University of Manitoba
Asper School of Business
3500 DBMS
Bob Travica
Chapter 7
Database Transactions, Integrity &
Accuracy
Based on G. Post, DBMS: Designing & Building Business Applications
Updated 2018
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Outline
 Concepts of Database Transaction,
Integrity & Accuracy
 Code in DB system
 Triggers
 Transactions
 Concurrent Access
 Locks & Deadlocks
 ACID Transactions
 Keys creation
 DB cursor
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Transaction & Integrity Concepts
 Database transaction is a set of processing steps that succeed
or fail altogether. A database transaction behaves like a whole.
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 Differentiate from business transaction – one instance of a
business operation (a purchase, a sale, an inventory input…);
refer to TPS.
 Workings of a DBS is usually in the form of database
transactions.
 Database Integrity is about data consistency. Inside a DBS,
integrated data are “correct” data since a particular piece of
data is the same across the system. But it might be wrong
(inaccurate), that is not corresponding to reality.
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Transaction & Integrity Concepts
 Database integrity is sometimes mixed up with the concept of
database accuracy (data corresponding to/reflecting reality);
keep this in mind when reading the textbook.
 Better: think of data integrity AND data accuracy as properties of
a quality database.
 Example: A $100 million figure is stored and referred to
consistently in a DB system (integrity), but the real income is
$90 million (accuracy, the lack of it).
 Properly designed and executed transactions ensure data
integrity and accuracy.
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Code as Part of DB System
 Create code
 As triggers = procedures
that support biz processes;
 In forms and reports
(localized effects)
 Functions combined with
embedded SQL
 User-defined functions
Function combined with
SQL statements
Tables
DB System
Trigger
If inventory < Minimum
Then
CREATE POrder
End If
Form
C++ code:
if (. . .) {
// embedded SQL
SELECT …
}
If (Click) Then
MsgBox . . .
End If
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User-Defined Functions*
• A program for increasing a salary for a certain amount (pseudo code).
CREATE FUNCTION IncreaseSalary
(EmpID INTEGER, Amt CURRENCY)
BEGIN
INPUT EmpID, Amt
IF (Amt > 50000)
THEN RETURN -1
-- error flag, data validation
ELSE
UPDATE Employee SET Salary = Salary + Amt
WHERE EmployeeID = EmpID;
PRINT Salary
-- new values
END
* A function is the code (piece of application software) that returns a result of
processing.
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Events and Triggers
(The topic from pre-midterm)
• An event starts (initiates) a trigger (programming code):
• Common events:
• SQL-based on rows: INSERT, DELETE, UPDATE
• SQL-based on tables: ALTER, CREATE, DROP
• Based on user’s action: LOGOFF, LOGON
• Based on server action: SERVERERROR, SHUTDOWN, STARTUP
• Example on slide 4: BEFORE UPDATE of Employee table
rows, run a trigger that checks if a salary ceiling is respected.
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Table
• SQL events support triggers at two points in time - BEFORE and AFTER data change
Row
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Table and Row Triggers
Before Update
on a table (all rows)
Before Update
of a row
Update
point
After Update
on the table
After Update
of the row
time
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An After Row Trigger Example
• The trigger logs employee ID, date, user’s ID, old salary,
and new salary into table SalaryChanges, after each update
on the salary column in the Employee table. Useful for auditing.
CREATE TRIGGER LogSalaryChanges
AFTER UPDATE OF Salary ON Employee
REFERENCING
OLD ROW As oldrow
NEW ROW As newrow
FOR EACH ROW
INSERT INTO tblSalaryChanges
(EmpID, ChangeDate, UserID, OldValue, NewValue)
VALUES
(newrow.EmployeeID, CURRENT_TIMESTAMP, CURRENT_USER,
oldrow.Salary, newrow.Salary);
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Cascading Triggers
• Increase scope of automation. Take care of dependencies.
Sale(SaleID, SaleDate, …)
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SaleItem(SaleID, ItemID, Quantity, …)
AFTER INSERT
UPDATE Inventory
SET Inventory.QOH=Inventory.QOH –
newrow.Quantity;
Inventory(ItemID, QOH, …)
AFTER UPDATE
WHEN Inventory.QOH < minQuantity
INSERT {new Order}
INSERT {new OrderItem};
Order(OrderID, OrderDate, …)
OrderItem(OrderID, ItemID, Quantity, …)
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Transactions
 Definition: Transaction is a sequence of
processing tasks (a process) that
succeed or fail altogether.
 Functions, procedures and triggers
(preceding slides) can be defined as
transactiosi.
Transaction
1. Subtract $1000 from Savings.
(Then, system crashes)
2. Add $1000 to Checking.
(Money not added)
 Reason: to protect database accuracy 1. SavingsAccount Checking
against system failures.
Joe Doe
Account
 Example on right:
1. customer starts transferring money
from savings account to checking
account.
2. System crashes after subtracting
amount from Savings and the
amount is not added to Checking.
Bal.:
5340.92 Joe Doe
Subtract: 1000.00 Bal.: 1424.27
New Bal.: 4340.92 Add: 1000
2. Transfer
Fails
$1000 subtracted from
Savings but Checking
Balance not increased!
inaccurate data!___
Checking
Account
Joe Doe
Bal.: 1424.27
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Designing Transactions
 Transactions are programmed
 Mark a transaction start - START TRANSACTION
 Determine a point of temporary saving of
data changes
 Mark a transaction end - COMMIT end-result of
processing to database, permanent save.
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Designing Transactions: Error Correction
Full
ROLLBACK
Start
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SAVEPOINT
StartOptional
Run simple steps
COMMIT (Save)
Riskier steps
Partial
ROLLBACK
processing
sequence
START TRANSACTION;
SELECT * FROM tbl_Customer WHERE CustID=…
UPDATE tbl_Customer.Address
SAVEPOINT StartOptional
UPDATE tbl_Order.newrecord
UPDATE tbl_Inventory.QOH
IF error THEN
ROLLBACK TO SAVEPOINT StartOptional
END IF
COMMIT;
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Concurrent Access
 Concurrent Access
 Multiple transactions competing for the same data at the
same time.
 If Order process reads Balance before Payment process
ends, the end balance will be incorrect.
Table Customer Account
read and update
Payment transaction
1) Read Balance
800
2) Subtract pmt
-200
3) Read old balance=800
4) Save new bal.
600
5) Add order
150
6) Write balance 950
Order transaction
Integrity
violation
Accuracy
violation
$950 is the end balance instead of $750 (600+150). Customer at loss.
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Pessimistic Locks (Serialization)
 One answer to problems of concurrent access is to
force transactions to run in a sequence one after
another.
 A transaction places a SERIALIZABLE lock on data so
that no other transaction can access it before the first
transaction is completed.
SET TRANSACTION SERIALIZABLE READ, WRITE
Payment transaction
locks the table
1) Read balance
2) Subtract pmt
3) Save new bal.
800
-200
600
Order transaction, trying to
interfere with Payment at step 3
is locked out.
3) System’s message :
“Table is locked, try
later”.
The integrity problem avoided and the
transaction will work with accurate data.
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Deadlock!
Transaction 1 (T1)
 Deadlock = problem with serialized locks
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when different transactions use multiple tables
concurrently. T1 locks tbl A and requests
access to tbl B, while tbl B is locked by T2
that, in turn, requests access to T1. Neither
transaction can be completed.
1) Lock tbl A
2) Read tbl B
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Tbl A
Tbl B
• Transactions lock out each other, creating
a “deadly embrace” (deadlock).
2
• Solution 1 to deadlock:
Each transaction waits random time, tries
again, releases locks if unsuccessful, and runs
itself all over again.
• Solution 2: automated lock manager (next).
4
1) Lock tbl B
2) Update tbl A
Transaction 2 (T2)
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Deadlock Loop
Sometimes a lock manager must intervene: A closed lock-wait loop that
blocks many transactions.
Table A
Transaction 1
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Table B
Table C
Lock
Table D
Table E
Wait
(needs tbl_B, tbl_D)
Transaction 2*
(needs tbl_D, tbl_A)
Lock
Wait
Transaction 3
Transaction 4
(needs tbl_A, tbl_E)
Lock
Wait
Transaction 6
(needs tbl_E, tbl_C)
Wait
Lock
Transaction 7
(needs tbl_C, tbl_D)
Lock
Wait
Lock Manager (part of DBMS) monitors all transactions and disables
some temporarily to clear the deadlock.
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Optimistic Locks
 Opposite to serialization lock (pessimistic lock); a solution
against deadlocks and wait times with pessimistic locks.
 Logic:
 Assuming that collisions are rare
 Read transactions are frequent
 Record state of DB at Read time of any transaction (no locks)
 If a transaction tries to write a new value, system reads data
again (current data), and compares it with the initial read.
 If there is a difference, system sends error message and calls
the transaction to execute itself again, so it works with the current
data.
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ACID Transactions
 The ACID standard ensures data integrity and accuracy (in part) in a
relational database. Differentiates DBS on quality and the relational DB tech.
from non-relational tech.
 Atomicity: all changes succeed or fail together (as “an atom”), i.e., DBS
supports transactions definition and execution.
 Consistency: all data remain internally consistent (when transactions are
executed) and can be validated, i.e., referential integrity supported).
 Isolation: each transaction is processed separately, i.e., DBS has locks
management.
 Durability: When a transaction is committed, data changes are permanently
saved (using log files and other techniques), i.e., DBS has a recovery
capability.
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Methods to Generate Keys
1.
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The DBMS can generate key values automatically whenever a
row is inserted into a table (surrogate key).
Drawback: concurrent access to DB system can mix up
keys that are generated at the same time (e.g., CustomerID
In Customer table, which needs to be reused in the Order
table).
2.
A separate key generator is called by a programmer to create
a new key for a specified table.
Prevents mix-ups but requires that programmers write code
to generate a key for every table and each row insertion.
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Key Generator
• Appropriate key creation also ensures
database integrity and accuracy.
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Customer Table
Create an order for a new customer:
(1) Create new key for CustomerID
(2) INSERT row into Customer
(3) Create key for new OrderID
(4) INSERT row into Order
CustomerID, Name, …
Order Table
OrderID, CustomerID, …
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Database Cursor
 Cursor = A type of variable (memory space) that
holds entire records. A relic from old DB systems
(Declare Cursor… Close Cursor) – no relationship with
screen cursor (user interface).
 Purposes:
 Complex calculations on rows
 Comparisons between rows
Year
1998
1999
2000
2001
Sales
Diff.
104,321
145,998
276,004
362,736
A cursor would read values of sales for the
current and previous year, and calculate the
difference.
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