Download Lecture Slides DBTransactions

Database Security
Transactions
• Database
• An essential corporate resource
• Data
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is a valuable resource
Must be strictly controlled, managed and secured
May have strategic importance
Should be kept secure and confidential
DBMS Functions and Services
• Authorization:
• DBMS must ensure that only authorized users can access the database
• Secure the database against unauthorized access (intentional or accidental)
• Database security:
• The mechanisms that protect the database against intentional or accidental
threats
• Security considerations apply not only to the data, considerations apply to
other parts as well, which may in turn affect the database. E.g.
• Hardware
• Software
• People
DBMS Function and Services
Three closely related functions of the DBMS that ensure reliability and
consistency.
The reliability and consistency must be maintained in the presence of failure
of both hardware and software components and when multiple users are
accessing the database.
• Transaction support
• A logical unit of work on the database
• Concurrency control services
• Controls simultaneous operations on the database
• Recovery services
• The process of restoring the database to a correct state
Transaction support
• Transaction
• A logical unit of work on the database
• A transaction should always transform a database from one consistent state
to another
• Consistency may be violated while the transaction is in progress.
• A transaction can have one of two outcomes
• Committed : if it completes successfully and the database reaches a new consistent state
• Aborted: if the transaction does not execute successfully.
• Rolled back/undone : If a transaction is aborted the database must be restored to the
consistent state it was in before the transaction started.
• Compensating transaction: A committed transaction cannot be aborted. If we decide
that the committed transaction was a mistake we must perform a compensating
transaction to revers its effect.
Properties of transactions (ACID)
ACID: The four conditions in which transactions ensure data integrity
• Atomicity
• The all or nothing property. The queries that make up the transaction must either all be
carried out, or none at all.
• Consistency
• This refers to the rules of the data. For example, an article body may have an associated
article heading. During the transaction, this rule may be broken, but the state of affairs
should never be visible from outside of the transaction. A transaction must transform the
database from one consistent state to another consistent state.
• Isolation
• Transactions execute independently of one another. Simply put, data being used for one
transaction cannot be used by another transaction until the first transaction is complete.
• Durability
• Once a transaction has completed, its effects should remain, and not be reversible.
(ACID)Isolation Example
• Take this example below, where an account balance starts at 900.
There is a single deposit of 100,and a withdrawal of 100, so the
balance at the end should remain the same.
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Connection 1: SELECT balance FROM account1;
Connection 2: SELECT balance FROM account1;
Connection 1: UPDATE account1 SET balance = 900+100;
Connection 2: UPDATE account1 SET balance = 900-100;
• The balance is now 800, so we have lost 100. These two transactions
should have been isolated, and the result supplied to Connection 2
only when the transaction from Connection 1 was complete.
Database Architecture
• Transaction manager : coordinates transactions on behalf of application
programs. It communicates with the scheduler
• Scheduler: the module responsible for implementing a particular strategy
for concurrency control. The objective of the scheduler is to maximize
concurrency without allowing concurrently executing transactions to
interfere with one another.
• Lock manager: the scheduler is sometimes called the lock manager if the
concurrency control protocol is a locking based.
• Recovery manager: Ensures that the database is restored to the state it
was in before the start of the transaction i.e. the consistent state.
• Buffer manager: is responsible for the transfer of data between disk
storage and main memory.
Concurrency Control
• The process of managing simultaneous operations on the database
without having them interfere with one another
• Potential problems caused by concurrency
• Lost update problem
• Uncommitted dependency problem
• Inconsistent analysis problem
Lost update problem
An apparently successfully completed operation by one user can be overwritten by another user
Uncommitted dependency problem/dirty read
When one transaction is allowed to see the uncommitted intermediate results of another transaction
Reading Dirty data
Inconsistent analysis problem
When a transaction reads several values from the database but a second transaction updates some of them
during the execution of the first.
Serializability
• Objective of concurrency control
• To avoid interference between transactions
• Solution
• Allow only one transaction to execute at a time
• However the aim is to maximize the d egree of concurrency and parallelism
so that transactions that can execute without interfering with one another.
• Serializability is the right solution
• Serializability identifies those execution of transactions that are guaranteed to
ensure consistency
Serializability
• First some difinitions
• Schedule
• Fine the definition in Database Systems by Thomas M.connoly Chapter 19
• Serial schedule
• Fine the definition in Database Systems by Thomas M.connoly Chapter 19
• In serial execution there is no interference between transactios, since only one is executing at any given
time.
• Nonserial schedule
• Fine the definition in Database Systems by Thomas M.connoly Chapter 19
• Now the objective of Serializability
• Is to find non serial schedules that allow transactions to execute concurrently without
interfering with one another, and thereby produce a database state that could be produced
by a serial execution.
• Serializable schedule
• Nonserial schedule that produces the same results as some serial execution.