Advanced Database Technologies Lecture 6: Transactions and Database Recovery

Advanced Database Technologies

Lecture 6: Transactions and Database Recovery

CG096 Advanced Database Technologies Lecture 6: Transaction Processing and Recovery 2

Content

What is a Transaction? ACID properties Transaction Processing Database Recovery


1. What is a Transaction?Definition The sequence of a logically linked actions that access a

shared database, usually on behalf of on-line usersExamples Airlines operation: Reserve an airline seat. Buy an

airline ticket. Assemble cabin crew. Fly. ATM Cash operation: Check credentials. Check

money. Withdraw amount. Pay amount. Credit card sale: Log on the card. Verify credit card

details. Check money. Handle. Issue withdrawal. Internet sale: Request an item from an on-line

catalogue. Check availability. Provide credit card details. Check details. Issue order. Dispatch. Issue withdrawal.


DatabaseApplication

DatabaseServer

UserUser...

DataSchema

DatabaseApplication

DatabaseApplication

DataSchema Data

Schema

SystemDictionary

UserDirectory

DBMS

DB Clients

Origin and Needs for Transactions in DB


Automated Teller Machines (ATM)

ATMn

Bank Server

BankNetwork

ATM1

AccountsDatabase

AmountRequested

BalanceChecked

WithdrawalConfirmed

CashReceived

AccountDebited

Cash Withdrawal Transaction

Start Finish


2. A.C.I.D. properties

Transactions have 4 main properties Atomicity - all or nothing Consistency - preserve database integrity Isolation - execute as if they were run alone Durability - results aren’t lost by a failure


2.1 Atomicity

All-or-nothing, no partial results. E.g. in a money transfer, debit one account, credit the

other. Either both debiting and crediting operation succeed, or neither of them do.

Successful completion is called Commit (Commit statement in SQL), unsuccessfull - Rollback (statement Rollback in SQL)

Transaction failure is called Abort Commit and abort are irrevocable actions. An Abort undoes operations that already executed

For database operations, restore the data’s previous value from before the transaction (Rollback-it);

But some real world operations are not undoable.Examples - transfer money, print ticket, fire missile


2.2 Consistency

Every transaction should maintain DB consistency Referential integrity - E.g. each order references an

existing customer number and existing part numbers

The books balance (debits = credits, assets = liabilities)

Consistency preservation is a property of a transaction, not of the database mechanisms for controlling it (unlike the A, I, and D of ACID)

If each transaction maintains consistency, then serial executions of transactions do too.


2.3 Isolation

Intuitively, the effect of a set of transactions should be the same as if they ran independently. Formally, an interleaved execution of transactions is

serializable if its effect is equivalent to a serial one. Implies a user view where the system runs each user’s

transaction stand-alone.

Of course, transactions in fact run with lots of concurrency, to use device parallelism. Transactions can use common data (shared data) They can use the same data processing mechanisms (time sharing)


2.4 Durability

When a transaction commits, its results will survive failures (e.g. of the application, OS, DB system … even of the disk).

Makes it possible for a transaction to be a legal contract.

Implementation is usually via a log DB system writes all transaction updates to its log to commit, it adds a record “commit(Ti)” to the

log when the commit record is on disk, the transaction

is committed. system waits for disk ack before acking to user


3. Transaction Processing

Identifying critical points for database changes through set of database states

Preparation for the control over transaction progress using labels of the transaction states

Management of the transactions using explicit manipulation of transaction states and enforcing transaction operations

Can be automatic (controlled by the RDBMS) or programmatic (programmed using SQL or other supported programming languages, like PL/SQL)


3.1 Database State and Changes

D1, D2 - Logically consistent states of the database data

T - Transaction for changing the databaset1, t2 - Absolute time before and after the transaction

State D1 State D2

T

t1 t2


Transaction Parameters

diff D = D2 D1 can have different scale: single data item in one memory area many items across several files and databases structural changes like new database schema, etc.

t = t2 - t1 is the time for executing T

T occupies real physical resources between D1 and D2 there may be intermediate states D11, D12

…; some of them can be inconsistent the final state D2 state could be unreachable

When T fails we should first come back to D1 (recovery) then try again to reach D2 (redo)


Transaction Operations For recovery purposes the system needs to keep track of when a transaction starts, terminates and commits. begin: marks the beginning of a transaction execution; end: specifies that the read and write operations have ended

and marks the end limit of transaction execution; commit: signals a successful end of the transaction. Any

updates executed by the transaction can be safely committed to the database and will not be undone;

rollback: signals that the transaction has ended unsuccessfully. Any changes that the transaction may have applied to the database must be undone;

undo: similar to rollback but it applies to a single operation rather than to a whole transaction;

redo: specifies that certain transaction operations must be redone to ensure that all the operations of a committed transaction have been applied successfully to the database;


Reading and WritingSpecify read or write operations on the database items that areexecuted as part of a transaction read (X): reads a database item named X into a program

variable also named X.1. find the address of the disk block that contains item X2. copy that disk block into a buffer in the main memory3. copy item X from the buffer to the program variable

write (X): writes the value of program variable X into the database

1. find the address of the disk block that contains item X2. copy that disk block into a buffer in the main memory3. copy item X from the program variable named X into its

current location in the buffer 4. store the updated block in the buffer back to disk (this

step updates the database on disk)


active partially committed committed

aborted terminated

BEGIN

READ , WRITE

END

ROLLBACKROLLBACK

COMMIT

3.2 Transaction State and Progress

A transaction reaches its commit point when all operations accessing the database are completed and the result has been recorded in the log. It then writes a [commit, <transaction-id>] and terminates.

When a system failure occurs, search the log file for entries[start, <transaction-id>]

and if there are no logged entries [commit, <transaction-id>]then undo all operations that have logged entries

[write, <transaction-id>, X, old_value, new_value]


Logging transaction states

Saving the initial database state D1 before starting the transaction T: D1->D2 (transaction begins)

Saving all intermediate states D11, D12 … (checkpoint logs) In the case of a failure at an intermediate state D1i before reaching

D2, restore D1 (rollback); the simplest strategy is to apply a series of atomic actions R which change the state to the initial state R: D1i->D1

In the case of successful reach of the last intermediate state D2,

force-write or flush the log file to disk and change the database state to it (transaction ends);

Note: if the transactions are controlled in SQL(using COMMIT), the rollback operation should be initiated explicitly (using ROLLBACK)


17

Entries in the log file [start, <transaction-id>]: the start

of execution of the transaction identified by transaction-id

[read, <transaction-id>, X]: the transaction identified by transaction-id reads the value of database item X

[write, <transaction-id>, X, old-value, new-value]: the transaction identified by transaction-id changes the value of database item X from old-value to new-value

[commit, <transaction-id>]: the transaction identified by transaction-id has completed all data manipulations and its effect can be recorded

[rollback, <transaction-id>]: the transaction identified by transaction-id has been aborted and its effect lost

Procedure Credit ( trans_id INTEGER, accno INTEGER, bcode CHAR(6), amount NUMBER) old NUMBER; new NUMBER; begin SELECT balance INTO old FROM account WHERE no = accno and branch = bcode;

new := old + amount; UPDATE account SET amount = new WHERE no = accno and branch = bcode;

COMMIT;

EXCEPTION WHEN FAILURE THEN ROLLBACK; END credit;


Controlling Subtransactions

All intermediate states of the transaction which are end states of the subtransactions included should become consistent database states

In the case of successful reach of an intermediate state of such kind the actions are

temporary suspension of transaction execution forced writing of all updated database blocks in main

memory buffers to disk and flush the log file resuming of transaction execution

Note: If the transactions are controlled in SQL, the rollback operation can be stepped to an intermediate state which is labeled (using ROLLBACK TO <label>)


Adding checkpoints to the log file

A [checkpoint, <label>] record is created each time new checkpoint is encountered

[commit,<transaction-id>] entries for the active subtransactions are automatically written when the system writes out to the database the effect of write operations of the successful transaction

In the case of a rollback to a given checkpoint within a transaction an entry [commit,<transaction-id>] is logged against this subtransaction

In the case of a rollback of the global transaction to a given checkpoint all subtransactions will not be committed either


4. Database Recovery

Need for recovery from failure during transaction for preventing the loss of data to avoid global inconsistency of the database to analyze the possible reasons for failure

Factors to be accounted for database recovery: what is the nature of the failure when it happened in the transaction what we need to recover


4.1 Categories of Transactions at Failure

tcheck tfa il

T1

T2

T3

T4

T5

T1 - Can be ignored (committed before the previous checkpoint)T2 - Must Redo complete (the database will be rolled back to a state when the transaction was not committed)T3 - Must Undo (not finished, and rollback to a state when not finished)T4 - Must Redo if possible (finished, but not committed)

T5 - Must Undo (did not finish and the rollback will lead to a state before it was even started)


4.2 Types of Failure

Catastrophic failure Restore a previous copy of the database from archival

backup Apply transaction log to copy to reconstruct more current

state by redoing committed transaction operations up to failure point

Incremental dump + log each transaction Non-catastrophic failure

Reverse the changes that caused the inconsistency by undoing the operations and possibly redoing legitimate changes which were lost

The entries kept in the system log are consulted during recovery.

No need to use the complete archival copy of the database.

If an error or hardware/software crash occurs between the begin and end of transaction, the database will be inconsistent


4.3 Recovery Strategy

Mirroring keep two copies of the database and maintain them

simultaneously Backup

periodically dump the complete state of the database to some form of tertiary storage

System Logging the log keeps track of all transaction operations

affecting the values of database items. The log is kept on disk so that it is not affected by failures except for disk and catastrophic failures.


Deferred Update: no actual update of the

database until the transaction reaches its commit point

1. Updates recorded in log 2. Transaction commit

point 3. Force log to the disk 4. Update the database

Immediate Update: the database may be updated

by some operations of a transaction before it reaches its commit point.

1. Update X recorded in log 2. Update X in database 3. Update Y recorded in log 4. Transaction commit point 5. Force log to the disk 6. Update Y in database

FAILURE!• REDO database from log entries• No UNDO necessary because database never altered

FAILURE!• UNDO X

FAILURE!• REDO Y

FAILURE!• UNDO in reverse order in log• REDO in committed log order (uses the write log entry)

Write-ahead Logging

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Advanced Database Technologies Lecture 6: Transactions and Database Recovery