* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download DBMS Functions Data, Storage, Retrieval, and Update
Survey
Document related concepts
Global serializability wikipedia , lookup
Microsoft SQL Server wikipedia , lookup
Entity–attribute–value model wikipedia , lookup
Oracle Database wikipedia , lookup
Commitment ordering wikipedia , lookup
Open Database Connectivity wikipedia , lookup
Ingres (database) wikipedia , lookup
Serializability wikipedia , lookup
Relational model wikipedia , lookup
Extensible Storage Engine wikipedia , lookup
Functional Database Model wikipedia , lookup
Microsoft Jet Database Engine wikipedia , lookup
Versant Object Database wikipedia , lookup
Database model wikipedia , lookup
Clusterpoint wikipedia , lookup
Transcript
DBMS Functions Data, Storage, Retrieval, and Update • Provides users with the ability to store, retrieve, and update the data that are in the database. DBMS Functions Shared Update • Ensures accuracy when several users are updating the database at the same time. Avoiding Inconsistent Data • Prohibit shared update. • Use batch processing • Lock other users from accessing records that are being updated by another. Concurrency Control • The DBMS must prevent or overcome the interference of transactions of multiple users • Locking – different levels of granularity (records to tables) Avoiding the Problem • Two-Phase Locking – Locks are held until all the required updates have been completed. • Deadlock – Occurs when two users hold more than one lock at a time. • Locking on PC-Based DBMSs – Usually much more limited than locking facilities provided by mainframe DBMSs. Record Locking • Deadlock prevention • Deadlock resolution Avoiding the Problem • Timestamping – The DBMS assigns to each database update the unique time when the update started, called a timestamp. – Avoids the need to lock rows and eliminates the processing time needed to apply and release locks; also detects and resolves deadlocks. Backup and Recovery • Provides a mechanism for recovering the database in the event that the database is damaged in any way. • The process of returning the database to a correct state is called recovery. • Periodically making a copy of the database is called backup. Replication • A way to manage copies of the same data at multiple locations. Security • The protection of the database against unauthorized access. – Passwords – Encryption – Views Integrity • Integrity constraints, or conditions, fall into four categories: – – – – Data type Legal values Format Key constraints Integrity Constraints in a DBMS Data Independence • Facilities that allow programs to be independent of the structure of the database. – – – – Addition of a field Changing the length of a field Creating a new index Adding or changing a relationship ACID properties • Atomic – all of a transaction should be executed or none • Consistent – the database should be in a “consistent” state after the transaction is completed • Isolated – Updates of concurrent transactions should be concealed from each other, till committed • Durable – once a transaction commits, its updates survive even if there is a subsequent system crash