# Tapir — Architecture (C4 + sequences)

Diagrams describe **current assumptions**, not a frozen design. They are diffable Mermaid
so they live in version control and render in Gitea. When a decision here changes, record it
in `DECISIONS.md` and update the relevant diagram in the same commit.

Altitude: C4 Level 1 (Context) and Level 2 (Container), plus sequence diagrams for the two
behaviours that carry the most design risk. Clean Architecture layering is described after
the diagrams.

---

## C4 L1 — System Context

Who and what Tapir talks to. (Brain and external AI are dashed — both optional.)

```mermaid
graph TB
    user["User<br/>(maintainer; later, trusted users)"]
    tapir["Tapir<br/>watches subscriptions,<br/>summarizes new videos"]
    yt["YouTube<br/>(Data API + captions)"]
    vimeo["Vimeo<br/>(API + text tracks)"]
    local["Local AI stack<br/>(LiteLLM / piguard alias)"]
    byo["User's BYO AI<br/>(Claude / OpenAI / Gemini)"]
    brain["brain<br/>(brain-mcp)"]

    user -->|connects accounts,<br/>reads summaries| tapir
    tapir -->|subscriptions,<br/>new-video events,<br/>captions| yt
    tapir -->|subscriptions,<br/>text tracks| vimeo
    tapir -->|summarize<br/>PRIMARY| local
    tapir -.->|summarize<br/>FALLBACK, opt-in| byo
    tapir -.->|optional sink:<br/>brain_ingest| brain

    classDef opt stroke-dasharray: 5 5;
    class byo,brain opt;
```

---

## C4 L2 — Containers

Inside Tapir. The **engine** is provider- and sink-agnostic; everything external is an
adapter behind an interface (Clean Architecture ports & adapters).

```mermaid
graph TB
    subgraph tapir["Tapir (Go)"]
        http["HTTP server<br/>OAuth callbacks +<br/>user-facing API"]
        watcher["Watcher<br/>detects new videos<br/>(WebSub + poll)"]
        engine["Summarization engine<br/>(use-case core)"]
        resolver["Transcript resolver<br/>(captions-first)"]
        router["AI router<br/>(llm.Router:<br/>Primary -> Fallback)"]
        store[("User store<br/>(Postgres,<br/>per-user isolated)")]

        subgraph sinks["Sink adapters (Sink interface)"]
            sink_store["Store sink<br/>(primary)"]
            sink_brain["Brain sink<br/>(HTTP brain-mcp)"]
        end

        subgraph providers["Provider adapters (VideoSource interface)"]
            p_yt["YouTube adapter"]
            p_vimeo["Vimeo adapter"]
        end
    end

    http --> store
    watcher --> p_yt
    watcher --> p_vimeo
    watcher -->|NewVideo event| engine
    engine --> resolver
    resolver --> p_yt
    resolver --> p_vimeo
    engine --> router
    engine --> sink_store
    engine -.-> sink_brain
    sink_store --> store

    classDef opt stroke-dasharray: 5 5;
    class sink_brain opt;
```

**Interfaces that keep the engine pure (the ports):**

- `VideoSource` — list subscriptions, detect new videos, fetch transcript. Implemented by
  YouTube and Vimeo adapters.
- `Summarizer` — turn a transcript + context into highlights/takeaways. Backed by the
  `llm.Router` (Primary local, Fallback BYO).
- `Sink` — deliver a summary. Implemented by the store sink (primary) and brain sink
  (optional). New sinks add an implementation, nothing else.
- `SecretStore` — fetch/store per-user OAuth tokens and BYO keys. Backed by ESO/1Password.

The engine depends only on these interfaces, never on YouTube, brain, or a concrete model.
This is what makes "standalone vs homelab" a configuration of which adapters are wired, not
two codebases (ADR-003).

---

## Sequence — core use case: new video summarized

```mermaid
sequenceDiagram
    participant Src as VideoSource<br/>(YouTube/Vimeo)
    participant W as Watcher
    participant E as Engine
    participant R as Transcript resolver
    participant AI as AI router
    participant S as Sink(s)

    Src->>W: new upload (WebSub push / poll)
    W->>E: NewVideo{user, channel, videoID}
    E->>R: resolve transcript(videoID)
    R->>Src: fetch captions
    alt captions available
        Src-->>R: captions text
        R-->>E: Transcript{source: captions}
        E->>AI: summarize(transcript, userContext)
        AI-->>E: Summary{highlights, takeaways}
        E->>S: deliver(summary)
        S-->>E: ok
    else no transcript
        Src-->>R: none
        R-->>E: NoTranscript
        E->>S: deliver(skipped: no transcript)
    end
```

---

## Sequence — AI routing (local-first, BYO fallback)

```mermaid
sequenceDiagram
    participant E as Engine
    participant R as llm.Router
    participant P as Primary<br/>(local: LiteLLM/piguard)
    participant F as Fallback<br/>(user BYO key)

    E->>R: summarize(transcript)
    R->>P: complete(prompt)
    alt local succeeds
        P-->>R: summary
        R-->>E: summary (fallback_used = false)
    else local fails (error/timeout/unavailable)
        P-->>R: error
        alt user has BYO configured
            R->>F: complete(prompt)
            F-->>R: summary
            R-->>E: summary (fallback_used = true)
        else no BYO
            R-->>E: error (queued for retry)
        end
    end
```

> "Reliably" (VISION / the BYO trigger) is operationalized as: **Primary returned without
> error within timeout.** Richer quality scoring can layer on later without changing the
> interface. `fallback_used` is recorded per summary so "is the local stack good enough?"
> becomes a query, not a guess.

---

## Clean Architecture layering

```mermaid
graph LR
    subgraph domain["Domain (entities)"]
        d["User, Subscription, Video,<br/>Transcript, Summary"]
    end
    subgraph usecase["Use cases (engine)"]
        u["SummarizeNewVideo,<br/>ConnectAccount,<br/>RouteAI"]
    end
    subgraph ports["Ports (interfaces)"]
        po["VideoSource, Summarizer,<br/>Sink, SecretStore"]
    end
    subgraph adapters["Adapters (infra)"]
        a["YouTube, Vimeo, llm.Router,<br/>Store sink, Brain sink,<br/>ESO secret store, HTTP, Postgres"]
    end

    a --> po
    po --> u
    u --> d
```

Dependencies point inward only. Domain knows nothing of YouTube, brain, Postgres, or any
model. Adapters are swappable; tests target the use-case core through fake adapters (see the
Gherkin features in `docs/use-cases/`).

---

## Out of scope in these diagrams (deferred per ADRs)

- Audio-download + speech-to-text resolver (ADR-007) — would be an additional `VideoSource`
  fallback path, drawn when built.
- Multi-tenant isolation primitives (per-tenant Postgres role, NetworkPolicy, tenant label)
  — activate at Stage 1 (ADR-002); single-user Stage 0 doesn't exercise them.
- Public SaaS surface (sign-up, billing) — Future C, not built (ADR-008).
