PROCEDURAL DATABASE PROGRAMMING ( PL/SQL AND T-SQL)

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1 Technology & Information Management Instructor: Michael Kremer, Ph.D. Class 10 Database Programming PROCEDURAL DATABASE PROGRAMMING ( PL/SQL AND T-SQL)

2 AGENDA 14. Advanced Topics 14.1 Optimistic/Pessimistic Concurrency Models 14.2 Sending Other Advanced Topics

3 Procedural Database Programming (PL/SQL and T-SQL) 14. ADVANCED TOPICS

4 263 Concurrency: Multiple users attempt to modify data at the same time. Minimize the occurrence of concurrency problems, this is called concurrency control

264 To resolve the various concurrency problems, databases systems use locking mechanism to restrict concurrent access to records by various transactions.

5 264 To resolve the various concurrency problems, databases systems use locking mechanism to restrict concurrent access to records by various transactions. Two types of locks are used: Shared Locks: Used to protect reads of records. Shared locks are compatible with other Shared locks but not with Exclusive locks (see below). Exclusive Locks: Used to protect writes to records. Only one transaction may hold an Exclusive lock on a record at any time, and furthermore, Exclusive locks are not compatible with Shared locks.

6 265 Data concurrency: Many users can access data at the same time. Data consistency: Each user sees a consistent view of the data, including visible changes made by the user's own transactions and transactions of other users. The different lock modes offer different levels of data consistency, trading off performance and concurrency for greater consistency. Single/Multi Version Databases SVDB: Writers block Readers. Readers also block Writers except in Read Uncommitted and Read Committed. MVDB: Prevent problems of Writers blocking Readers and vice versa.

7 266 A MVDB needs to provide the following features: The DBMS must be able to retrieve older versions of a row. The DBMS must have a mechanism to determine which version of a row is valid on the context of a transaction. Usually, a DBMS will only consider a version that was committed prior to the start of the transaction that is running the query. Multi-Version Concurrency improves database concurrency, its impact on data consistency is more complex. Two approaches to MVCC: Store multiple versions of a record in the database, and garbage collect records when they are no longer required (SQL Server). Keep only the latest version of a record in the database, and reconstruct older versions of data dynamically as needed by exploiting information in the Rollback(Undo) logs (Oracle). Optimistic: No locking of resources, rather you have means to identify incompatible actions against the data. Pessimistic: Locking of resources to guarantee incompatible actions against the data.

8 267 Read Uncommitted Readers do not request shared Locks Read locked data (dirty read) Worst in terms of isolation level Best in terms of concurrency

9 268 Read Committed Default isolation level When reading data a shared lock is requested (only during the actual read process, not when statement or transaction finishes) Can only read data that has been committed no dirty read

10 269 Repeatable Read Also request a shared lock when reading data. But shared locks are maintained until transaction finishes. Guaranteed repeatable reads, no lost updates.

11 270 Serializable Isolation Similar to repeatable reads, but transactions acquire key-range locks. Lock not only data that exists but also prevent inserts meeting the query s filter. Avoids phantom reads.

12 271 Multi-Version, Snapshot Isolation Implemented in SQL Server 2008, before only SVDB. Oracle was never a SVDB. MVDB maintains older versions of rows that resulted from committed transactions.

272 Snapshot Isolation is similar to serializable isolation in terms of problems than can or cannot arise. But it is based on row versioning instead of blocking and locking.

13 272 Snapshot Isolation is similar to serializable isolation in terms of problems than can or cannot arise. But it is based on row versioning instead of blocking and locking. Conflict Detection Snapshot isolation uses optimistic concurrency, meaning when reading data you do not acquire a lock. Between reading data and then updating the same data SQL Server detects conflicts if someone else has updated the data in the meantime.

14 273 Multi-Version, Read Committed Snapshot Read committed isolation level based on multi-version database. Readers get latest consistent version of the data when the statement and not when the transaction started. Does not detect update conflicts, just like read committed Lost Updates

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275 MV, Read Committed is default isolation level in Oracle. Oracle always operates as MVDB. Oracle explicitly supports Read Committed and Serializable isolation levels.

16 275 MV, Read Committed is default isolation level in Oracle. Oracle always operates as MVDB. Oracle explicitly supports Read Committed and Serializable isolation levels. SET TRANSACTION ISOLATION LEVEL {SERIALIZABLE READ COMMITTED} SERIALIZABLE: A dml statement that attempts to modify a table already modified in an uncommitted transaction will fail. READ COMMITTED: A dml statement that requires row-level locks held by another transaction will wait until those row locks are released.

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21 280 One more option in Oracle for setting a transaction: SET TRANSACTION READ ONLY In a read-only transaction, all subsequent queries see only those changes that were committed before the transaction began. This statement is useful when you are executing long-running, multiple query reports, and you want to make sure that the data used is consistent at one point in time. To prevent Lost Updates in Oracle: select statement FOR UPDATE This row is now locked, even though you are only reading data. Even if no update is performed, you need to issue a rollback or commit command to release locks.

281 14.2 Sending E-mail SQL Server possesses built-in e-mail capabilities, but not Express.

22 Sending SQL Server possesses built-in capabilities, but not Express. Oracle contains built-in packages: Utl_smtp, utl_tcpip, utl_mail (newer version) 14.3 Other Advanced Topics

Are You OPTIMISTIC About Concurrency?

Are You OPTIMISTIC About Concurrency? SQL Saturday #399 Sacramento July 25, 2015 Kalen Delaney www.sqlserverinternals.com Kalen Delaney Background: MS in Computer Science from UC Berkeley Working exclusively