The BDR provides thumbnails for image and PDF objects. The thumbnail service is set up to check for a thumbnail in storage, then try to generate one from the IIIF image server, and fall back to an icon if needed. Thumbnails are cached by the thumbnail service for 30 days, up to a maximum of 5000 thumbnails. The maximum number of thumbnails was the limiting factor on the thumbnails in the cache – they weren’t being purged from the cache because they were older than 30 days, but because the cache filled up.
We recently had the need to purge some thumbnails, because we updated the images for some objects. We decided to update the thumbnail caching, so that the cache is timestamped. We already use our API to check for permissions on an object before displaying the thumbnail, and we added an API check for when the object in storage was last modified. If the object was modified more recently than the cache timestamp, the cache is stale and we grab an updated thumbnail. This should keep the thumbnail cache from serving stale images.
