In order to support the use case of cross-catalogue search of datasets there needs to be a mechanism on how to transfer DCAT data form one catalog to an aggregator. Given that there may be many catalogs that are interested in aggregating DCAT data it would be useful with a mechanism that support harvesting of data.
The aggregator needs information about new datasets, updated datasets and deleted datasets (the CUD in CRUD) to make sure the aggregator doesn't provide incorrect information.
A method to solve this in practice is to use the Atom Syndication Format (RFC 4287) to carry links to DCAT metadata. Atom is comparable to RSS, has widespread use and has multiple implementations and various platforms. This makes it easy to implement and debug. However, Atom only supports information about Created and Updated objects and needs to be enhanced with a method to desribe deleted items. A proposal for "tombstones" enhancement of atom is available here.
A catalog provides a feed for aggregators to harvest. The aggregator harvests those periodically and builds its own index for searching etc.