Mistify: Augmenting cloud storage with delay-tolerant cooperative backup

A variety of personal backup services now allow users to synchronise their files across multiple devices such as laptops and smartphones. These applications typically operate by synchronising each device with a centralised storage service across the Internet. However, access to the Internet may occasionally not be available, leaving any unsynchronised content in a vulnerable state. To address this, applications could alternatively make use of storage capacity provided by other devices within close proximity, using ad-hoc or local network connectivity. Such devices can provide a secondary storage tier in case of Internet connectivity issues, and could also be used to forward files to central storage at a later time. In our proposed design, we delegate the task of propagating information across locally networked devices to a lower layer, by making use of a content-centric opportunistic network platform (Haggle). This allows our application, Mistify, to treat the neighbourhood of peers as a single distributed content repository (or “mist”), in a manner similar to the way in which existing applications interface with the cloud. Mistify employs a differentiated replication strategy, with the aim of improving the safety of items in the mist. In our evaluation of the prototype, we have found that in a simulated network of locally-connected peers, the prototype was able to achieve a high level of availability for stored content, without resorting to flooding. Furthermore, Mistify was able to deliver a high proportion of content to the cloud, even when only a small proportion of nodes were given Internet connectivity.