Concurrency Control

R/RR/W W/W User 2 ReadWrite User 1 Read Write R/W: Inconsistent Read problem. W/W: Lost Update problem.

Example Husband/Wife joint account with $1000 balance. Transactions: –Husband: Withdraw 800 –Wife: Withdraw 100 Processing: –Read Balance, Calculate New Balance, Write New Balance

Husband:ReadBalanceCalNewBalanceWriteNewBalance (In memory)(On disk) 1000New= Wife:ReadBalanceCalNewBalance WriteNewBalance 1000New=

Serializable Schedule Serial Schedule: The operations of each concurrent transaction are executed consecutively without any interleaved operations from other transactions. Nonserial Schedule: The operations from a set of concurrent transactions are interleaved. –Maximizing database availability Serializable schedule: If a set of transactions executes concurrently, the nonserial schedule is called serializable if it produces the same results as a serial schedule.

Locking Locking is the most widely used approach to ensure serializability of concurrent transactions. Shared lock: read only access Exclusive lock: for both read and write access.

Two-Phase Locking A transaction must acquire a lock (read or write) on an item before operating on the item. Once the transaction releases a lock, it cannot acquire any new locks. All locking operations precede the first unlock operation in the transaction: –Growing phase –Shrinking phase

Lock Granularity The size of data items protected by a lock. –Entire database –Entire table –A page –A record –A Field The coarser the data item size, the lower the degree of concurrency permitted.

Dead Lock Two transactions wait for locks on items held by the other. T1T2 DataItem 1 DataItem 2 Lock Wait For Lock

Transaction An unit of work on database that is either completed in its entirety or is not performed at all.

Transaction ACID Properties Atomic –Transaction cannot be subdivided –All or nothing Consistent –Constraints don’t change from before transaction to after transaction –A transaction transforms a database from one consistent state to another consistent state. Isolated –Transactions execute independently of one another. –Database changes not revealed to users until after transaction has completed Durable –Database changes are permanent and must not be lost.

Transaction Commands Begin Transaction Update commands Commit RollBack End Transaction

Begin Transaction Active State Read/ Write Partially Committed Failed aborted Abort Commit

DefiningTransaction in An Application Truck Rental System: –Vehicle Table:VID, VType, VStatus » V1PickUp Available » V2TowTruck Booked –VReservation:RID, VID, Date » R1V21/2/04

Transaction Example (Pseudo Code) Sub Rent(RID, VID, RDate) Begin Transaction Insert (RID, VID, RDate) into VReservation table If No Error Then Update Vehicle Status If No Error Then Commit Transaction Else Roll Back End if Else Roll Back End if End Sub

Log File (Journal) A file that contains all information about all updates to the database. It may contain the following data: –Transaction records: Transaction ID Type of action: –Begin, Insert,Delete, Modify, Commit, Rollback, End Before-image After-image –Checkpoint records The point of synchronization between the database and the transaction log file.

Completed Transaction All calculations done by the transaction in its work space (RAM) must have finished, and a copy of the results of the transaction must have been written in a secure place (log file). The action of committing the transaction must also be written in the log.

Two Phase Commit A transaction cannot commit until it has recorded all its changes in the log. A transaction cannot write into the database until it has committed.

To Recover In the event of a failure, examine the log starting from the most recent checkpoint record. Any transaction with Transaction Start and Transaction Commit records should be redone: –Perform all the writes to the database using the after-image log records in the order in which they were written to the log.

Back-up Facilities Automatic dump facility that produces backup copy of the entire database Periodic backup (e.g. nightly, weekly) Cold backup–database is shut down during backup Hot backup–selected portion is shut down and backed up at a given time Backups stored in secure, off-site location

Database Security

Database Security: Protection of the data against accidental or intentional loss, destruction, or misuse Increased difficulty due to Internet access and client/server technologies

Threats to Data Security Accidental losses attributable to: –People Users: using another person’s means of access, viewing unauthorized data, introduction of viruses Programmers/Operators Database administrator: Inadequate security policy –Software failure DBMS: security mechanism, privilege Application software: program alteration –Hardware failure Theft and fraud Improper data access: –Loss of privacy (personal data) –Loss of confidentiality (corporate data) Loss of data integrity Loss of availability (through, e.g. sabotage)

Figure 12-3 Possible locations of data security threats

Countermeasures to Threats Authorization –Authentication Access controls: privileges Database views BackUp and Recovery Enforcing integrity rules Encryption –Symmetric encryption:use same key for encryption and decryption –Asymmetric encryption: Public key: for encryption Private key: decryption RAID

Authorization Rules Controls incorporated in the data management system  Restrict: –access to data –actions that people can take on data  Authorization matrix for: –Subjects –Objects –Actions –Constraints

Figure 12-5 Authorization matrix

SQL Injection "SQL Injection" is an unverified/unsanitized user input vulnerability, and the idea is to convince the application to run SQL code that was not intended. Exploits applications that use external input for database commands.

SQL Injection Demo On a web page that takes customer ID entered in a textbox as input, then displays the customer’s data. In the textbox, enter: ‘ OR 1=1 OR CID = ‘ SQLInjectionDemo Other SQL injection examples:

Demo Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click Dim strConn As String = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source = c:\salesDB.mdb" Dim objConn As New OleDbConnection(strConn) Dim strSQL As String = "select * from customer where cid = '" & TextBox1.Text & "'" Dim objComm As New OleDbCommand(strSQL, objConn) Try objConn.Open() Dim objDataReader As OleDbDataReader objDataReader = objComm.ExecuteReader() GridView1.DataSource = objDataReader GridView1.DataBind() Catch except As SystemException Response.Write(except.Message) End Try End Sub