Chapter 21 Transaction Processing Concepts Part I Transaction Concepts The concept of transaction provides a mechanism for describing logical units of database processing A transaction is typically implemented by a computer program that includes database commands such as retrievals, insertions, deletions, and updates. A transaction groups SQL statements so that they are either all applied to the database, or they are undone from the database. Oracle Database assigns every transaction a unique identifier called a transaction ID. Transaction Processing System Definition: Is a system with large database and hundreds of users executing transactions concurrently Examples: An airline reservations, banking system, purchasing system, and many others These systems require high availability and fast response time for hundreds of concurrent users Introduction to Transaction Processing A database system is classifying according to the number of users who can use the system to: Single-User System: Multiuser System: At most one user at a time can use the system. Many users can access the system concurrently. Concurrent execution of processes is either: Interleaved processing: Concurrent execution of processes is interleaved in a single CPU Parallel processing: Processes are concurrently executed in multiple CPUs. Interleaved processing vs. Parallel Processing Introduction to Transaction Processing A Transaction: A transaction (set of operations) may be stand-alone specified in a high level language like SQL submitted interactively, or may be embedded within a program. Transaction boundaries: Logical unit of database processing that includes one or more access operations (read -retrieval, write - insert or update, delete). Begin and End transaction. An application program may contain several transactions separated by the Begin and End transaction boundaries. Introduction to Transaction Processing SIMPLE MODEL OF A DATABASE (for purposes of discussing transactions): A database is a collection of named data items Granularity of data - a field, a record , or a whole disk block (Concepts are independent of granularity) Basic operations are read and write read_item(X): Reads a database item named X into a program variable. To simplify our notation, we assume that the program variable is also named X. write_item(X): Writes the value of program variable X into the database item named X. Introduction to Transaction Processing READ AND WRITE OPERATIONS: Basic unit of data transfer from the disk to the computer main memory is one block. In general, a data item (what is read or written) will be the field of some record in the database, although it may be a larger unit such as a record or even a whole block. read_item(X) command includes the following steps: Find the address of the disk block that contains item X. Copy that disk block into a buffer in main memory (if that disk block is not already in some main memory buffer). Copy item X from the buffer to the program variable named X. Introduction to Transaction Processing READ AND WRITE OPERATIONS (cont.): write_item(X) command includes the following steps: Find the address of the disk block that contains item X. Copy that disk block into a buffer in main memory (if that disk block is not already in some main memory buffer). Copy item X from the program variable named X into its correct location in the buffer. Store the updated block from the buffer back to disk (either immediately or at some later point in time). Sample Transactions Sample Transactions Concurrent Transactions Introduction to Transaction Processing Why Concurrency Control is needed: The Lost Update Problem This occurs when two transactions that access the same database items have their operations interleaved in a way that makes the value of some database item incorrect. Concurrent execution is uncontrolled: (a) The lost update problem. Execute the schedule by assuming that X=50 and Y=50 Introduction to Transaction Processing Why Concurrency Control is needed: The Temporary Update (or Dirty Read) Problem This occurs when one transaction updates a database item and then the transaction fails for some reason (see Section 21.1.4). The updated item is accessed by another transaction before it is changed back to its original value. Concurrent execution is uncontrolled: (b) The temporary update problem. Introduction to Transaction Processing Why Concurrency Control is needed: The Incorrect Summary Problem If one transaction is calculating an aggregate summary function on a number of records while other transactions are updating some of these records, the aggregate function may calculate some values before they are updated and others after they are updated. Concurrent execution is uncontrolled: (c) The incorrect summary problem. Introduction to Transaction Processing Why Concurrency Control is needed: Unrepeatable Read Problem If one transaction reads different values for the same database item within the same session Introduction to Transaction Processing Why recovery is needed: Whenever a transaction is submitted to a DBMS for execution, the system is responsible for making sure that either all the operations in the transaction are completed successfully and recorded permanently in the database, or that the transaction does not have any effect on the database or any other transactions. Types of Failures 1. A computer failure (system crash): A hardware, software, or network error occurs in the computer system during transaction execution. Hardware crashes are usually media failures—for example, main memory failure. 2. A transaction or system error: Some operation in the transaction may cause it to fail, such as integer overflow or division by zero. Failure may also occur because of erroneous parameter values or may be interrupted by the user. Introduction to Transaction Processing Types of Failures (Cont.) 3. Local errors or exception conditions detected by the transaction: Certain conditions that may cancel the transaction for example, data may not be found or a condition such as insufficient account balance in a banking database. 4. Concurrency control enforcement: The concurrency control method may decide to abort the transaction to be restarted later, because it violates serializability or because several transactions are in a state of deadlock (see Chapter 22). 5. Disk failure: Some disk blocks may lose their data because of a read or write malfunction or because of a disk read/write head crash. This may happen during a read or a write operation of the transaction. 6. Physical problems and catastrophes: Such problems include; power or air-conditioning failure, fire, theft, overwriting disks or tapes by mistake. Transaction and System Concepts A transaction is an atomic unit of work that is either completed in its entirety or not done at all. For recovery purposes, the system needs to keep track of when the transaction starts, terminates, and commits or aborts. Transaction states: Active state Partially committed state Committed state Failed state Terminated State State Transition Diagram Illustrating the States for Transaction Execution Transaction and System Concepts Recovery manager keeps track of the following operations: begin_transaction: This marks the beginning of transaction execution. read or write: These specify read or write operations on the database items that are executed as part of a transaction. end_transaction: This specifies that read and write transaction operations have ended and marks the end limit of transaction execution. At this point it may be necessary to check whether the changes introduced by the transaction can be permanently applied to the database or whether the transaction has to be aborted because it violates concurrency control or for some other reason. Transaction and System Concepts Recovery manager keeps track of the following operations (cont): commit_transaction: This signals a successful end of the transaction so that any changes (updates) executed by the transaction can be safely committed to the database and will not be undone. rollback (or abort): This signals that the transaction has ended unsuccessfully, so that any changes or effects that the transaction may have applied to the database must be undone. Transaction and System Concepts Recovery techniques use the following operators: undo: Similar to rollback except that it applies to a single operation rather than to a whole transaction. redo: This specifies that certain transaction operations must be redone to ensure that all the operations of a committed transaction have been applied successfully to the database. Transaction and System Concepts The System Log Log or Journal: The log keeps track of all transaction operations that affect the values of database items. This information may be needed to permit recovery from transaction failures. The log is kept on disk, so it is not affected by any type of failure except for disk or catastrophic failure. In addition, the log is periodically backed up to archival storage (tape) to guard against such catastrophic failures. Transaction and System Concepts The System Log (cont): T in the following discussion refers to a unique transaction-id that is generated automatically by the system and is used to identify each transaction: Types of log record: [start_transaction,T]: Records that transaction T has started execution. [write_item,T,X,old_value,new_value]: Records that transaction T has changed the value of database item X from old_value to new_value. [read_item,T,X]: Records that transaction T has read the value of database item X. [commit,T]: Records that transaction T has completed successfully, and affirms that its effect can be committed (recorded permanently) to the database. [abort,T]: Records that transaction T has been aborted. Transaction and System Concepts Commit Point of a Transaction: Definition a Commit Point: A transaction T reaches its commit point when all its operations that access the database have been executed successfully and the effect of all the transaction operations on the database has been recorded in the log. Beyond the commit point, the transaction is said to be committed, and its effect is assumed to be permanently recorded in the database. The transaction then writes an entry [commit,T] into the log. Transaction and System Concepts Commit Point of a Transaction (cont): Force writing a log: Before a transaction reaches its commit point, any portion of the log that has not been written to the disk yet must now be written to the disk. This process is called force-writing the log file before committing a transaction. Desirable Properties of Transactions ACID properties: Atomicity: A transaction is an atomic unit of processing; it is either performed in its entirety or not performed at all. Consistency preservation: A correct execution of the transaction must take the database from one consistent state to another. Isolation: A transaction should not make its updates visible to other transactions until it is committed; this property, when enforced strictly, solves the temporary update problem and makes cascading rollbacks of transactions unnecessary (see Chapter 21). Durability or permanency: Once a transaction changes the database and the changes are committed, these changes must never be lost because of subsequent failure.