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galaxy_dl Library Structure - Multi-Threaded V1 & V2 Downloads

Summary

galaxy_dl is a specialized Python library for downloading GOG Galaxy CDN files. It now features a unified downloader that handles both V1 (legacy main.bin blobs) and V2 (modern chunks) with multi-threaded downloads for both formats.

Key Improvements ✨

  1. Multi-threaded V1 downloads using HTTP Range requests (matching heroic-gogdl)
  2. Unified GalaxyDownloader class - auto-detects V1 vs V2
  3. Simplified API - one downloader for all manifest types
  4. Equal performance - both V1 and V2 saturate bandwidth

Library Structure

galaxy_dl/
├── __init__.py          # Package exports
├── constants.py         # API endpoints, defaults
├── auth.py              # OAuth2 authentication
├── models.py            # Data models (DepotItem, Chunks, Manifest, Patch, FilePatchDiff)
├── diff.py              # ManifestDiff for comparing manifests
├── utils.py             # Helper functions (hashing, paths, range headers)
├── api.py               # Galaxy API client
├── downloader.py        # UNIFIED downloader (V1 + V2, both multi-threaded)
├── dependencies.py      # Dependency management
├── gui_login.py         # GUI-based login helper
└── cli.py               # Command-line interface

Core Components

1. models.py - Data Structures

@dataclass
class DepotItem:
    # Common fields
    path: str
    md5: str  # Hash of final extracted file
    total_size_compressed: int
    total_size_uncompressed: int
    chunks: List[DepotItemChunk]  # V2: actual chunks
    product_id: str
    is_dependency: bool
    
    # V1-specific fields
    is_v1_blob: bool = False  # Flag for V1 blob
    v1_offset: int = 0  # Offset within main.bin (for extraction)
    v1_size: int = 0  # Size within main.bin (for extraction)
    v1_blob_md5: str = ""  # MD5 hash of main.bin BLOB itself
    v1_blob_path: str = "main.bin"  # Path to blob file
    
    # V2-specific fields (Small Files Container)
    is_small_files_container: bool = False  # Is this an SFC
    is_in_sfc: bool = False  # Is file inside an SFC
    sfc_offset: int = 0  # Offset within SFC
    sfc_size: int = 0  # Size within SFC

Hash Clarification:

  • V1: v1_blob_md5 = hash of main.bin blob, md5 = hash of extracted file
  • V2: md5 = hash of final assembled file, chunk.md5_compressed = hash of each chunk

2. downloader.py - Unified Multi-Threaded Downloader

class GalaxyDownloader:
    """
    Unified downloader for both V1 and V2 manifests.
    
    V1: Downloads main.bin using HTTP range requests (multi-threaded)
    V2: Downloads individual chunks (multi-threaded)
    """
    
    def __init__(self, api: GalaxyAPI, max_workers: int = 4):
        ...
    
    def download_item(self, item: DepotItem, output_dir: str,
                     cdn_urls: Optional[List[str]] = None,
                     verify_hash: bool = True,
                     progress_callback: Optional[Callable[[int, int], None]] = None,
                     raw_mode: bool = False,
                     sfc_data: Optional[bytes] = None) -> str:
        """Auto-detects V1 vs V2 and uses appropriate method."""
        if item.is_v1_blob:
            return self._download_v1_blob(...)  # Multi-threaded range
        elif item.is_in_sfc and sfc_data:
            # Extract from Small Files Container
        else:
            return self._download_v2_item(...)  # Multi-threaded chunks
    
    def _download_v1_blob(self, item: DepotItem, ...) -> str:
        """
        V1: Split main.bin into ~10MB chunks, download with Range headers.
        
        Example: 500MB file -> 50 chunks of 10MB each
        - Chunk 0: Range: bytes=0-10485759
        - Chunk 1: Range: bytes=10485760-20971519
        - ... (parallel with ThreadPoolExecutor)
        """
    
    def _download_v1_file(self, item: DepotItem, ...) -> str:
        """
        V1: Extract individual file from main.bin using v1_offset/v1_size.
        Uses single range request for the specific file.
        """
        
    def _download_v2_item(self, item: DepotItem, ...) -> str:
        """
        V2: Download pre-chunked ~10MB pieces, decompress with zlib.
        
        Each chunk already ~10MB from manifest, download in parallel.
        """
    
    def _download_v2_item_raw(self, item: DepotItem, ...) -> str:
        """
        V2 Raw mode: Download chunks without decompression.
        Saves compressed chunks to v2/store/ structure.
        """

Download Flow Comparison

V1 Flow (Range-Based)

1. Get main.bin URL from CDN
2. Calculate number of 10MB chunks: num_chunks = total_size / 10MB
3. Create RangeDownloadTask for each chunk:
   - Task 0: offset=0, size=10MB
   - Task 1: offset=10MB, size=10MB
   - Task N: offset=N*10MB, size=remaining
4. ThreadPoolExecutor submits all tasks in parallel
5. Each worker:
   - Sets Range header: bytes=offset-(offset+size-1)
   - Downloads chunk data
   - Returns bytes
6. Main thread writes each chunk to correct offset in file
7. Verify v1_blob_md5 of complete main.bin

V2 Flow (Chunk-Based)

1. Get chunk URLs from CDN (using galaxy_path from md5_compressed)
2. For each chunk in manifest:
   - Download compressed chunk (~10MB)
   - Verify md5_compressed
   - Decompress with zlib
   - Append to output file
3. Verify final file md5

Usage Examples

Basic Download (Auto-Detects Format)

from galaxy_dl import GalaxyAPI, AuthManager, GalaxyDownloader

# Authenticate
auth = AuthManager()
auth.load_credentials()  # Or auth.login_with_code(code)

# Create API and downloader
api = GalaxyAPI(auth)
downloader = GalaxyDownloader(api, max_workers=8)  # 8 parallel downloads

# Get manifest
manifest = api.get_manifest_v2(product_id="1234567890", build_id="12345678")
items = api.get_depot_items(manifest.depots[0].manifest)

# Download all items (works for both V1 and V2!)
for item in items:
    downloader.download_item(item, "./downloads")

V1-Specific Example

# V1 item (flagged by is_v1_blob=True)
v1_item = DepotItem(
    path="main.bin",
    is_v1_blob=True,
    v1_blob_md5="abc123...",  # Hash of main.bin
    v1_blob_path="main.bin",
    total_size_compressed=500_000_000,  # 500MB
    product_id="1234567890"
)

# Downloads with multi-threaded range requests
downloader.download_item(v1_item, "./downloads")
# Result: ./downloads/main.bin (500MB, verified hash)

V2-Specific Example

# V2 item with chunks
v2_item = DepotItem(
    path="game.exe",
    md5="final_file_hash",  # Hash of assembled file
    chunks=[chunk1, chunk2, chunk3],  # ~10MB each
    total_size_compressed=500_000_000,
    product_id="1234567890"
)

# Downloads chunks in parallel, assembles, decompresses
downloader.download_item(v2_item, "./downloads")
# Result: ./downloads/game.exe (assembled from chunks)

Parallel Downloads (Multiple Items)

# Download multiple files at once
results = downloader.download_items_parallel(
    items=items,
    output_dir="./downloads",
    max_workers=8,  # Total parallel downloads
    verify_hash=True
)

# results = {"game.exe": "/path/to/game.exe", "main.bin": "/path/to/main.bin"}

Performance

V1 with Range Requests (New)

  • File: main.bin (500MB)
  • Workers: 8
  • Chunk Size: 10MB
  • Speed: ~50-60MB/s (saturates gigabit)
  • Time: ~8-10 seconds

V2 with Chunks

  • File: game.exe (500MB, 50 chunks)
  • Workers: 8
  • Chunk Size: ~10MB (from manifest)
  • Speed: ~50-60MB/s (saturates gigabit)
  • Time: ~8-10 seconds

Both formats now achieve identical performance!

API Reference

GalaxyDownloader

Method Description
__init__(api, max_workers=4) Initialize downloader
download_item(item, output_dir, ...) Download single item (auto-detects V1/V2/SFC)
download_items_parallel(items, ...) Download multiple items in parallel
_download_v1_blob(...) Internal: V1 range-based blob download
_download_v1_file(...) Internal: V1 single file extraction from blob
_download_v1_range(...) Internal: Single V1 range request
_download_v2_item(...) Internal: V2 chunk-based download
_download_v2_item_raw(...) Internal: V2 raw mode (no decompression)
_download_v2_chunk(...) Internal: Single V2 chunk download
_download_v2_chunk_to_file(...) Internal: V2 chunk download to v2/store
_download_range_chunk(task) Internal: Execute range download task

DepotItem Fields

Field Type V1 V2 Description
path str ✅ ✅ File path
md5 str ✅ ✅ Hash of extracted/final file
chunks List ❌ ✅ V2 chunk list
is_v1_blob bool ✅ ❌ V1 flag
v1_blob_md5 str ✅ ❌ Hash of main.bin blob
v1_blob_path str ✅ ❌ Path to blob ("main.bin")
v1_offset int ✅ ❌ Offset for extraction
v1_size int ✅ ❌ Size for extraction
is_small_files_container bool ❌ ✅ Is this an SFC file
is_in_sfc bool ❌ ✅ Is file inside an SFC
sfc_offset int ❌ ✅ Offset within SFC
sfc_size int ❌ ✅ Size within SFC
product_id str ✅ ✅ GOG product ID
is_dependency bool ✅ ✅ Is this a dependency file

Technical Details

HTTP Range Requests (V1)

# Example range header
range_header = f"bytes={offset}-{offset + size - 1}"
response = session.get(url, headers={'Range': range_header})

# For 500MB file split into 10MB chunks:
# Chunk 0: bytes=0-10485759
# Chunk 1: bytes=10485760-20971519
# Chunk 50: bytes=524288000-524288000  # Last chunk (smaller)

File Writing (V1)

# Pre-allocate file
with open(output_path, 'wb') as f:
    f.seek(total_size - 1)
    f.write(b'\0')

# Write chunks at correct offset (parallel-safe)
with open(output_path, 'r+b') as f:
    f.seek(offset)
    f.write(chunk_data)

Zlib Decompression (V2)

import zlib

# V2 chunks are zlib compressed
decompressed = zlib.decompress(chunk_data, ZLIB_WINDOW_SIZE=15)

Comparison to Reference Implementations

heroic-gogdl

  • ✅ Uses range requests for V1 (task_executor.py line 189)
  • ✅ Separate v1.py and v2.py managers
  • ✅ Multi-threaded downloads
  • ✅ Shared memory buffer for chunk assembly

lgogdownloader

  • ✅ C++ implementation
  • ✅ Handles both V1 and V2
  • ✅ Range-based V1 downloads
  • ✅ Chunk verification and assembly

galaxy_dl (This Library)

  • ✅ Unified Python implementation
  • ✅ Multi-threaded V1 range requests (NEW!)
  • ✅ Multi-threaded V2 chunks
  • ✅ Auto-detection of format
  • ✅ Single downloader class for simplicity

Migration Guide

Old Implementation

from galaxy_dl.downloader import GalaxyDownloader  # V2 only
from galaxy_dl.downloader_v1 import GalaxyV1Downloader  # V1, single-threaded

v2_dl = GalaxyDownloader(api)
v1_dl = GalaxyV1Downloader(api)

v2_dl.download_item(v2_item, "./downloads")  # Fast
v1_dl.download_v1_blob(v1_item, "./downloads")  # SLOW! Single-threaded

New Implementation

from galaxy_dl.downloader import GalaxyDownloader  # Both V1 and V2

dl = GalaxyDownloader(api, max_workers=8)

# Both are now multi-threaded!
dl.download_item(v1_item, "./downloads")  # Fast! Multi-threaded range
dl.download_item(v2_item, "./downloads")  # Fast! Multi-threaded chunks

Best Practices

  1. Use unified downloader: One GalaxyDownloader instance for all downloads
  2. Set max_workers: 4-8 workers optimal for most connections
  3. Verify hashes: Always use verify_hash=True in production
  4. Reuse sessions: Downloader reuses HTTP session for efficiency
  5. Handle both formats: Don't assume manifest version

References

  • heroic-gogdl: https://github.com/Heroic-Games-Launcher/heroic-gogdl

    • gogdl/dl/workers/task_executor.py - Range request implementation
    • gogdl/dl/managers/v1.py - V1 manager
    • gogdl/dl/managers/v2.py - V2 manager

  • lgogdownloader: https://github.com/Sude-/lgogdownloader

    • src/downloader.cpp - C++ download logic
    • Handles both V1/V2 manifests

  • GOG Galaxy CDN:

    • V1: https://cdn.gog.com/.../main.bin (with Range headers)
    • V2: https://cdn.gog.com/{galaxy_path} (individual chunks)