In distributed system environments, ensuring high availability of databases poses multiple challenges, including network latency, node failures, and the maintenance of data consistency. Addressing these challenges requires not only advanced technical solutions but also flexible architectural design and refined management strategies. High availability plays a crucial role in maintaining data integrity and consistency, as well as in improving system performance and enhancing fault tolerance. This study provides a comprehensive review of the current challenges and issues associated with high availability in distributed databases. Important concepts, theoretical foundations, and technical approaches are examined, and the current state of research is analyzed across three levels: system and network, data and computing, and application and service. The study aims to deepen the understanding of the difficulties to be addressed and the existing solutions while offering recommendations for future research and technological advancements in the field.