This directory contains the logic for serving, processing, and validating static assets used in the GitHub Docs application. While the actual asset files (images, CSVs, etc.) reside in the root assets/ directory, src/assets houses the code that manages how these assets are delivered to the user.

The primary responsibilities of this module are:

• Dynamic Image Processing: Converting PNGs to WebP on-the-fly and resizing images based on URL parameters.
• Caching Strategy: Setting appropriate cache headers and surrogate keys for assets, especially those with checksums in their URLs.
• Validation & Maintenance: Scripts to ensure assets are used correctly, identifying orphaned files, and validating image dimensions.

The core logic resides in src/assets/middleware:

• dynamic-assets.ts: Intercepts requests for /assets/. It handles:
  • WebP Conversion: If a request ends in .webp but the source is a .png, it converts the image using sharp.
  • Resizing: Supports virtual path segments like /mw-1000/ to resize images to a maximum width (e.g., 1000px).
  • Security: Validates requested widths against an allowed list (VALID_MAX_WIDTHS) to prevent DoS attacks.
• static-asset-caching.ts: Detects if an asset URL contains a checksum (e.g., /assets/cb-12345/...) and sets aggressive caching headers (Surrogate-Key: manual-purge) because the content is immutable.

Located in src/assets/scripts, these tools help maintain the asset library:

• find-orphaned-assets.ts: Identifies assets present in the disk but not referenced in the content or code.
• validate-asset-images.ts: Checks for issues like invalid file types or corruption.
• list-image-sizes.ts: Utility for analyzing image dimensions.

• src/assets/lib/image-density.ts: Utilities for handling high-density (Retina) images.

Place static files (images, PDFs, etc.) in the root assets/ directory.

• Images: assets/images
• Data: assets/ (e.g., CSV files)

The application (via src/frame) often rewrites asset URLs to include a checksum for cache busting.

• Source: /assets/images/foo.png
• Served: /assets/cb-123456/images/foo.png (The cb-xxxxx part is ignored by the file system lookup but used for caching).

To request a WebP version of a PNG: GET /assets/images/foo.webp (Server looks for foo.png and converts it).

To request a resized version: GET /assets/mw-1000/images/foo.webp (Resizes to max-width 1000px).

• sharp: Used for high-performance image processing (resizing, format conversion).
• assets/ directory: The source of truth for static files.

• Team: @github/docs-engineering
• Escalation: If image serving fails or performance degrades, check the dynamic-assets middleware and sharp processing.

• On-the-fly Processing: We currently process images on request (cached by CDN). This avoids a massive build-time step but requires CPU resources on the server for uncached requests.
• WebP: We prefer WebP for performance but maintain PNGs as the source of truth.