satellite-provider/_docs/02_document/components/03_tile_downloader/description.md

TileDownloader

1. High-Level Overview

Purpose: Acquires satellite imagery tiles from Google Maps, stores them on disk, and persists metadata to the database. Handles session tokens, concurrent downloads, retry logic, and tile deduplication. Since AZ-488 it also hosts the UAV upload pipeline: the UavTileQualityGate 5-rule validator and the UavTileUploadHandler that persists source='uav' rows via ITileRepository.InsertAsync.

Architectural Pattern: Service + Gateway (wraps external API with retry/throttling) + per-source quality gate (UAV upload path)

csproj: SatelliteProvider.Services.TileDownloader/SatelliteProvider.Services.TileDownloader.csproj (split out of the monolithic SatelliteProvider.Services project in epic AZ-309)

Upstream dependencies: Common (DTOs, Enums — TileSource + TileSourceConverter since AZ-484, plus UavTileMetadata / UavTileBatchUploadResponse / UavTileRejectReasons since AZ-488; GeoUtils; configs MapConfig, StorageConfig, ProcessingConfig, UavQualityConfig; RateLimitException), DataAccess (TileEntity, ITileRepository), SixLabors.ImageSharp 3.1.11 (UAV decode + variance check).

Downstream consumers: RegionProcessing and WebApi — both via ITileService from Common; WebApi also resolves IUavTileQualityGate and IUavTileUploadHandler from this component (DI only — no compile-time ProjectReference from any consumer to this project's concrete types).

2. Internal Interfaces

Class: GoogleMapsDownloaderV2

Method	Input	Output	Async	Error Types
DownloadSingleTileAsync	lat, lon, zoomLevel, CancellationToken	DownloadedTileInfoV2	Yes	ArgumentException, RateLimitException, HttpRequestException
GetTilesWithMetadataAsync	center, radiusM, zoom, existingTiles, CancellationToken	List<DownloadedTileInfoV2>	Yes	ArgumentException, RateLimitException, HttpRequestException

Service: TileService (implements ITileService)

Method	Input	Output	Async	Error Types
DownloadAndStoreTilesAsync	lat, lon, sizeM, zoom, CancellationToken	List<TileMetadata>	Yes	propagated from downloader
GetTileAsync	Guid	TileMetadata?	Yes	NpgsqlException
GetTilesByRegionAsync	lat, lon, sizeM, zoom	IEnumerable<TileMetadata>	Yes	NpgsqlException
GetOrDownloadTileAsync (AZ-310)	z, x, y, CancellationToken	TileBytes	Yes	propagated from downloader
DownloadAndStoreSingleTileAsync (AZ-311)	lat, lon, zoom, CancellationToken	TileMetadata	Yes	propagated from downloader

Service: UavTileQualityGate (implements IUavTileQualityGate, AZ-488)

Method	Input	Output	Async	Error Types
Validate	imageBytes, contentType, UavTileMetadata	UavTileQualityResult (accept + reason code)	No	none (decode exceptions caught and translated to INVALID_FORMAT)

Rules run in fixed order (Format → Size band → Dimensions → Captured-at age → Blank/uniform); first failure short-circuits. Thresholds come from UavQualityConfig. Time comes from injected TimeProvider (defaults to TimeProvider.System) for deterministic tests.

Service: UavTileUploadHandler (implements IUavTileUploadHandler, AZ-488)

Method	Input	Output	Async	Error Types
HandleAsync	metadataJson, IReadOnlyList<UavUploadFile>, CancellationToken	UavTileUploadHandlerResult (envelope error OR per-item response)	Yes	propagated IOException/UnauthorizedAccessException per item, translated to per-item STORAGE_FAILURE

Per-item flow: parse metadata JSON → reject envelope (mismatch, oversize, malformed JSON) OR run each item through IUavTileQualityGate → for accepted items, write JPEG to {StorageConfig.TilesDirectory}/uav/{zoom}/{x}/{y}.jpg then call ITileRepository.InsertAsync (per-source UPSERT) with source='uav' and the request-supplied capturedAt. File-before-row ordering keeps an orphan file (rather than a row pointing at nothing) when persistence fails.

4. Data Access Patterns

Caching Strategy

Data	Cache Type	TTL	Invalidation
Tile bytes	In-memory (IMemoryCache, owned by TileService since AZ-310)	1h absolute, 30min sliding	None (manual restart)
Tile metadata	Database	Append-by-source per cell (AZ-484); reads return most-recent across sources	Per-source UPSERT keyed on (latitude, longitude, tile_zoom, tile_size_meters, source) overwrites the existing same-source row and refreshes captured_at
Active downloads	ConcurrentDictionary	Duration of download	Removed on completion

5. Implementation Details

Algorithmic Complexity: Tile grid calculation is O(w×h) where w×h is the number of tiles covering the bounding box.

State Management: _activeDownloads (ConcurrentDictionary) prevents duplicate concurrent downloads. _downloadSemaphore limits parallelism.

Key Dependencies:

Library	Version	Purpose
Newtonsoft.Json	13.0.4	Serialize session creation request body
IHttpClientFactory	built-in	Create HttpClient instances per request

Error Handling:

• Exponential backoff retry for 429 (rate limit) and 5xx errors: 1s → 2s → 4s → 8s → 16s, max 30s, 5 retries
• Immediate throw for 401/403 (auth errors) and cancellation
• RateLimitException thrown after exhausting retries on 429

7. Caveats & Edge Cases

• GoogleMapsDownloaderV2 is registered behind ISatelliteDownloader (resolved by AZ-310 cleanup); the previous concrete-type coupling is gone.
• User-Agent header spoofs Chrome — could be rejected if Google changes detection
• Allowed zoom levels hardcoded to [15,16,17,18,19] — throws for others
• Session token rotation threshold (100 tiles) is an educated guess; Google's actual limit is not documented
• Static _activeDownloads dictionary means deduplication is process-wide, surviving service scope boundaries

8. Dependency Graph

Must be implemented after: Common, DataAccess Can be implemented in parallel with: nothing (needs both foundations) Blocks: RegionProcessing

9. Logging Strategy

Log Level	When	Example
ERROR	Download failure, session token failure	Tile download failed. Tile: (X, Y), Status: {StatusCode}
WARN	Rate limiting retry	Rate limited (429). Waiting {Delay}s before retry
INFO	—	(no INFO-level logs in this component)