Ruby asset-pipeline library that became official in Rails 3.1 (2011-ish). Concatenates files for compression, minifies them, and fingerprints URLs with content hashes so cache invalidation happens automatically when a file changes. Adopted as a gem in 2009 before being absorbed by Rails. Largely superseded by Prop Shaft in modern Rails setups since HTTP/2 removes most of the need for concatenation and native ES modules remove the need for minification-heavy toolchains.