What a CDN in front forgets when a file is deleted: the file's URLs, its variants' (Storehouse.Variants), and its cache tag. Keys are never reused, so a file kept for a year never changes; it only goes, and a CDN would go on serving it. A purger tells the CDN.

config :storehouse, Storehouse.Purge,
  purger:
    {Storehouse.Purge.Cloudflare,
     zone_id: System.fetch_env!("CLOUDFLARE_ZONE_ID"),
     api_token: System.fetch_env!("CLOUDFLARE_API_TOKEN")},
  base_url: "https://example.com"

After Storehouse.delete/2 of a file (a file replaced is the old one deleted), the purger is given its URLs, its variants' URLs and its cache tag, once. After Storehouse.delete_prefix/2 (an account's files), the prefix's URLs, for a CDN that purges by prefix. It runs as a job of the app's runner (Storehouse.Runner), tried again when the CDN fails, and its job holds the URLs, never the purger's credentials, which are read from the config when it runs.

Configuration

• :purger - a module of this behaviour, or {module, options}. Without it, nothing is purged.
• :base_url - put before a URL that is a path (Storehouse.Local's, Storehouse.Variants.Plug's): the CDN's host.
• :urls - a function of a key that gives more URLs of it: the app's own routes to it, through Storehouse.Proxy or Storehouse.Redirect.

Cache tags

Storehouse.Proxy, Storehouse.Variants.Plug and Storehouse.Local.Plug send a file's Cache-Tag (cache_tag/1) with cache_tag: true, its variants' with the original's: a purge by tag forgets them all, however many URLs they were served at.