This study seeks to understand how network failures affect the availability of service delivery across wide area networks and to evaluate classes of techniques for improving end-to-end service availability. Using several large-scale connectivity traces, we develop a model of network unavailability that includes key parameters such as failure location and failure duration. We then use trace-based simulation to evaluate several classes of techniques for coping with network unavailability. We find that caching alone is seldom effective at insulating services from failures but that the combination of mobile extension code and prefetching can improve average unavailability by as much as an order of magnitude for classes of service whose semantics support disconnected operation. We find that routing-based techniques may provide significant improvements, but that the improvements of many individual techniques are limited because they do not address all significant categories of network failures. By combining the techniques we examine, some systems may be able to reduce average unavailability by as much as one or two orders of magnitude.

Commercial distributed database systems generally support an optional protocol that provides loose consistency of replicas, allowing replicas to be inconsistent for some time. In such a protocol, each replicated data item is assigned a primary copy site. Typically, a transaction updates only the primary copies of data items, with updates to other copies deferred until after the transaction commits. After a transaction commits, its updates to primary copies are sent transactionally to the other sites containing secondary copies. We investigate the transaction model underlying the above protocol. We show that global serializability in such a system is a property of the placement of primary and secondary copies of replicated data items. We present a polynomial time algorithm to assign primary sites to data items so that the resulting topology ensures serializability. 1 Introduction A widely used method for improving the reliability and availability of data in distributed databases is ...

OceanStore is a utility infrastructure designedto span the globe and provide continuous access to persistent information. Since this infrastructure is comprised of untrusted servers, data is protected through redundancy and cryptographic techniques. To improve performance, data is allowedtobe cached anywhere, anytime. Finally, monitoring of usage patterns allows adaptation to regional outages and denial of service attacks; monitoring also enhances performancethrough pro-active movement of data. A prototype implementation is currently under development.

Abstract not found

In distributed computing environments, replication of data provides improved availability, isolation between workloads with different characteristics, and improved performance through local access to data. The "real data" is server resident and by "local data" we refer to cached client data. We examine which data should be cached on behalf of a cached query. The minimum requirement for cached data for a query Q is that it enables answering Q locally. We consider the following: � Definitions of what data is cached for a cached query. � Deciding whether cached data can be used to solve a "new" query. � Deciding whether a "new" query to be cached is already effectively cached due to caching of other queries. � A simple class of caching rules.