  Similarity caching systems have recently attracted the attention of the
scientific community, as they can be profitably used in many application
contexts, like multimedia retrieval, advertising, object recognition,
recommender systems and online content-match applications. In such systems, a
user request for an object $o$, which is not in the cache, can be (partially)
satisfied by a similar stored object $o$', at the cost of a loss of user
utility. In this paper we make a first step into the novel area of similarity
caching networks, where requests can be forwarded along a path of caches to get
the best efficiency-accuracy tradeoff. The offline problem of content placement
can be easily shown to be NP-hard, while different polynomial algorithms can be
devised to approach the optimal solution in discrete cases. As the content
space grows large, we propose a continuous problem formulation whose solution
exhibits a simple structure in a class of tree topologies. We verify our
findings using synthetic and realistic request traces.
