Publish/subscribe model does not scale?

on Wiki someone posted "...For relatively small installations, pub/sub provides the opportunity for better scalability than traditional client-server, through parallel operation, message caching, tree-based or network-based routing, etc. However, as systems scale up to become datacenters with thousands of servers sharing the pub/sub infrastructure, this benefit is often lost; in fact, scalability for pub/sub products under high load in large deployments is very much a research challenge." Does anyone have something to say regarding scaling Publish/subscribe models?

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counting # of views, calculating most/least viewed

I'm seeking for a design pattern or advice or directions. I need to count views/downloads of a set of resources, let them to be identified by their respective URLs. This is not a big problem. I also need to keep a list of viewed/downloaded resources in the last X days. This list needs to be updated every now and then to reflect real last X days of usage. So resources that were requested prior to X days get evicted from it. So it's sort of a black box, you feed messages (download request) in and it gives you that list of URLs with counters on the other end. How would you go about designing it?

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