# Attachment Domain

Handles file sharing between peers over WebRTC data channels. Files are announced, chunked into 64 KB pieces, streamed peer-to-peer as base64, and optionally persisted to disk (Electron) or kept in memory (browser).

## Module map

```
attachment/
├── application/
│   ├── facades/
│   │   └── attachment.facade.ts                     Thin entry point, delegates to manager
│   └── services/
│       ├── attachment-manager.service.ts        Orchestrates lifecycle, auto-download, peer listeners
│       ├── attachment-transfer.service.ts       P2P file transfer protocol (announce/request/chunk/cancel)
│       ├── attachment-transfer-transport.service.ts  Base64 encode/decode, chunked streaming
│       ├── attachment-persistence.service.ts    DB + filesystem persistence, migration from localStorage
│       └── attachment-runtime.store.ts          In-memory signal-based state (Maps for attachments, chunks, pending)
│
├── domain/
│   ├── logic/
│   │   └── attachment.logic.ts                  isAttachmentMedia, shouldAutoRequestWhenWatched, shouldPersistDownloadedAttachment
│   ├── models/
│   │   ├── attachment.model.ts                  Attachment type extending AttachmentMeta with runtime state
│   │   └── attachment-transfer.model.ts         Protocol event types (file-announce, file-chunk, file-request, ...)
│   └── constants/
│       ├── attachment.constants.ts              MAX_AUTO_SAVE_SIZE_BYTES = 10 MB
│       └── attachment-transfer.constants.ts     FILE_CHUNK_SIZE_BYTES = 64 KB, EWMA weights, error messages
│
├── infrastructure/
│   ├── services/
│   │   └── attachment-storage.service.ts        Electron filesystem access (save / read / delete)
│   └── util/
│       └── attachment-storage.util.ts           sanitizeAttachmentRoomName, resolveAttachmentStorageBucket
│
└── index.ts                                     Barrel exports
```

## Service composition

The facade is a thin pass-through. All real work happens inside the manager, which coordinates the transfer service (protocol), persistence service (DB/disk), and runtime store (signals).

```mermaid
graph TD
    Facade[AttachmentFacade]
    Manager[AttachmentManagerService]
    Transfer[AttachmentTransferService]
    Transport[AttachmentTransferTransportService]
    Persistence[AttachmentPersistenceService]
    Store[AttachmentRuntimeStore]
    Storage[AttachmentStorageService]
    Logic[attachment.logic]

    Facade --> Manager
    Manager --> Transfer
    Manager --> Persistence
    Manager --> Store
    Manager --> Logic
    Transfer --> Transport
    Transfer --> Store
    Persistence --> Storage
    Persistence --> Store
    Storage --> Helpers[attachment-storage.util]

    click Facade "application/facades/attachment.facade.ts" "Thin entry point" _blank
    click Manager "application/services/attachment-manager.service.ts" "Orchestrates lifecycle" _blank
    click Transfer "application/services/attachment-transfer.service.ts" "P2P file transfer protocol" _blank
    click Transport "application/services/attachment-transfer-transport.service.ts" "Base64 encode/decode, chunked streaming" _blank
    click Persistence "application/services/attachment-persistence.service.ts" "DB + filesystem persistence" _blank
    click Store "application/services/attachment-runtime.store.ts" "In-memory signal-based state" _blank
    click Storage "infrastructure/services/attachment-storage.service.ts" "Electron filesystem access" _blank
    click Helpers "infrastructure/util/attachment-storage.util.ts" "Path helpers" _blank
    click Logic "domain/logic/attachment.logic.ts" "Pure decision functions" _blank
```

## File transfer protocol

Files move between peers using a request/response pattern over the WebRTC data channel. The sender announces a file, the receiver requests it, and chunks flow back one by one.

```mermaid
sequenceDiagram
    participant S as Sender
    participant R as Receiver

    S->>R: file-announce (id, name, size, mimeType)
    Note over R: Store metadata in runtime store
    Note over R: shouldAutoRequestWhenWatched?

    R->>S: file-request (attachmentId)
    Note over S: Look up file in runtime store or on disk

    loop Every 64 KB chunk
        S->>R: file-chunk (attachmentId, index, data, progress, speed)
        Note over R: Append to chunk buffer
        Note over R: Update progress + EWMA speed
    end

    Note over R: All chunks received
    Note over R: Reassemble blob
    Note over R: shouldPersistDownloadedAttachment? Save to disk
```

### Failure handling

If the sender cannot find the file, it replies with `file-not-found`. The transfer service then tries the next connected peer that has announced the same attachment. Either side can send `file-cancel` to abort a transfer in progress.

```mermaid
sequenceDiagram
    participant R as Receiver
    participant P1 as Peer A
    participant P2 as Peer B

    R->>P1: file-request
    P1->>R: file-not-found
    Note over R: Try next peer
    R->>P2: file-request
    P2->>R: file-chunk (1/N)
    P2->>R: file-chunk (2/N)
    P2->>R: file-chunk (N/N)
    Note over R: Transfer complete
```

## Auto-download rules

When the user navigates to a room, the manager watches the route and decides which attachments to request automatically based on domain logic:

| Condition | Auto-download? |
|---|---|
| Image or video, size <= 10 MB | Yes |
| Image or video, size > 10 MB | No |
| Non-media file | No |

The decision lives in `shouldAutoRequestWhenWatched()` which calls `isAttachmentMedia()` and checks against `MAX_AUTO_SAVE_SIZE_BYTES`.

## Persistence

On Electron, completed downloads are written to the app-data directory. The storage path is resolved per room and bucket:

```
{appDataPath}/{serverId}/{roomName}/{bucket}/{attachmentId}.{ext?}
```

Room names are sanitised to remove filesystem-unsafe characters. The bucket is either `attachments` or `media` depending on the attachment type. The original filename is kept in attachment metadata for display and downloads, but the stored file uses the attachment ID plus the original extension so two uploads with the same visible name do not overwrite each other.

`AttachmentPersistenceService` handles startup migration from an older localStorage-based format into the database, and restores attachment metadata from the DB on init. On browser builds, files stay in memory only.

## Runtime store

`AttachmentRuntimeStore` is a signal-based in-memory store using `Map` instances for:

- **attachments**: all known attachments keyed by ID
- **chunks**: incoming chunk buffers during active transfers
- **pendingRequests**: outbound requests waiting for a response
- **cancellations**: IDs of transfers the user cancelled

Components read attachment state reactively through the store's signals. The store has no persistence of its own; that responsibility belongs to the persistence service.