapter layer vs fedwiki-style homogeneous network | UC-02, UC-26 |
| Remix primitives | Fork vs overlay vs import vs link-only | UC-04, UC-26, UC-29 |
| Equivalent page identity | Title/path/alias/graph matching; chorus vs canonical | UC-27, UC-07 |
| History model | Per-shard journal vs Git coordination journal vs both | UC-29, UC-33 |
| Union composition | Server orchestrator vs client composition | UC-05, UC-27 |
| Change notification | Git ping, ActivityPub, poll — adapter transports | UC-31 |
| Space lifecycle | Permanent vs ephemeral; archive and carry-forward | UC-28, UC-30 |
| Transclusion depth | Whole-page projection vs inline span | UC-03, UC-32 |
| Consensus presets | Spread, merge, designated canonical — policy not core | UC-07, UC-27 |
| Capability matrix | Which federation ops require which adapter capabilities | UC-02–UC-07 |

### Adapter-contract topics (T11–T15, from engine deep dives)

| Topic | Key tradeoff | Primary UCs |
|-------|--------------|-------------|
| Adapter contract & capabilities | Versioned interface + capability vocabulary; write granularity | UC-02, UC-35, UC-38 |
| Structured page model | Typed frontmatter vs sidecar vs blob; bodiless / multi-record pages | UC-34, UC-39 |
| History portability | Supplement (DB-internal) vs import (open file history) | UC-36, UC-41 |
| Adapter binding | Runtime API vs on-disk store; engine-hosted vs external; backend-swap | UC-38, UC-40, UC-43 |
| Syntax translation | Lossless non-Markdown round-trip vs read-only degradation | UC-42, UC-03 |

---

## Architecture positioning and boundaries

```task
id: SHARD-WP-0002-T1
status: todo
priority: high
state_hub_task_id: "ea8fdb22-6c7f-4ac1-9799-1346abf3c3b7"
```

Write the opening sections of `spec/FederationArchitecture.md`:

- shard-wiki as **orchestration layer** over heterogeneous shards (contrast
  Federated Wiki homogeneous JSON sites, ikiwiki homogeneous git wikis,
  ActivityPub activity streams).
- Explicit **compare, do not equate** mapping from federation research §6.
- Architectural boundaries: what core owns vs adapters vs UI vs policy config.
- Relationship to `SHARD-WP-0001` outputs (resolution, namespaces, overlays).

**Tradeoffs to document:** Central Git coordination journal vs fully
decentralized peer sync; browser-composed union vs server-side orchestrator for
agents/CLI.

---

## Remix primitives: fork, overlay, import, reference

```task
id: SHARD-WP-0002-T2
status: todo
priority: high
state_hub_task_id: "fb7d4bce-5d2e-4602-9b63-85934d90e82d"
```

Define when each remix primitive applies and how they interact:

| Primitive | Typical trigger | Writes to remote? |
|-----------|-----------------|-------------------|
| **Reference** | Link only | No |
| **Projection** | Read remote page | No (cache optional) |
| **Overlay** | Edit read-only shard | No until explicit apply |
| **Import / fork** | Copy into writable shard | Source unchanged |

Resolve federation research open question #1. Cover capability-limited shards
(read-only, no diff/merge). Map to UC-26, UC-04, UC-29.

**Tradeoffs:** Fedwiki fork-as-default vs overlay-before-mutation; copy cost
and attribution portability vs link-only federation.

---

## Equivalent page identity and multi-version presentation

```task
id: SHARD-WP-0002-T3
status: todo
priority: high
state_hub_task_id: "8f2a333d-ddcc-4cc6-b6ed-1ba9b178eee3"
```

Specify how shard-wiki identifies "the same topic" across shards:

- Matching signals: normalized title, path, explicit alias table, link-graph
  equivalence, manual curator binding.
- Presentation: chorus-of-voices (UC-27) vs designated canonical (policy).
- Link to UC-07 divergence detection and reconciliation triggers.

**Tradeoffs:** Automatic matching false positives vs manual curation burden;
showing all versions vs default canonical with alternates visible.

---

## History, attribution, and coordination journal

```task
id: SHARD-WP-0002-T4
status: todo
priority: high
state_hub_task_id: "5f39f48d-5142-4078-a84f-3245ec1add7e"
```

Define how edit history and attribution flow across federation operations:

- Per-shard revision model (Git commit, engine history, fedwiki-style journal
  where applicable).
- Information-space **coordination journal** role for cross-shard operations
  (fork, import, reconcile, space branch).
- Portable attribution requirements for UC-29 (frictionless remix).

**Tradeoffs:** Fedwiki journal-per-page vs Git-only; duplication of history vs
reconstructibility from coordination journal; storage cost of embedded media on
import.

---

## Union composition layer

```task
id: SHARD-WP-0002-T5
status: todo
priority: medium
state_hub_task_id: "3ff71e11-d0e9-4fda-b916-d6c34c51aa51"
```

Decide where the **union view** is assembled:

- Orchestrator API (server-side graph for agents, CI, non-browser clients).
- Optional client-side composition (fedwiki-style browser pull) as a consumer
  pattern, not the only path.
- Caching, freshness, and invalidation interaction with UC-03, UC-05, UC-31.

**Tradeoffs:** Single composition point (simpler provenance) vs distributed
composition (fedwiki resilience); cache staleness vs live-pull latency.

---

## Change notification and subscription transports

```task
id: SHARD-WP-0002-T6
status: todo
priority: medium
state_hub_task_id: "9596e5e8-8d6b-4ed4-bcbc-ebb45e3168be"
```

Specify change-notification as an **optional adapter capability**:

- Transports: git hook / pinger (ikiwiki), ActivityPub Create/Update (XWiki),
  WebDAV ETag, polling fallback.
- What a subscription triggers: projection refresh, reconciliation queue,
  RecentChanges union entry (UC-31, UC-17).
- Out-of-scope vs in-scope for v1.

**Tradeoffs:** Push freshness vs implementation complexity; ActivityPub as
notification-only vs content-bearing share; dependency on external fediverse
infrastructure.

---

## Information space lifecycle

```task
id: SHARD-WP-0002-T7
status: todo
priority: medium
state_hub_task_id: "38134064-51ce-4f5a-80bf-b2cfbe381c59"
```

Model lifecycle states for root entities / information spaces:

- Active, read-only archived, retired (detached), merged-into-successor.
- Carry-forward workflow (UC-28, UC-30): selective import from archived shard.
- Space-level fork / branch (UC-33): coordination journal + shard config branch.

**Tradeoffs:** First-class ephemeral "happenings" vs permanent spaces only;
automatic archive vs manual; whether retirement deletes projections or preserves
read-only union entries.

---

## Transclusion and projection depth

```task
id: SHARD-WP-0002-T8
status: todo
priority: medium
state_hub_task_id: "5607732b-612a-4550-bb17-b8cd34979cf4"
```

Distinguish projection levels:

| Level | UC | Behavior |
|-------|-----|----------|
| Whole-page projection | UC-03 | Lazy full page from remote shard |
| Block/span transclusion | UC-32 | Inline embed with origin + freshness |
| Link reference | UC-08 | Pointer only |

Define provenance display requirements and staleness semantics for UC-32.
Reference Xanadu transclusion patterns without adopting unbuilt Xanadu scope.

**Tradeoffs:** Live transclusion fragility (link rot, remote down) vs snapshot
on import; core orchestrator support vs Markdown extension + adapter.

---

## Consensus and reconciliation policy catalog

```task
id: SHARD-WP-0002-T9
status: todo
priority: medium
state_hub_task_id: "adfca8b3-eb21-497d-9f51-65dc9269c810"
```

Document **configurable policy presets** (mechanism over policy):

- Fedwiki-spread (versions coexist; popularity implicit).
- Designated canonical shard (explicit write target).
- Git-merge reconciliation (ikiwiki-style).
- Overlay-only (no destructive merge on read-only sources).
- Vote-to-merge / editorial gate (optional, UI-layer).

Map presets to UC-07, UC-27. Core provides primitives; policy selects preset.

**Tradeoffs:** Default policy for L0 open mode vs team mode; exposing conflicts
vs auto-merge when backends support it.

---

## Federation operations capability matrix

```task
id: SHARD-WP-0002-T10
status: todo
priority: medium
state_hub_task_id: "c7a93d06-8631-43b4-bc7f-1b0a1cd1436f"
```

Produce a capability matrix: which federation operations require which adapter
capabilities (read, write, diff, merge, lock, version, publish, notify,
transclude-source).

Cross-check against INTENT capability-aware adapters principle. Identify
graceful degradation paths for limited backends (read-only participant,
projection-only, patch target).

This matrix **consumes the capability vocabulary defined in T11** — keep the two
in sync. Feeds `spec/TechnicalSpecificationDocument.md` adapter contract section.

---

# Shard adapter contract (folded from engine deep dives, 2026-06-13)

These tasks distill the cross-cutting **adapter-contract constraints** surfaced by
the four engine deep dives (wiki-engines landscape, XWiki, TWiki, Foswiki). They
answer *what a backend must expose to participate as a shard*, complementing the
federation tasks above (which answer *what the union does*). Output target:
`spec/TechnicalSpecificationDocument.md` (adapter contract section), cross-linked
from `spec/FederationArchitecture.md`.

Recurring cross-engine insight to honor: **structure and history in text files
federate far more easily than in a database**, and **Foswiki::Store already proves
the versioned-interface-with-swappable-backends pattern** the contract should adopt.

## Adapter contract: capability model & versioned interface

```task
id: SHARD-WP-0002-T11
status: todo
priority: high
state_hub_task_id: "a7379621-6694-488b-94ca-846b8e27e745"
```

Define the **capability-aware shard adapter contract** as a *versioned* interface,
taking `Foswiki::Store` + `Foswiki::Meta` as direct prior art (a real engine that
already separates store backend from core behind a stable interface).

- Capability vocabulary: `read, write, diff, merge, lock, version, publish, notify,
  transclude-source, translate-syntax, structured-payload` (reconcile with T10).
- **Write granularity** as a first-class capability dimension: per-page / per-file /
  per-space / append-only (TiddlyWiki single-file vs DB whole-space vs TWiki per-topic).
- Versioning/compatibility rules for the contract itself; capability discovery.

**Inputs:** `260608-wikiengines-overview` §3, §5; `260613-foswiki-deep-dive` §2, §6.
**Provides** the vocabulary T10's matrix consumes. **UCs:** UC-02, UC-35, UC-38.

**Tradeoffs:** Rich capability set (precise degradation) vs contract complexity;
static profile vs runtime capability negotiation.

---

## Page model: structured / typed payload representation

```task
id: SHARD-WP-0002-T12
status: todo
priority: high
state_hub_task_id: "7334a4a4-ba75-4fac-a8b4-8350d342b299"
```

Decide how the **backend-neutral, Markdown-first page model carries structured data
without flattening**:

- Source shapes: XWiki XObjects against an XClass; TWiki/Foswiki DataForms stored as
  `%META:FIELD%` in text; Foswiki MetaDataPlugin **multiple records per topic**;
  Semantic MediaWiki annotations.
- Representation choice: typed frontmatter vs sidecar file vs opaque provenance blob.
- **Bodiless pages** (UC-39): may a page be purely structured, no Markdown body?
- **N typed records per page** (not one form) must be representable.

**Inputs:** `xwiki` §2.3; `twiki` §2.3; `foswiki` §4. **UCs:** UC-34, UC-39.

**Tradeoffs:** Lossless fidelity vs a uniform queryable model; diff/search over
structured fields vs prose.

---

## History portability: supplement vs import

```task
id: SHARD-WP-0002-T13
status: todo
priority: high
state_hub_task_id: "6837862a-8f57-410d-9200-a6a5dcf1a7b9"
```

Define how the **coordination journal** relates to an engine's native history (aligns
with and refines T4):

- **Supplement** — engine history is DB-internal and non-portable
  (Confluence, MediaWiki, XWiki `xwikircs`): the journal begins now / mirrors forward
  (UC-36).
- **Import** — engine history is an open file format (TWiki/yawex RCS `.txt,v`,
  Foswiki PlainFile timestamped copies): backfill into Git preserving author/timestamp
  (UC-41).
- Authoritative journal vs mirror-reconciled-back; one-time vs continuous.

**Inputs:** `xwiki` §2.4; `twiki` §2.1; `foswiki` §2.2. **UCs:** UC-36, UC-41.

**Tradeoffs:** Import fidelity vs effort; duplicated history vs single source of truth.

---

## Adapter binding: attach path, hosting, backend-swap

```task
id: SHARD-WP-0002-T14
status: todo
priority: medium
state_hub_task_id: "f8835969-d118-4738-952a-5e67e5209f3d"
```

Decide **how and where an adapter binds to a backend**:

- **Dual-path attach** — runtime API vs reading the on-disk store directly
  (consistency vs offline/fidelity); capability-gate the choice (UC-40).
- **Hosting** — engine-side adapter via native extension API (XWiki components/REST/
  UIX; TWiki/Foswiki plugin handlers + REST + `registerTagHandler`) vs external
  adapter (no engine change, lower fidelity) (UC-38).
- **Backend-swap tolerance** — shard identity/provenance survives a storage-format
  change (Foswiki RCS↔PlainFile; folder→Git) (UC-43).

**Inputs:** `twiki` §5, §6; `xwiki` §3; `foswiki` §2. **UCs:** UC-38, UC-40, UC-43.

**Tradeoffs:** Fidelity vs deploy access; consistency vs offline capability.

---

## Syntax translation & content fidelity

```task
id: SHARD-WP-0002-T15
status: todo
priority: medium
state_hub_task_id: "22b57b3a-b06b-4ff0-a34a-667a0386bf25"
```

Specify how **non-Markdown shards** participate in the Markdown-first model:

- Read native markup (TWiki/Foswiki TML, XWiki syntax) into the page model and accept
  Markdown overlays back via **bidirectional, lossless translation** (UC-42).
- Feasibility proof: Foswiki **WysiwygPlugin** (TML→HTML for editing, HTML→TML
  losslessly on save).
- **Graceful degradation:** when lossless translation is unavailable, the shard is a
  read-only/projection participant (UC-03), never silently corrupted.
- Open: may overlays be stored in Markdown, or must they round-trip in native syntax?

**Inputs:** `foswiki` §5; `twiki` §2.4; `xwiki` §2.5. **UCs:** UC-42, UC-03.

**Tradeoffs:** Translation coverage vs read-only floor; Markdown overlays (portable)
vs native-syntax overlays (safe round-trip).

---

## Acceptance criteria

- `spec/FederationArchitecture.md` exists with all ten federation topic sections
  (T1–T10) and an explicit **decisions / deferred / open** table per topic.
- The **adapter contract** section of `spec/TechnicalSpecificationDocument.md` exists,
  populated by T11–T15, with the capability vocabulary (T11) reconciled against the
  T10 federation-ops matrix.
- Each decision honors INTENT: mechanism over policy, union without erasure,
  overlay before mutation, no silent remote mutation, shard sovereignty,
  capability-aware adapters, Markdown-first / backend-neutral.
- UC-26–UC-43 are traceable to architecture / adapter-contract sections
  (UC-34–UC-43 to T11–T15 specifically).
- Conflicts or dependencies on `SHARD-WP-0001` outputs are listed (e.g.
  namespace model affects equivalent-page identity; page model T12 affects
  resolution/overlays).
- `SHARD-WP-0001` and `SHARD-WP-0002` can proceed in parallel where topics
  are independent; merge conflicts in spec are resolved before implementation
  workplan starts.

## Suggested task order

**Federation architecture (T1–T10):**
1. T1 positioning (frames everything)
2. T2 remix primitives + T3 equivalent identity (parallel)
3. T4 history model
4. T5 composition + T6 notification (parallel)
5. T7 lifecycle + T8 transclusion (parallel)
6. T9 policy catalog + T10 capability matrix (parallel, finalize doc)

**Adapter contract (T11–T15)** — can start in parallel with T1; T11 first as it
frames the rest, and pair it with T10 (shared capability vocabulary):
7. T11 contract & capabilities (frames T12–T15; sync with T10)
8. T12 page model + T13 history portability (parallel; T13 aligns with T4)
9. T14 binding + T15 syntax translation (parallel, finalize adapter-contract spec)