File replication and consistency maintenance mechanism in a trusted distributed environment

Abstract

An effort has been made to propose an adaptive, trust based, on demand and reliable file replication approach for distributed environment. The work proposes an active replication mechanism for file replication; file access and performance transparency to system, thereby ensuring replication decisions such as when to replicate, where to replicate, load status of the node selected for replication and avoidance of redundant replica on that node. The proposed model introduces File Replication Server (FRS) that replicates the file, when total number of request for it reaches the threshold value, adhering to replication decision. If all replicas of file are not synchronized with each other, this drives system into inconsistent state. To keep files consistent, changes made at one replica of file should be reflected on other replicas in minimum possible time. Unlike conventional approaches viz., Write Update and Write Invalidate, in which either all or any fixed master replica is updated respectively, the proposed approach transfers the role of master replica to the last modified replica. File replica is updated on-demand by only propagating the required partial updates. The master replica immediately computes the file content modifications but propagates them only on-demand. Once a node gets itself registered, it becomes the part of elite FRS community. This paper introduces basic trust parameters and adaptive factors in computing trustworthiness of peer FRS, namely, frequency of the request for a particular file that a FRS perform, registration type of node i.e., paid or unpaid, write operation not permitted if trust value of a FRS is less than threshold, feedback that a FRS gives about other FRS and authenticity of the session key. Simulation results show that during high file request scenario for a particular file, frequently accessed files are replicated on other FRSs dynamically and file request is redirected in transparent manner, thus reducing request completion time by about 28.78–47.24 % as compared to FTP. Experimental results shows that various factors viz., file size, size of modifications and number of replicas to be updated affects the time to propagate the changes to other replicas. Percentage reduction in time for propagating these changes varies from 31.56 to 78.1 %. Similarly, the reduction in time for updating replicas simultaneously varies from 78.17 to 85.37 %. Though the percentage increase in time required for acquiring the keys to access the services with trust based security model varies from 11.94 to 17.49 %, the same is compensated by reduction in time for updating replicas. In order to ascertain stability of proposed model, Calculus of Communicating System (CCS) for proposed model is compiled on CWB-NC tool and bisimulation equivalence is proved for proposed file replication model.