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Cloud Storage Services Have Become Commercially Popular Due To Their Overwhelming Advantages. To Provide Ubiquitous Always-on Access, A Cloud Service Provider (CSP) Maintains Multiple 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 To Achieve Strong Consistency On A Worldwide Scale. In This Paper, We First Present A Novel Consistency As A Service (CaaS) Model, Which Consists Of A Large Data Cloud And Multiple Small Audit Clouds. In The CaaS Model, A Data Cloud Is Maintained By A CSP, And A Group Of Users That Constitute An Audit Cloud Can Verify Whether The Data Cloud Provides The Promised Level Of Consistency Or Not. We Propose A Two-level Auditing Architecture, Which Only Requires A Loosely Synchronized Clock In The Audit Cloud. Then, We Design Algorithms To Quantify The Severity Of Violations With Two Metrics: The Commonality Of Violations, And The Staleness Of The Value Of A Read. Finally, We Devise A Heuristic Auditing Strategy (HAS) To Reveal As Many Violations As Possible. Extensive Experiments Were Performed Using A Combination Of Simulations And Realcloud Deployments To Validate HAS.