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blender-cloud-addon/README.md
Sybren A. Stüvel 8fbdf456cd Change of plans: caching is done with CacheControl. 
Requests-Cache ignores HTTP cache control headers, violating the HTTP
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Blender Cloud addon

Installation requires the Pillar SDK to be installed. It can either be installed regularly somewhere on the Python PATH, or be stored as a wheel file at blender_cloud/pillar_sdk-*.whl.

Communication & File Structure

Assumptions

  • Cache is user-global, stored in an OS-specific location, and can be removed/recreated. This document refers to that location as $CACHE, and is typically a directory like $HOME/.cache/blender/blender_cloud. Also see T47684. This directory should not be tied to the version of Blender -- malformed/invalid caches should just be ignored or removed.
  • At the moment, versioning of files is limited to re-syncing to get the latest versions. More extensive versioning falls under the umbrella of "asset management" and is out of scope of this addon.
  • Users can download texture nodes. Such a download may result in multiple files on the local filesystem (think of one texture node containing diffuse, bump and specular maps).

Caching

Caching is performed at different levels:

  • Caching of HTTP GET requests is performed by CacheControl.

    • Cache is stored in $CACHE/blender_cloud_http/; by using the file backend, we ensure cache persistence across Blender runs. This does require the lockfile package to be installed or bundled.
    • The code should be made cache-aware and use the Requests-Cache CachedSession sessions. This allows for more granular control over where in the code cache is (not) used.

  • Downloaded thumbnails are cached (by us) in $CACHE/thumbnails/node_uuid/filename.

    • Use a non-cached HTTP GET to download these (using session.cache_disabled()).
    • HTTP headers are stored as JSON key/value pairs in $CACHE/thumbnails/node_uuid/filename.headers.
    • Use the ETag and If-Modified-Since headers to prevent unnecessary re-downloading.
    • Check Content-Length header against actual file size to detect partially-downloaded files that need re-downloading.

  • Downloaded files (such as textures) are handled in the same way as thumbnails (described above), but have their metadata stored somewhere else (described below).

Filesystem Layout

  • Top-level texture directory, referred to as $TEXTURES in this document, can be configured per Blender scene. It defaults to a global addon preference, which in turns defaults to //textures.

  • Texture files are stored in $TEXTURES/{project_name}/{node_name}/{node_name}/.../{file_variant}-{file_name}. file_variant is "col" for colour/diffuse textures, "nor" for normal/bump textures, etc.

  • Metadata is stored in $TEXTURES/.blender_cloud, with the following subdirectories:

    • projects/{project_uuid}.json containing the project document from Pillar
    • nodes/{node_uuid}.json containing the node document from Pillar
    • files/{file_uuid}.json containing the file document from Pillar
    • index.sqlite containing a mapping from {project_name}/{node_name}/{node_name}/... to the UUID of the last-named path component (project, node, or file). This allows us to map the path of a filesystem path to its Pillar document.