即時性物件導向資料庫的協同控制

Abstract

即時資料庫系統中的每個異動都有明確的時間限制,亦即一個異動必須在 將來的某一時間點前完成。因此,在一即時資料庫系統中異動執行的正確 性,不只須遵守資料的一致性限制並產生正確的結果,同時也要使異動能在 時限內完成。為了使異動得以在時限內完成,所以便須對其執行的順序加 以規劃;目前大多數即時資料庫的排程方法都是利用現有的協同控制協定, 尤其是兩段式鎖定法,並配合以個別異動的死限為主要考慮因素的排程方 法以提升效能。在一物件導向資料庫系統中,資料庫為一組抽象資料型態 集合,系統中的每一物件均透過定義在此型態內的操作來存取。由於物件 導向資料模型的一些特性,如類別、繼承等觀念在關聯式資料庫中並不存 在。因此,傳統的排程法並不適用於即時性物件導向資料庫系統。在本論 文中首先對傳統的協同控制技術與物件導向資料庫中的作法做歸類與分 析,並根據此部份的結果提出一即時性物件導向協同控制協定。在設計此 協定時,係以傳統資料庫的作法為基礎 ,並配合物件導向資料庫系統的特 性而成。利用兩段式鎖定法及新的優先序指定策略,可提高異動的有效性 並增加系統的同作程度。最後,在本文中以模擬的方法建構一個封閉式的 等候模式,從模擬的結果可以發現此協定可維護系統中資料的一致性並使 更多的異動皆能如期完成。 A real-time database system is a transaction processing system where each trnasaction has an explicit timing constraint. Typically, a timing constraint is expressed in term of a deadline, a certain time in the future by which a trnasaction needs to be completed. In the real- time database system, the correctness of an transaction processing depends not only on maintaining consistency constraints and producing correct results, but also on the time at which a transaction is completed. Transactions must be scheduled in such a way that they can be completed before their corresponding deadline expires. Most real-time transaction scheduling methods use the existing concurrency control protocols, especially the two- phase locking algorithm, in conjunction with the time-critical transaction scheduling method which favors more urgent transactions. In an object- oriented database (OODB), the database is a collection of abstract data types, with operations defined on the type, and with objects being instances of their corresponding type. Each object can be accessed and manipulated only by invoking those operations defined within its type. Nevertheless, these cbaracteristics of OODB were never introduced in the relational database system, Therefore, the traditional scheduling methods in the use are not suitable for the real-time object-oriented transactions. In this thesis, we propose a real-time object-oriented concurrency control protocol. Based on the traditional approach, this protocol takes account the object-oriented characteristics. With two-phase locking and priority assignment, it tries to meet the timing constraint of each transaction and to maximize the consurrency level of the system. A simulation model, based on the closed queuing system, is built. The simulation results show that this protocol can maintain the OODB consistency while at the same time minimizing the number of transactions missing their deadlines.