Database Management Ch 10

Atomicity

Requires that all operations of a transaction be completed ; if not, the transaction is aborted. If a transaction T1 has four SQL requests, all four requests must be successfully completed; otherwise, the entire transaction is aborted. In other words, a transaction is treated as a single, indivisible, logical unit of work

Deadlock Detection

The Database management system periodically test the database for deadlocks. If a deadlock is found, the victim transaction is aborted and the other transaction continues

Database Request

The equivalent of a single SQL statement in an application program or transaction

Lock grandularity

The level of lock use. Locking can take place at the following levels; database, table, page, row, and field

Database Recovery

The process of restoring a database to a previous consitent state

Deadlock Avoidance

The transaction must obtain all of the locks necessary before it can be executed which avoids rollbacks from conflicting transactions. This increases action response time

Pessimistic Locking

The use of loced based on the assumption that conflict between transaction is likely

Uncommitted Data

When you are trying to achieve concurrency control, uncommitted data causes problems with data integrity and consistency. These problems occur when two transactions are executed concurrently and the first transaction is rolled back after the second transaction has already accessed the uncommitted data, thus violating the isolation property of transactions

Immediate Update

Write-through technique - The database is immediately updated by transaction operations during the transaction's execution, even bore the transaction reaches its commit point.

Redundant transaction logs

ensure that a physical disk failure will not impair the DBM's ability to recover data

Write-ahead-log protcol

ensures that transaction logs are always written before any database data are actually updated.

Isolation

means that the data used during the execution of a transaction cannot be used by a second transaction until the first one is completed. In other words, if transaction T1 is being executed and is using the data item X, that data item cannot be accessed by any other transaction until T1 ends. This property is particulary useful in multiuser database environments because several users can access and update teh database at the same time

Deadlock happens when...

one of the transactions wants to obtain an exclusive lock on the data item; NO DEADLOCK CONDITION CAN EXIST AMONG SHARED LOCKS!!!

Lock Manager

A DBMS componet that is responsible for assigning and releasing locks

Concurrency Control

A DBMS feature that coordinates the simultaneous execution of transactions in a multiprocessing database systems while preserving data integrity

Wait/die

A concurrency conteol scheme in which an older transaction must wait for the younger transaction to complete and release that locks before requesting the locks itself. Otherwise, the newer transaction dies and is rescheduled.

Lost Update

A concurrency control problem in which data updates are lost during the concurrent execution of transactions

Inconsistent retrievals

A concurrency control problem that arises when a transaction calculating summary functins over a set of data while other transactions are updating the data while other transactions are updating the data, yielding erraneous results

Wound/wait

A concurrency control scheme in which an older transaction can request the lock, preempt the younger transaction, and reschedule it. Otherwise, the newer transaction waits until the older transaction finishes

Deadlock

A condition in which two or more transactions wait indefinately for the other to release the lock on a previously locked data item

Consistent Database State

A database state in which all data integrity constraints are satisfied

Lock

A device that gurantees unique use of a data item in a particular transaction operation. A transaction requires a lock prior to data access; the lock is released after the operations execution to enable other transactions to lock thedata item for their own use

Transaction Log

A feature used by the DBMS to keep track of all transaction operations that update the database. The information stored in this log is used by the DBMS for recovery purposes

Row-level lock

A less restrictive database lock in which the DBMS allows concurrent transactions to access different rows of the same table, even when the rows are on the same page

Field-level lock

A lock that allows concurrent transactions to access the same row as they require the use of different fields ithin that row. This type of lock yields that most flexible multiuser data access but requires a high level of computer overhead

Binary Lock

A lock that has only two states; locked and unlocked. If a data item is locked by a transaction, no other transaction can use that data item

Exclusive lock

A lock that is reserved

Table-level Lock

A locking scheme that allows only one transaction at a time to access a table. A table-level lock locks an entire table, preventing access to any row by transaction T2 while transaction T1 is using the table

Atomic Transaction Property

A property that requires all parts of a transaction to be treated as a single, logical unit of work in which all operations must be completed to produce a consistent database

Monotonicity

A quality that ensures that timestamp values always increase.

Two-phase locking

A set of rules that governs how transactions acquire and relinquish locks. Two-phase locking gurantees serializinability, but it does not prevent deadlocks. Incl: Growing Phase and shrinking phase

Transaction

A transaction is a logical unit of work; that is, it must be entirely completed or aborted - no intermediate ending states are accepted. All transactions must have the properties of atomicity, consistency, isolation, and durability.

Deadly Embrace

Also known as deadlock

Checkpoints

Are operations in which in which the DBMS writes all of its updated buffer to disk .

Buffers

Are temporarily storage areas in primary memory used to speed up disk operations.

Deadlock Prevention

Avoids the conditions leading to deadlocking by noticing the possibility of a deadlock occurrence. Once realized, the transaction requesting a new lock is aborted, changes are rolled back, and all locks obtained by the transaction are then released

Deferred Update

Deferred-write technique - The transaction operations do not immediately update the physical database

Durability

Ensures that once transaction changes are done and committes, they cannot be undone or lost even in the event of a system failure

Serializability

Ensures that the schedule for the concurrent execution of the transactions yields consistent results. This property is important in the multiuser and distributed databases in which multiple transactions are likely to be executed concurrently. Naturally, if only a single transaction is executed, serializability is not an issue

Uniqueness

In concurrency control, a property of timestamping that ensures no equal timestamp values can exist.

Page-level lock

In this type of lock, the database management system locks an entire diskpage, or section of a disk. A diskpage came contain data for one or more rows and from one or more tabels

Optimistic Approach

In transaction management, a concurrency control technique based on the assumption that most database operations do not conflict.

Timestamping

In transaction management, a technique used in scheduling concurrent transactions that assigns a global unique timestamp to each transaction

Consistency

Indicates the performance of the database's consistent state. A transaction takes a database from one consistent state of another. When a transaction is completed, the database must be in consistent state; if any of the transaction parts diolates an integrity constraint, the entire transaction is aborted.

Rollback Statement

Is reached when all changes are aborted and the database is rolled back to its previous consistent state

Commit Statement

Is reached when all changes are permanetely recorded within the database. Automatically ends the the SQL transaction