Publication details
Distributed Storage Framework with Offline Support
| Authors | |
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| Year of publication | 2007 |
| MU Faculty or unit | |
| Citation | |
| Description | While the computer networks are gradually becoming more reliable and ubiquitous, mobile clients often receive unfair treatment because of not being online all the time. With network-based distributed storage systems, the offline clients are either not supported, their functionality is severely limited, or they are exposed to the risk of update conflicts occurring during the storage synchronization after offline to online state transition. This thesis introduces distributed storage framework with versioning support, suitable for unreliable environments such as mobile networks or networks with fluctuating quality. Incorporating disconnected mode poses great advantage for clients in such networks, being able to work with distributed storage system regardless of network availability and quality. Our system provides the same opportunities to both connected and disconnected clients. Such approach has several consequences such as practical impossibility to use distributed blocking mutual exclusion. The system also needs to be designed in a way that offline clients must not prevent (block) other clients to use the system. Previous approaches to offline support explicitly distinguish between online and offline state. Distributed systems without distributed mutual exclusion must deal with conflicts resulting from parallel data writes and updates. Our proposed system avoids these conflicts by implementing file versioning. Since we aim to build a fault tolerant system, we have also incorporated replication and we propose a simple non-blocking replication algorithm. Besides immutable versioned files, our distributed system supports mutable files, too. The mutable files may cause update conflicts but these conflicts are automatically detected and resolved. The mutable files are implemented using a redo log---an approach commonly used for file versioning, but in our case it is used to detect update conflicts. The update conflicts are resolved using relaxed last-writer-wins rule. A specialized variant of mutable files are the so called streamed files, which are continuously appended and their beginning expires in time. A part of our proposed distributed system is a security model based on Grid authentication and authorization services. Our proposed architecture guarantees that user is authenticated to all services included in the network storage stack; the authorization granularity is also at the service level and all authorizations can be revoked at any moment by service providers. |
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