Discovery systems aim to provide a more modern and user-friendly search experience compared to traditional ILS-based OPACs. They index content from a library's ILS as well as other sources to allow patrons to search across local and remote resources from a single interface. Early discovery systems depended on regular synchronization with the ILS, but the goal became developing web-scale discovery that could harvest and index content in real-time similarly to search engines like Google. Features such as faceted navigation, relevance ranking, and recommendations are now expected in discovery interfaces to meet user expectations.