Abstract: Cloud storage services have become very popular due to their well security, availability and cost effective. To provide presence always-on access, a cloud service provider (CSP) maintains several replicas for each piece of data on geographically distributed servers. A key problem of using the replication Technique in clouds is that it is very expensive and crude to achieve strong consistency on everywhere. So first present a causal consistency as a service (CaaS) model, which consists of a large data cloud and several small audit clouds. In the CaaS model, a data cloud is maintained by a CSP, that constitute an audit cloud can verify whether the data cloud provides the promised level of consistency or not. So propose a two-level auditing, which only requires a loosely synchronized clock in the audit cloud. Then design different algorithms to quantify the seriousness of violations with two metrics: the commonality of violations, and the staleness of the value of a read. Finally, a heuristic auditing strategy (HAS) method is used to reveal as many violations as possible.

Keywords: Two level auditing, CaaS model, commonality, staleness, heuristic auditing strategy (HAS), cloud storage.