The rapid growth of data and workload makes centralized database systems less and less favorable to today's applications. A distributed database system can scale out dynamically to satisfy the business development. As a result, it has gained much more attention from applications. Since the needs for distributed DB became apparent, an increasing number of products have emerged and been adopted by the Web. However, due to the complexity of distributed DB systems, their designers have to trade off among several desired properties, resulting in dramatic difference in their designs and advantages. To the best of public knowledge, no one has performed a comprehensive analysis on the design space and the tradeoff choices of modern distributed DB systems. After reviewing and understanding a significant number of real world DB products, this study believe that a distributed DB system can be generally described using three dimensions-operational consistency, transactional consistency and availability. While these dimensions are not new, their concepts are somehow blurred in the literature. This paper clarifies the three concepts in the context of database, based on which can draw a sensible landscape of the existing products and technologies. The paper also provides an analysis of the relationship among the three dimensions, intending to help developers make right choice when designing new distributed DB systems.