Clients fetch content from stratum 1 servers as needed. Multiple layers of cache are possible/recommended, including 1) a local SQUID proxy (in the case of large infrastructure) 2) the client's local disk 3) a networked/parallel filesystem

Cache invalidation happens through checksums and catalogs (each revision has one or multiple nested catalogs, content is checksummed, and clients can know if one or multiple catalogs are outdated by comparing checksums they have to upstream checksums).