You are mixing concerns. Varnish is a caching reverse proxy, and memcached is an in-memory cache for like, unstructured data. If you're pulling from some sort of backend that speaks HTTP, I guess you could consider Varnish something akin to memcached for pull-through, but keep in mind that in both instances your most significant concern will be cache invalidation rather than getting the cache working to begin with. (Famously one of the two "hard problems in CS.")