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In Analogie zu den Elektrizitätsnetzen electricity grid), von denen die technische Revolution ausging, wurde der Begriff Computational Grid (kurz Grid) geprägt. Ein wichtiger Bestandteil des Systems liegt im benutzerfreundlichen Zugang und der koordinierten Nutzung der weltweit verteilten Speicherressourcen und Rechnerkapazitäten. Bei der Entwicklung dazu notwendiger Technologien und Software (Middleware) profitiert man von Kenntnissen und Erfahrungen bei der Entwicklung verteilter Algorithmen, dem Software-Engineering und dem Supercomputing.
Key/value stores which are built on structured overlay networks often lack support for atomic transactions and strong data consistency among replicas. This is unfortunate, because consistency guarantees and transactions would allow a wide range of additional application domains to benefit from the inherent scalability and fault-tolerance of DHTs. The Scalaris key/value store supports strong data consistency and atomic transactions. It uses an enhanced Paxos Commit protocol with only four communication steps rather than six. This improvement was possible by exploiting information from the replica distribution in the DHT. Scalaris enables implementation of more reliable and scalable infrastructure for collaborative Web services that require strong consistency and atomic changes across multiple items.
Transactional DHT Algorithms
(2009)
We present a framework for transactional data access on data stored in a DHT. It allows to atomically read and write items and to run distributed transactions consisting of a sequence of read and write operations on the items. Items are symmetrically replicated in order to achieve durability of data stored in the SON. To provide availability of items despite the unavailability of some replicas, operations on items are quorum-based. They make progress as long as a majority of replicas can be accessed. Our framework processes transactions optimistically with an atomic commit protocol that is based on Paxos atomic commit. We present algorithms for the whole framework with an event based notation. Additionally we discuss the problem of lookup inconsistencies and its implications on the one-copy serializability property of the transaction processing in our framework.