  Data distribution for opportunistic users is challenging as they neither own
the computing resources they are using or any nearby storage. Users are
motivated to use opportunistic computing to expand their data processing
capacity, but they require storage and fast networking to distribute data to
that processing. Since it requires significant management overhead, it is rare
for resource providers to allow opportunistic access to storage. Additionally,
in order to use opportunistic storage at several distributed sites, users
assume the responsibility to maintain their data. In this paper we present
StashCache, a distributed caching federation that enables opportunistic users
to utilize nearby opportunistic storage. StashCache is comprised of four
components: data origins, redirectors, caches, and clients. StashCache has been
deployed in the Open Science Grid for several years and has been used by many
projects. Caches are deployed in geographically distributed locations across
the U.S. and Europe. We will present the architecture of StashCache, as well as
utilization information of the infrastructure. We will also present performance
analysis comparing distributed HTTP Proxies vs StashCache.
