数据库课件lec1overview幻灯片.ppt

Strict Two-Phase (2PL) Locking Protocol It is the most widely used locking protocol. Two rules (1) If a transaction T wants to read (respectively, modify) a database object, it first requires a shared (respectively, exclusive) lock on the object. (2) All locks held by a transaction are released when the transaction is completed. Locking example T1 (Bill): Savings – = 100; Checking + = 100 T2 (Bill’s wife): Print(Checking); Print(Savings) T1 and T2 both lock Savings and Checking objects. If T1 locks Savings Checking first, T2 must wait. An Overview of Transaction Properties A DBMS must ensure four properties of transactions to keep database consistent in the face of concurrent executions and system crashes. Atomicity Consistency Isolation Durability So Called ACID Properties Ensuring Atomicity DBMS ensures atomicity (all-or-nothing property) even if system crashes in the middle of a Xact (i.e., transaction). Idea: Keep a log (history) of all actions carried out by the DBMS while executing a set of Xacts: Before a change is made to the database, the corresponding log entry is forced to a safe location. (WAL protocol; OS support for this is often inadequate.) After a crash, the effects of partially executed transactions are undoneusing the log. STRUCTURE OF A DBMS Databases make these folks happy ... End users and DBMS vendors DB application programmers E.g., smart webmasters Database administrator (DBA) Designs logical /physical schemas Handles security and authorization Data availability, crash recovery Database tuning as needs evolve Must understand how a DBMS works! Summary DBMS used to maintain, query large datasets. Benefits include recovery from system crashes, concurrent access, quick application development, data integrity and security. Levels of abstraction give data independence. A DBMS typically has a layered architecture. DBAs hold responsible jobs and are well-paid! DBMS RD is one of the broadest, most exciting areas in CS. We focus on Relationa