Dbms module iii
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Dbms module iii Dbms module iii Document Transcript

  • ! "! "! "! " #### TRANSACTION PROCESSING: - It is small unit of database processing, which consist set of operation to perform on the data item. - Every transaction consist two operation ,Read and Write - Read(x): it read the value of data item x from the database to the buffer associated with transaction. - Write(x): it write the value of data item x from the buffer associated with the transaction to the database. - Note: x is the data item, Buffer: is the temporary memory location. - Example transaction are - A) Booking of air ticket - B) Transaction of money from one a/c to another a/c. - A transaction include one or more database operations i) Insertion. ii) Deletion iii) Modification iv) Retrieve - If a database operation in a transaction do not update the database but only retrieve data, the transaction is called as read only operation. - Read-item(x): i) Find the address of disk block that contain x. ii) Copy that disk into buffer in main memory( if the disk block is not already in memory) - Copy item(x) from buffer to the program variable name(x). - Write-item(x): I) Find the address of disk block that contain x. II) Copy the disk block into buffer in main memory. III) Copy x item from program variable need x into correct location in the buffer. IV) Store the updated block from the buffer back to disk.
  • ! "! "! "! " $$$$ Note: the d.b.m.s will generally maintain number of buffer in main memory that hold database disk block contain database items being processed. - When buffer is full, some replacement policy are used to replace the current buffer, this replacement policy is called page replacement. - Example: write transaction for transfer Rs.1000 from a/c A to a/c B. T1: Read (A) A=A-1000 Write (A) Read (B) B=B+1000 Write (B) Transaction Initialization In the transaction most important active agent is user, who will responsible for all initialization process. During the process a transaction has covered some predefined stages. Commit Db modified No Error System Error Begin Error Abort unmodified
  • ! "! "! "! " !!!! A transaction that complete it executation successfully is said to be commmited. - Once the transaction is commied we cannot undo its effect by aborting it. - The only way to undo is to execute a compensating transaction. - Active:it is the intial stage of transaction. A transaction goes in active state after it start executing. - Partially Commited; When all the operation of the transaction has been completed with no error, it goes partially committed state. At this stage some update may still reside in buffer. - Commite: At this point all the updated made by transaction has been properly reflected on the disk from the buffer. Once the transaction is committed, the changes will persist even if system is failure. - Failed: if the normal execution of transaction could not proceed the transaction goes to failed state. - Aborted: once the transaction is fail to complete exection all the changes made by the transaction on D.B must be undone. At this point transaction goes into abort stage. - Terminated: Once the transaction leaves the system, it is said to be terminated. A transaction may be terminated if it has either committed or aborted.
  • ! "! "! "! " Properties of Transaction To maintain dataintegrity and consistency when concurrent takes place.each transaction satisfy the ACID property. ACID ATOMICITY CONSISTENCY ISOLATION DURABILITY Atomicity: this property basically shows the behaviour of transaction inter of unique. -Atomicity means all the operation of the transaction are reflected properly in the database or none of the operation are reflected. - it has to rechecked to termination stage,either followed by commit stage or followed by abort stage. - the basic behind the atomicity is the database system keeps tracks of old value of any data on which transaction perform write and if the transaction doesnot complete its exection, the old value are restored to make it appear as though the transaction never executed. Consistency: This property will basically shows the behaviour of transaction in term of consistency, i.e if the transaction is successful that what ever the changes made by the user should be refleted on the database. - If the transaction is failed/unsucccessfull then the database shows the prexiting thing but discarding the current transaction.
  • ! "! "! "! " %%%% Isolation: it indicates that unique in nature of an transaction i.e if a transaction is carried out (under execution) then the other transaction should not start in between execution,until and unles ist one is terminted. Ex: there are two transaction T1 & T2. It appear to T1 that either T1 finished execution before T1 or T2 started execution after T1 finished. - The isolation property is the responsible of a components of the database system called Concurrency controlled components. Durability: once the execution of the transaction complete successful all the updates that is carriedout on the datbase persist even if there is asystem failure after the transaction is complete execution. It is responsible for recovery management. Note: database must consistent for a long period. It should not hampered lossees of data due to any circumstances. Concurrency Control Concurrency control is the mechanisam of transaction processing system that allow multiple transaction to run at a time inefficent maner so that database should not be inconsistent. As we know that there are numoruous of queries in DBMS insert,delete,update etc, hence the most complexity will get arised then the DB is updated hence the concurrency control is the mechanisam using which we can carryout simoultneously without any repeation.
  • ! "! "! "! " &&&& Problems/Disadvantages: As we know that when two transaction attempts to odify/update the same time,and on some data item.some time it may suffer the following problems susch as. 1) Lost update problem 2) Dirty read problem 3) Unpredictable read Lost Update Problem: This problem occurs when two transaction attempts to mainly update/modify at same time on same data. Then the update made by one transaction T1 on data item ‘x’ is lost due to the update made by another transaction T2 on data item ‘x’. Ex: let us consider two transaction T1 & T2 T1 T2 Read(A) Read(A) A=A+5000 A=A+10000 Write(A) Write(A) Let us consider the schedualr transaction T1 & T2 at same time,intially value of A=3000, what is the value after transaction T2. Ans: T1: Read (A) A=A+5000=3000+5000=8000 Wrie(A) T2: Read(A) A=A+10000 now the value of A is 8000, then
  • ! "! "! "! " '''' A=a=A+10000 A=8000+10000 18000 After the transaction T2 value 18000 Due to loss update problem: Read(A)----- T1 (A=3000) A=A+5000 A=3000+5000=8000 Write(A) Read(A)------ T2 A=A+10000 (A=13000) Write(A) The above schedule the actual value of A is 18000, but it comes to 13000 because transaction T2 Read(A) before the update of T1 is stored in the database. Dirty Read Problem: This problem occurs when one transaction T1 update dataitem X and then transaction fails, the value of X will be rollback to original value.but it means that another transaction T2 has the read the value of X before it can Rollback . The value of X read by T2 is dirty read of data. Since it is the value created by the transaction that has been aborted this problem is known as Dirty read problem.
  • ! "! "! "! " (((( Unpredictable Problem: When more than one transactiobn are occuring cocurrently in the same database and try to manipulate the same attribute of database, in due course of time let 1st transaction makes some changes then after,next transaction make the same changes again and again,by default non of them can reflect on the database.then the database hold with previous value. Or if the transaction makes same changes and before commiting it the other transaction applies a rollback command the database cannot be consistent. Solution for the problem occur due to cocurrency: To avoid the problem occur due to concurrency , uses the protocol. 1) Lock-Based Protocol 2) Timestamp protocol 3) Validation based protocol 4) Multiversion Protocol Characterstics of cocurrency control: 1) Sharing of data maong users 2) Keep tight control on data redudancy 3) It represent the complex relation between different dataitem. 4) Has different interface through which user manipulate data. 5) Enforces data access authorization.
  • ! "! "! "! " )))) Locking Mechanisam it is a variable associated with adata item X that reflects the status of data item X with respect to possible opeartion that can applied to it. - A data item X can be locked in two modes. Or It is the basic mechanisam which is applied to the problem of concurrent schedular that is ,it helps up to make the dat5abase consistent. Types of Lock: 1) Shared Lock (S-Lock) 2) Execlusive Lock (X-Lock)0 Shared Lock: if a transaction T1 has obtained a shared mode lock then T1 can read but write. - it is locking mechanisam which also refered as S-Lock Mechanisam. - This lock mechanisam basically perform the read mechanisam in which if more than one one transaction tries to perform the s-lock mechanisam than it is acceptable (No-Conflict arises). Execlusive Lock: if a transaction has obtained an execlusive mode lock then T1 can both read and write -It is denoted by symbol X - whenever a transaction T has to access a data item, it first request for lock in appropiate mode on data item X. if the lock manager of
  • ! "! "! "! " #*#*#*#* DBMS grant the lock on X to transaction T, then only T can proceed its operation on X. T1: T2 Lock S(A) Lock X(A) Read(A) Read(A) Unlock(A) A=A-1000 Lock D(B) Write(A) Read(B) Unlock(A) Unlock(B) Lock X(B) Display(A+B) Read(B) B=B+1000 Unlock(B) Grant Lock: When a transaction request a lock on a database in a particular mode and no other transaction has lock on same dataitem in a conflicting mode then the lock can be granted. Binary Lock: it is locking process in which the possibility of acquiring the lock over the attributes are either 0’s and 1’s. - Here 0’s represent shared lock which is used for Read Only operation. - 1’s represent both read and write operations.
  • ! "! "! "! " ######## Two-Phase locking Protocol: it is locking protocol ensure that the serializability of multiple operation. - This protocol states that each transaction in a schedule must issue the lock and unlock request in two phases. - 1) Growing Phase - 2) Shrinking Phase Growing Phase: in this phase a transaction request for lock the data but cannot unlock any data item/may not realease lock. Shrinking Phase: in this process a transaction can only release lock on dataitem but cannot request for a lock. - Intially a transaction is in the growing stage, at that time transaction acquire lock on dataitem. - Once the transaction release the lock it enter into shrinking phase and it cannot acquire more lock request. Example: T1: Lock X(A) Read(A) A=A-1000 Write(A) Growing Phase Lock X(B) Read(B) Write(A) B=B+100 Unlock(A) Shrinking Phase Unlock(B)
  • ! "! "! "! " #$#$#$#$ T2: Lock X(A) Read(A) Growing Phase Lock X(B) Read(B) Display (A+B) Unlock(A) Shrinking Phase Unlock(B) Time-Stamp Based Protocol: - In such cases each transaction Ti in the system associated with unique identifier.this unique identifier is called timestamp of Ti denoted as Ts(Ti). - If the transaction Tj enter the system after the transaction Ti then Ts(Ti)<Ts(Tj). There are two simple method of implementing this scheme. - 1) use the value of the system clock as the timestamp - 2) use a logical counter i.e incrment by one after a new timestamp has been assigned. - Ther are two time-stamp a) W-timestamp( ) b) R-timestamp( ) - W-Timestamp( )-it denotes largest timestamp of any transaction that execute write( ) successfully. - R-timestamp( ):- it denotes that the largest timestamp of any transaction that executed Read( ) successfully. - These timestamp are updated whenever a new read( ) or write( ) instruction is executed.
  • ! "! "! "! " #!#!#!#! - When a transaction Ti is rejected and rollback by the system as base on timestamp ordering protocol, then the system assign anew timestamp and is restarted. - The timestamp ensure that freedom from deadlock. Deadlock: it is a situation where two transaction are in wait state. - There are two principle method for dealing with deadlock problem. Deadlock Prevention: protocol ensure that the system never enter a deadlock state. - When a system enter in the deadlock state try to recover using a deadlock-detection and deadlock recovery. - Two approaches a) no cyclic wait can occur by ordering the request for locks.b) perform transaction rollback instead ofv waiting for lock. - In the first approaches each transaction locks all its data items before it begin execution that means either all are locked in one step or none are locked. Deadlock Detection: if a system deadlock occur in the system then the system must attempts to recover from deadlock and system must do the following actions. - A) maitain information about the current transaction of data item to transaction as well as any outstanding datitem request. - B) provide an algorithm that uses this information to determine whether the system has entered a deadlock state. Deadlock Avoidance: a) select the victims-given a setof deadlocked transaction, that must detremine which transaction to be rolledback to break the deadlock.
  • ! "! "! "! " #### b) we should rollback those transaction that will be minimum cost. c) the minimumn cost of transaction can be determined by following factor. -i) how long the transaction hascomputed ii) how many datitems the transaction needs to completee the process. iii) How many transaction will be involved in rollback. Rollback: once we have decided that a particular transaction must be rolledback,we must determine how far this transaction should be rolledback. Starvation: in a system where the selection of victim is based on cost factor . it may happen that the same transaction is always pickd as a victim.as a result the transaction never complete it designated task. Such situation is called starvation. Data Recovery System A data recovery is the process which is used to recover the valuable data of database then it is subjected to under failure. Types of failure: A database may undergoes failure state due to the following reason. a) Transaction failure: there are two types of error causes of transaction failure. i) Logical error: the transaction cannot executed because bad input data, data not found.
  • ! "! "! "! " #%#%#%#% ii) System failure: the system has entered undesirable state such as deadlock,as a result the transaction cannot be executed. b) System Crash: there is H/W problems or error the database softwares or error in operating system, that causes the loss of content of volitile storage and transaction execution is halt. c) Disk Failure: a diskblock losses its content a s result of either head crash or failure a data transfer operation. d) Storage Failure: it is one kind of natural failure where mainly envirnoment is responsible. Database Recovery: It is a technique that restore the databasec to the most recent consistent state that execited before failure. a) Forward recovery (Reading) b) Backward recovery (Undoing) c) Media Recovery. Recovery Techniques: The most widely used scheme for the database recovery is log- based recovery. Log-based recovery: the structure used for recording the database modification is called the log. - The log is sequence of log records and maintain record of all update activities in the dtabase. - A log record has four field - A) Transaction identifier: it is unique identifier of the transacrtion that performed the write operation. - B) Data item identifier: it is unique identifier of the dataitem written.
  • ! "! "! "! " #&#&#&#& - C) Old-value: it is value of data item prior to write - D) New Value: t is value of that the data item will have after the write.