We consider the problem of ensuring high data availability in federated content sharing systems. Ideally, such a system would provide high data availability in a device transparent manner so that users are not faced with the time-consuming and error-prone task of managing data replicas across the constituent devices of the system. We propose a novel unified availability model and a decentralized replication algorithm to approximate this ideal. Our availability model addresses three different concerns: availability during connected operation (online), availability during disconnected operation (offline), and availability after permanent disconnection from the federated system (ownership). Our replication algorithm centers around the intuition that devices should selfishly use their local storage to ensure offline and ownership availability for their individual owners. Excess storage, however, is used communally to ensure high online availability for all shared content. Evaluation of an implementation shows that our algorithm rapidly reaches stable and communally desirable configurations when there is sufficient space. Consistent with the fact that devices in a federated system are owned by different users, however, as space becomes highly constrained, the system approaches a non-cooperative configuration where devices only hoard content to serve their individual owners' needs.
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