Pattern where processing (e.g. image resizing) happens when a derivative URL is requested for the first time, then the result is typically cached in a CDN. Active Storage and some other gems encode processing steps directly into the URL; Shrine instead uses a custom alias and arguments that map to a named processing block, avoiding URL bloat and supporting processing that cannot be serialized (e.g. compositing with blocks). Vulnerable to DDoS unless URLs are signed with a server-only secret so attackers cannot generate arbitrary variants.