Slideshow:
What is: database consistency ??
Definition:
database element (unit of update)
Definition:
state of database elements and
state of a database
Consistent and inconsistent database state
We will first define the transaction concept...
Introduction to Transaction
using an inprecise definition
Definition: Transaction
- Important fact:
- A database represents (a portion) of the reality
I.e.:
- The data stored in a database must reflect the state of the reality
Question:
- When is a database consistent ??
We need to define the notion of a consistent database state first before discussing failures.
- Database elements:
- Database element = the unit of data accessed by the database system
- Database:
- Database = a collection of database elements
- Note:
- Different DBMS uses
different notion
for database element
- Possible units:
- A relation
(kinda large)
- A disk block
(the most commonly used unit)
- A tuple in a relation (kinda small)
- A relation
(kinda large)
- Different DBMS uses
different notion
for database element
- State
of a database element:
- State of a database element = the current value stored in the database element
- State
of a database:
- Database state = the collection of values of all database elements in the database
- Change to the
(current) database state:
- Database state
can be changed by:
- Changing the value stored in one or more of the database elements in the database
- Database state
can be changed by:
- A database state can be:
- Consistent or
- Inconsistent
-
We need a mechanism
to define what
constitute a
consistent database state.
- This mechanism is a transaction
- Transaction:
- Transaction = a sequence of changes to one or more database elements
- Example: a transaction that
transfers
$100 from
A to
B
Read record A Set: A.balance = A.balance - 100 Write record A Read record B Set: B.balance = B.balance + 100 Write record B
- Example: a transaction that
deposits
$100 to
A
Read record A Set: A.balance = A.balance + 100 Write record A
- A more precise definition of
transaction is:
- Transaction =
a sequence of
changes to
one or more
database elements
-
When all changes in
a transaction are
made to the
database state:
- The resulting database state is a consistent state (if the initial state is consistent)
Graphically:
A transaction will transform a consistent database state to another consistent database state
- Transaction =
a sequence of
changes to
one or more
database elements