satellite-provider/_docs/02_document/architecture.md

# Satellite Provider — Architecture

## Architecture Vision

Satellite Provider is a self-hosted .NET 8.0 backend service that pre-downloads, caches, and composites Google Maps satellite imagery for offline use. It runs as a single containerized monolith with PostgreSQL, processing requests asynchronously via in-process queues. The dominant pattern is a layered architecture (API → Services → DataAccess → PostgreSQL) with background hosted services for long-running work.

**Components & responsibilities** (each owns its own `.csproj` since AZ-309):
- **Common** (`SatelliteProvider.Common`) — Shared contracts: DTOs, service interfaces, common exceptions, configuration models, geospatial math
- **DataAccess** (`SatelliteProvider.DataAccess`) — PostgreSQL persistence via Dapper + DbUp migrations
- **TileDownloader** (`SatelliteProvider.Services.TileDownloader`) — Provider-agnostic tile acquisition via `ISatelliteDownloader` interface (first implementation: Google Maps) with deduplication, concurrency control, and an in-memory tile-byte cache owned by `TileService`
- **RegionProcessing** (`SatelliteProvider.Services.RegionProcessing`) — Batch tile downloads for geographic areas, stitching, output generation
- **RouteManagement** (`SatelliteProvider.Services.RouteManagement`) — Route interpolation, geofenced region generation, consolidated map output

The three Layer-3 service components are compile-time siblings: each only references `SatelliteProvider.Common` and `SatelliteProvider.DataAccess`. Cross-component runtime calls flow exclusively through interfaces in `SatelliteProvider.Common.Interfaces`.

**Major data flows**:
- *Tile acquisition*: HTTP request → cache check → Google Maps download → disk + DB persistence
- *Region processing*: Request queued → background worker calculates tile grid → downloads all tiles → produces CSV/summary/stitched image
- *Route expansion*: Waypoints → interpolated points every ~200m → geofence filtering → region requests per point → optional ZIP archive

**Architectural principles** (inferred):
- Single-instance deployment, no horizontal scaling requirements (`inferred-from: Channel-based queue, no distributed state`)
- Append-by-source tile storage — multiple producers (Google Maps, UAV upload, future SatAR, …) can each persist a row per `(latitude, longitude, tile_zoom, tile_size_meters)` cell. Reads return the most-recent row across sources, ordered by `captured_at DESC` with deterministic `(updated_at DESC, id DESC)` tie-breaks. The single-row-per-cell-per-source invariant is enforced by the 5-column unique index `idx_tiles_unique_location_source` introduced in migration 013 (AZ-484). The `tiles.version` column is vestigial since AZ-357 dropped year-based cache invalidation in favour of cell-level overwrite. (`inferred-from: tiles table + AZ-484/AZ-357 migrations + tile-storage contract v1.0.0`)
- Fire-and-forget async processing with status polling (`inferred-from: queue + background service + status endpoint`)
- JWT-validated callers only — every HTTP endpoint requires a valid HS256-signed Bearer token, validated locally against a shared `JWT_SECRET` per the suite-level auth contract (`suite/_docs/10_auth.md`). Issuer/audience are intentionally not validated yet; signature + lifetime + ≥32-byte key are. Per-endpoint permission claims (e.g. `permissions: ["GPS"]` on the UAV upload) layer on top of this baseline.

**Authentication & Authorization** (AZ-487):
- Validation library: `Microsoft.AspNetCore.Authentication.JwtBearer` 8.0.21 (matches the rest of the ASP.NET Core 8 package set).
- Signing key: read from the `JWT_SECRET` environment variable (preferred) or the `Jwt:Secret` configuration key. Startup fails fast if the resolved secret is unset, empty, or shorter than 32 bytes (HMAC-SHA256 minimum per RFC 2104 §3).
- Token contract: `ValidateIssuerSigningKey = true`, `ValidateLifetime = true`, `RequireSignedTokens = true`, `RequireExpirationTime = true`, `ValidateIssuer/Audience = false`, `ClockSkew = 30s`. The 5-minute JwtBearer default is intentionally tightened.
- Authorization model: every endpoint registered in `Program.cs` is decorated with `.RequireAuthorization()`. AZ-488 adds `permissions`-claim policies on top of this baseline (UAV upload requires `GPS`).
- Test infrastructure: `JwtTokenFactory` (unit tests) and `JwtTestHelpers` (integration tests) mint deterministic tokens against the same `JWT_SECRET`; the integration test runner attaches a default Bearer token to its shared `HttpClient` so legacy non-auth tests continue to exercise the protected endpoints unchanged.

**Planned features** (confirmed by user, currently stubs):
- MGRS endpoint — tile access via Military Grid Reference System coordinates
- Upload endpoint — UAV nadir camera tile ingestion. Writes a row with `source='uav'` for the captured cell; the storage layer accepts it alongside any existing Google Maps row, and reads return whichever has the highest `captured_at`. AZ-484 has built the multi-source storage; the upload endpoint itself (T2 — AZ-485) and any quality-gate logic remain to be implemented.

The N-source storage contract is authoritative in `_docs/02_document/contracts/data-access/tile-storage.md` (v1.0.0). Anything that reads or writes `tiles` MUST follow that contract rather than re-deriving the rules from prose here.

**Drift signals**:
- `geofence_polygons` mentioned in AGENTS.md as a routes table column but does not exist in schema or entity — documentation drift

## 1. System Context

**Problem being solved**: A GPS-denied UAV navigation service requires satellite imagery for positioning and route planning without GPS. This service pre-downloads satellite tiles from one or more imagery sources (currently Google Maps; future sources including UAV nadir camera upload and additional providers such as SatAR) for specified regions and routes, stores them alongside each other under a per-source storage key, and serves the most-recent row across sources on access. Tiles are stitched into composite images and packaged for offline use.

**System boundaries**: The Satellite Provider is a self-contained backend service. It receives HTTP requests (region/route definitions), downloads tiles from Google Maps, stores them on disk and in PostgreSQL, and produces output files (images, CSVs, ZIPs).

**External systems**:

| System | Integration Type | Direction | Purpose |
|--------|-----------------|-----------|---------|
| Satellite imagery provider (e.g., Google Maps) | HTTPS (tile download) | Outbound | First implementation of the multi-source `tiles` storage; provider-agnostic via `ISatelliteDownloader`. Stamps `source='google_maps'` on every persisted row. |
| GPS-Denied Service (UAV) | REST API | Inbound | Future producer of `source='uav'` rows via the upload endpoint (T2 — AZ-485). The storage layer (AZ-484) is already in place; the endpoint itself is still a stub. |
| PostgreSQL | TCP (Npgsql) | Both | Tile metadata, region/route state |
| File System | Local disk | Both | Tile image storage, output artifacts |
| HTTP Clients | REST API | Inbound | Region/route requests, tile queries |

## 2. Technology Stack

| Layer | Technology | Version | Rationale |
|-------|-----------|---------|-----------|
| Language | C# | 12.0 | .NET ecosystem, strong typing |
| Framework | ASP.NET Core (Minimal API) | 8.0 | Lightweight HTTP hosting |
| Database | PostgreSQL | 15+ | Reliable RDBMS, spatial-friendly |
| ORM | Dapper | latest | Micro-ORM, raw SQL control |
| Migrations | DbUp | latest | Simple SQL-file-based schema migrations |
| Image Processing | SixLabors.ImageSharp | 3.1.11 | Cross-platform image manipulation |
| Logging | Serilog | 8.0.3 | Structured logging with file sinks |
| Hosting | Docker (docker-compose) | — | Containerized deployment |
| CI/CD | Woodpecker CI | — | Lightweight self-hosted CI |

## 3. Deployment Model

**Environments**: Development (docker-compose), Production (Docker)

**Infrastructure**:
- Docker-based containerized deployment
- PostgreSQL as a separate container
- Shared volumes for tile storage and output artifacts
- No cloud provider dependency (self-hosted capable)

**Environment-specific configuration**:

| Config | Development | Production |
|--------|-------------|------------|
| Database | localhost:5432 (Docker) | Container network `db:5432` |
| Secrets | appsettings.Development.json | Environment variables |
| Logging | Console + File | File (./logs/) |
| API URL | http://localhost:5100 | http://0.0.0.0:5100 |

## 4. Data Model Overview

**Core entities**:

| Entity | Description | Owned By Component |
|--------|-------------|--------------------|
| Tile | A single satellite image tile with coordinates and zoom | TileDownloader |
| Region | A square area request with processing status | RegionProcessing |
| Route | A named path with geofence polygons | RouteManagement |
| RoutePoint | An individual point (original or interpolated) on a route | RouteManagement |

**Key relationships**:
- Route → RoutePoint: one-to-many (a route has many sequential points)
- Route → Region: many-to-many via `route_regions` (each route point generates a region)
- Region → Tile: implicit (a processed region references tiles by coordinate/zoom)

**Data flow summary**:
- Client → API → Queue → BackgroundService → GoogleMaps → FileSystem + DB: tile acquisition pipeline
- Client → API → RouteService → PointInterpolation → RegionCreation → Queue: route-to-region expansion

## 5. Integration Points

### Internal Communication

| From | To | Protocol | Pattern | Notes |
|------|----|----------|---------|-------|
| WebApi | RegionProcessing | In-process queue (Channel) | Fire-and-forget | Request queued, status polled. Uses `IRegionService` / `IRegionRequestQueue` from Common. |
| WebApi | TileDownloader | `ITileService` (Common interface) | Request-Response | Single-tile reads (`GetOrDownloadTileAsync`) and writes (`DownloadAndStoreSingleTileAsync`) flow through `ITileService` since AZ-310 / AZ-311. No direct dependency on the concrete `GoogleMapsDownloaderV2`. |
| RegionProcessing | TileDownloader | `ITileService` (Common interface) | Request-Response | Per-tile within region processing. Resolved through DI; no compile-time `ProjectReference` between RegionProcessing and TileDownloader csprojs. |
| RouteManagement | RegionProcessing | `IRegionService` / `IRegionRequestQueue` (Common interfaces) | Fire-and-forget | Route regions submitted to queue. No compile-time `ProjectReference` between RouteManagement and RegionProcessing csprojs. |
| All Services | DataAccess | Direct method call (via repository interfaces) | Repository pattern | Dapper queries |

### External Integrations

| External System | Protocol | Auth | Rate Limits | Failure Mode |
|----------------|----------|------|-------------|--------------|
| Satellite imagery provider (abstracted via `ISatelliteDownloader`; first implementation: Google Maps) | HTTPS GET | Provider-specific (e.g., session token) | Configured concurrency (MaxConcurrentDownloads) | Retry with backoff, mark region failed |

## 6. Non-Functional Requirements

| Requirement | Target | Measurement | Priority |
|------------|--------|-------------|----------|
| Concurrent Downloads | 4 (configurable) | SemaphoreSlim limit | High |
| Concurrent Regions | 20 (configurable) | Processing config | Medium |
| Queue Capacity | 1000 requests | Channel bounded capacity | Medium |
| Tile Deduplication | 100% (no re-download) | DB lookup before fetch | High |
| Max Zip Size | 50 MB | Route zip output | Medium |

## 7. Security Architecture

**Authentication**: HS256 JWT Bearer tokens (AZ-487). Signing key from `JWT_SECRET` env var (≥ 32 bytes, validated at startup). `Microsoft.AspNetCore.Authentication.JwtBearer` validates signature, lifetime, and signing key; issuer and audience are intentionally not validated (suite contract does not specify expected values). ClockSkew tightened from JwtBearer default (5 min) to 30 s. Tokens are minted by the centralized Admin API per `suite/_docs/10_auth.md`.

**Authorization**: Every endpoint requires authentication via `.RequireAuthorization()`. Permission-claim enforcement (e.g. `permissions: ["GPS"]`) is added per-endpoint where needed — AZ-488 introduces it on `POST /api/satellite/upload`. Other endpoints accept any authenticated principal.

**Data protection**:
- At rest: No encryption (tiles stored as plain JPEG files)
- In transit: HTTPS for Google Maps calls; API itself runs HTTP behind Kestrel (TLS termination is a deployment-layer concern)
- Secrets management: `JWT_SECRET` and `GOOGLE_MAPS_API_KEY` from environment variables / `.env` (gitignored); `.env.example` documents the required keys. Production deployments MUST supply both via the host environment, never via the appsettings files.

**Audit logging**: Serilog writes to file; logs exceptions and processing state transitions. 401/403 responses are emitted by the JwtBearer middleware via the `WWW-Authenticate` header; no body leakage of internal details.

## 8. Key Architectural Decisions

### ADR-001: Minimal API over Controller-based

**Context**: Project needed a lightweight HTTP layer for a small set of endpoints.

**Decision**: Use ASP.NET Core Minimal APIs (no controllers, no MVC).

**Consequences**: Less ceremony, all routing in `Program.cs`, but less structure for future growth.

### ADR-002: Dapper over Entity Framework

**Context**: Database access is straightforward CRUD with some spatial queries.

**Decision**: Use Dapper for raw SQL control and performance, paired with DbUp for schema migrations.

**Consequences**: Full SQL control, no ORM overhead; trade-off is manual mapping and no change tracking.

### ADR-003: In-Process Queue over External Message Broker

**Context**: Region/route processing needs to be asynchronous but the system is a single service.

**Decision**: Use `System.Threading.Channels` as an in-process bounded queue.

**Consequences**: Simple, no external dependencies; but limited to single-instance deployment — no horizontal scaling of workers.

### ADR-004: File-Based Tile Storage

**Context**: Tiles are immutable JPEG images that need fast random access.

**Decision**: Store tiles as files in a directory hierarchy (`./tiles/{zoom}/{x}/{y}.jpg`) with metadata in PostgreSQL.

**Consequences**: Fast reads, easy backup/migration, but requires shared filesystem for multi-instance (which is not currently needed).

### ADR-005: Background Hosted Services for Processing

**Context**: Region and route processing is long-running and should not block HTTP requests.

**Decision**: Use `IHostedService` implementations that consume from the in-process queue.

**Consequences**: Clean separation of request handling and processing; lifecycle managed by the host.