1. DB Concurrency ITEC 340 Database I Dr. Ian Barland
(and, access privileges)
ITEC 340
Database I
Dr. Ian Barland
based on notes by Dr. Pittges, Dr. Phillips

2. Allowing Multi-user Access
GRANT select on Students to fred;
GRANT select, insert ON Customers to ALL;
GRANT ALL on Customers to fred;
CREATE ROLE manager_privs;
GRANT select, insert, update
ON Customers
TO manager_privs;
GRANT manger_privs TO amy, bob, fred;

3. grant syntax Grant Revoke Very effective when used with views
GRANT <objPriv> ON <object> TO <user>
GRANT <sysPriv> ANY TABLE TO <user>
<user> can also be PUBLIC, or a role
<objPriv> can be ALL
<sysPriv> includes all <objPrivs> plus CREATE
Revoke
REVOKE <objpriv> ON <object> FROM <user>
REVOKE <syspriv> FROM <user>
Very effective when used with views

4. Oracle Object Privs ALTER DELETE EXECUTE INDEX INSERT SELECT UPDATE
ALL

5. Problems with Concurrent Access
Lost updates
Uncommitted Data
Inconsistent retrievals
Solution
Resource Locking: While a user (transaction) holds a lock, no other user can access that resource.

6. Locking Granularity lock entire database lock one table lock one page
Not efficient
lock one table
Sometimes needed for batch updates
Not very efficient
lock one page
Usually contains multiple rows
Locks a unit corresponding to a the working unit of the OS
lock one row
Most commonly used locking level
Very efficient
lock one column
Locks only certain attributes of a row.

7. How to specify locking in Oracle
Table Lock Command
the 'FOR UPDATE' option of SELECT command:
Select *
From Customer
Where CustID = '123456'
For update; -- If I can't get lock, keep waiting until I can (“block”)
Select *
From Customer
Where CustID = '123456'
For update WAIT 5; -- Try to get lock for 5sec, before throw exception.
Select *
From Customer
Where CustID = '123456'
For update NOWAIT; -- If I can’t get lock, immediately throw exception.

8. Lock Types and Strategies
Exclusive vs. Shared Locks (lock type)
Exclusive
Will not allow another transaction to obtain a lock of any kind.
Shared
Allows multiple transactions to read the data.
Will not allow any other transaction to write the locked rows.
Can potentially be upgraded to an exclusive lock
Optimistic vs. Pessimistic Locking (locking strategy)
Pessimistic (“lock, ‘cause somebody’s almost surely going to interfere”)
Rows are locked at point of client retrieval of data
Client application will not proceed if locks cannot be obtained
Locks are not released until client application submits completed transaction
Optimistic (“nobody's likely to interfere w/ me, so minimize locking”)
No locking occurs until the transaction is ready for submission by the client program
Can use several techniques to avoid business rule violations
Check row image before updating (timestamp, sequence number, or compare fields)
Update only changed fields
Use incremental updates to balances
Use conditional transactions with a timer
Client refreshes (not typically an option on web applications)

9. Pessimistic Locking Strategy Steps
Request Lock(s)
Retry (wait) or Abort
Lock(s) Acquired
Read
ABORT/rollback
Ponder
Update
Commit/Release Lock(s)

10. Optimistic Locking Strategy Steps
Read
Ponder
Request Lock(s)
Retry (wait) or Abort
Lock(s) Acquired
ABORT/rollback
Check for Mutation
Mutation Detected
Update *
* update should include update to timestamp or mutation sequence number.
Commit/Release Lock(s)

11. Transactions Transaction Definition: Example:
A logical unit of work that consists of one or more SQL statements that moves the database from one consistent state to another consistent state. Therefore, the transaction must succeed or fail as a unit.
Example:
01/01/02 Journal Entry #87543
110 Accounts Receivable – R. Smith(123) 1,045
401 Service Revenue – tax services 645
402 Service Revenue – consulting services 400
(provided services on account)
Would cause multiple related inserts and updates

12. Oracle: No Transaction Defined
Begin
Update accounts set balance = balance where accountnum = ‘110’;
Commit;
Update accounts set balance = balance where accountnum = ‘401’;
Update accounts set balance = balance where accountnum = ‘402’;
Update customers set BalDue = BalDue where custid = 123;
Insert into JournalEntries (JENumber, JEDate, Description)
values (87543, ‘01-JAN-02’, ‘provided services on account’);
Insert into DrCrDetail (JENumber, LineNumber, Account, Amount)
values (87543, 1, ‘110’, );
values (87543, 2, ‘401’, );
values (87543, 3, ‘402’, );
End;
General guide: Commit as soon as you can (but no sooner).

13. Transaction Methods (Oracle)
BEGIN [transactName] is optional.
COMMIT [TRAN[SACTION] transactName] saves the changes and ends the current transaction.
SAVEPOINT [savepointName] optional, creates a point that can be rolled back to
ROLLBACK [TO savepointName] ends the current transaction and restores the databases in the Workspace object to the state they were in when the current transaction began.
The transactName is ignored – it’s purely for readability in nested commits.

14. Transaction Methods (Oracle with lock waits)
Begin
Begin Transaction;
Update accounts set balance = balance where accountnum = ‘110’;
Update accounts set balance = balance where accountnum = ‘401’;
Update accounts set balance = balance where accountnum = ‘402’;
Update customers set BalDue = BalDue where custid = 123;
Insert into JournalEntries (JENumber, JEDate, Description)
values (87543, ‘01-JAN-02’, ‘provided services on account’);
Insert into DrCrDetail(JENumber, LineNumber, Account, Amount)
values (87543, 1, ‘110’, );
Insert into DrCrDetail (JENumber, LineNumber, Account, Amount)
values (87543, 2, ‘401’, );
values (87543, 3, ‘402’, );
Commit;
Exception
rollback;
End;

15. Transaction/Locking Problem
Deadlock
Solutions
Prevention
e.g.: Two-phase locking with nowait
Dectection
Detect, then kill and rollback one transaction

16. Transaction Methods (Oracle with 2 stage locking)
Begin
Begin Transaction;
Select * from accounts
where accountnum in (‘110’, ‘401’, ‘402’) for update nowait;
Select * from customers
where custid = 123 for update nowait;
Update accounts set balance = balance where accountnum = ‘110’;
Update accounts set balance = balance where accountnum = ‘401’;
Update accounts set balance = balance where accountnum = ‘402’;
Update customers set BalDue = BalDue where custid = 123;
Insert into JournalEntries (JENumber, JEDate, Description)
values (87543, ‘01-JAN-02’, ‘provided services on account’);
Insert into DrCrDetail(JENumber, LineNumber, Account, Amount)
values (87543, 1, ‘110’, );
Insert into DrCrDetail (JENumber, LineNumber, Account, Amount)
values (87543, 2, ‘401’, );
values (87543, 3, ‘402’, );
Commit;
Exception
rollback;
End;

17. SELECT FOR UPDATE WAIT <seconds>
Begin
Begin Transaction;
Select * from accounts
where accountnum in (‘110’, ‘401’, ‘402’) for update wait 3;
Select * from customers
where custid = 123 for update wait 3;
Update accounts set balance = balance where accountnum = ‘110’;
Update accounts set balance = balance where accountnum = ‘401’;
Update accounts set balance = balance where accountnum = ‘402’;
Update customers set BalDue = BalDue where custid = 123;
Insert into JournalEntries (JENumber, JEDate, Description)
values (87543, ‘01-JAN-02’, ‘provided services on account’);
Insert into DrCrDetail(JENumber, LineNumber, Account, Amount)
values (87543, 1, ‘110’, );
Insert into DrCrDetail (JENumber, LineNumber, Account, Amount)
values (87543, 2, ‘401’, );
values (87543, 3, ‘402’, );
Commit;
Exception
rollback;
End;