多级多版本数据库管理系统全局串行化

摘要

The concurrency control mechanism in the multi-level DBMS is required to promise the serializability of transactions and the multi-level security properties, avoid possible covert channels and the starving problem of high-level transactions. The multi-level multi-version timestamp ordering mechanisms satisfy these requirements but may cause transactions read old version data, and the scheduler is required to be a trusted process. This paper presents a multi-level multi-version global timestamp ordering (MLS_MVGTO) mechanism and the basic global timestamps generation steps based on the transaction's snapshot. This paper also presents two improvements according to the pre-knowledge of the read-only transactions. In addition it can be implemented as a set of untrusted schedulers. Given the pre-knowledge about transactions' operations, transactions are able to read more recent version.%多级调度应该保证事务历史可串行化,满足多级安全特性,不会引入隐通道,并保证高级别事务不会因为无限等待而"饿死".与其他多级数据管理系统调度机制相比,多级多版本时戳调度机制满足上述要求,但该机制存在两个问题,一是事务可能读旧版本,二是要求调度器是可信进程.提出一种多级多版本全局时戳调度机制(MLS_MVGTO),以及依据事务快照生成其全局时戳的基本步骤.给出了预知只读事务信息时的两种改进方法.MLS_MVGTO机制生成的事务历史可串行化,不引入隐通道等,并且该方法避免引入一个全局可信的调度器,并通过对只读事务的深入分析,允许事务读新版本.