Transaction Management and Concurrency Control Chapter 9 Transaction Management and Concurrency Control Database Systems: Design, Implementation, and Management, Sixth Edition, Rob and Coronel

In this chapter, you will learn: What a database transaction is and what its properties are How database transactions are managed What concurrency control is and what role it plays in maintaining the database’s integrity What locking methods are and how they work How database recovery management is used to maintain database integrity Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

What is a Transaction? Any action that reads from and/or writes to a database may consist of Simple SELECT statement to generate a list of table contents A series of related UPDATE statements to change the values of attributes in various tables A series of INSERT statements to add rows to one or more tables A combination of SELECT, UPDATE, and INSERT statements Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

What is a Transaction? (continued) A logical unit of work that must be either entirely completed or aborted Successful transaction changes the database from one consistent state to another One in which all data integrity constraints are satisfied Most real-world database transactions are formed by two or more database requests The equivalent of a single SQL statement in an application program or transaction Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

The Relational Schema for the Ch09_SaleCo Database Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Evaluating Transaction Results Not all transactions update the database SQL code represents a transaction because database was accessed Improper or incomplete transactions can have a devastating effect on database integrity Some DBMSs provide means by which user can define enforceable constraints based on business rules Other integrity rules are enforced automatically by the DBMS when table structures are properly defined, thereby letting the DBMS validate some transactions Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Tracing the Transaction in the Ch09_SaleCo Database Figure 9.2 Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Transaction Properties Atomicity Requires that all operations (SQL requests) of a transaction be completed Durability Indicates permanence of database’s consistent state Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Transaction Properties (continued) Serializability Ensures that the concurrent execution of several transactions yields consistent results Isolation Data used during execution of a transaction cannot be used by second transaction until first one is completed Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Transaction Management with SQL ANSI has defined standards that govern SQL database transactions Transaction support is provided by two SQL statements: COMMIT and ROLLBACK ANSI standards require that, when a transaction sequence is initiated by a user or an application program, it must continue through all succeeding SQL statements until one of four events occurs Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

The Transaction Log Stores A record for the beginning of transaction For each transaction component (SQL statement) Type of operation being performed (update, delete, insert) Names of objects affected by the transaction (the name of the table) “Before” and “after” values for updated fields Pointers to previous and next transaction log entries for the same transaction The ending (COMMIT) of the transaction Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

A Transaction Log Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Concurrency Control Coordination of simultaneous transaction execution in a multiprocessing database system Objective is to ensure transaction serializability in a multiuser database environment Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Concurrency Control Important  simultaneous execution of transactions over a shared database can create several data integrity and consistency problems lost updates uncommitted data inconsistent retrievals Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Normal Execution of Two Transactions Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Lost Updates Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Correct Execution of Two Transactions Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

An Uncommitted Data Problem Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Retrieval During Update Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Transaction Results: Data Entry Correction Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Inconsistent Retrievals Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

The Scheduler Special DBMS program: establishes order of operations within which concurrent transactions are executed Interleaves the execution of database operations to ensure serializability and isolation of transactions Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

The Scheduler (continued) Bases its actions on concurrency control algorithms Ensures computer’s central processing unit (CPU) is used efficiently Facilitates data isolation to ensure that two transactions do not update the same data element at the same time Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Read/Write Conflict Scenarios: Conflicting Database Operations Matrix Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Concurrency Control with Locking Methods Guarantees exclusive use of a data item to a current transaction Required to prevent another transaction from reading inconsistent data Lock manager Responsible for assigning and policing the locks used by the transactions Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Lock Granularity Indicates the level of lock use Locking can take place at the following levels: Database Table Page Row Field (attribute) Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Lock Granularity (continued) Database-level lock Entire database is locked Table-level lock Entire table is locked Page-level lock Entire diskpage is locked Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Lock Granularity (continued) Row-level lock Allows concurrent transactions to access different rows of the same table, even if the rows are located on the same page Field-level lock Allows concurrent transactions to access the same row, as long as they require the use of different fields (attributes) within that row Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

A Database-Level Locking Sequence Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

An Example of a Table-Level Lock Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Example of a Page-Level Lock Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

An Example of a Row-Level Lock Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Lock Types Binary lock Exclusive lock Shared lock Has only two states: locked (1) or unlocked (0) Exclusive lock Access is specifically reserved for the transaction that locked the object Must be used when the potential for conflict exists Shared lock Concurrent transactions are granted Read access on the basis of a common lock Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

An Example of a Binary Lock Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Two-Phase Locking to Ensure Serializability Defines how transactions acquire and relinquish locks Guarantees serializability, but it does not prevent deadlocks Growing phase, in which a transaction acquires all the required locks without unlocking any data Shrinking phase, in which a transaction releases all locks and cannot obtain any new lock Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Two-Phase Locking to Ensure Serializability (continued) Governed by the following rules: Two transactions cannot have conflicting locks No unlock operation can precede a lock operation in the same transaction No data are affected until all locks are obtained—that is, until the transaction is in its locked point Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Two-Phase Locking Protocol Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Deadlocks Condition that occurs when two transactions wait for each other to unlock data Possible only if one of the transactions wants to obtain an exclusive lock on a data item No deadlock condition can exist among shared locks Control through Prevention Detection Avoidance Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

How a Deadlock Condition Is Created Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Concurrency Control with Time Stamping Methods Assigns a global unique time stamp to each transaction Produces an explicit order in which transactions are submitted to the DBMS Uniqueness Ensures that no equal time stamp values can exist Monotonicity Ensures that time stamp values always increase Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Wait/Die and Wound/Wait Schemes Older transaction waits and the younger is rolled back and rescheduled Wound/wait Older transaction rolls back the younger transaction and reschedules it Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Wait/Die and Wound/Wait Concurrency Control Schemes Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Concurrency Control with Optimistic Methods Optimistic approach Based on the assumption that the majority of database operations do not conflict Does not require locking or time stamping techniques Transaction is executed without restrictions until it is committed Phases are read, validation, and write Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Database Recovery Management Restores database from a given state, usually inconsistent, to a previously consistent state Based on the atomic transaction property All portions of the transaction must be treated as a single logical unit of work, in which all operations must be applied and completed to produce a consistent database If transaction operation cannot be completed, transaction must be aborted, and any changes to the database must be rolled back (undone) Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Transaction Recovery Makes use of deferred-write and write-through Transaction operations do not immediately update the physical database Only the transaction log is updated Database is physically updated only after the transaction reaches its commit point using the transaction log information Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Transaction Recovery (continued) Write-through Database is immediately updated by transaction operations during the transaction’s execution, even before the transaction reaches its commit point Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

A Transaction Log for Transaction Recovery Examples Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Summary Transaction Sequence of database operations that access the database Represents real-world events Must be a logical unit of work No portion of the transaction can exist by itself Takes a database from one consistent state to another One in which all data integrity constraints are satisfied Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Summary (continued) SQL provides support for transactions through the use of two statements: COMMIT and ROLLBACK Concurrency control coordinates the simultaneous execution of transactions Scheduler is responsible for establishing order in which concurrent transaction operations are executed Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

Summary (continued) Lock guarantees unique access to a data item by a transaction Database recovery restores the database from a given state to a previous consistent state Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel