Post on 30-May-2018
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Introduction to Transaction
Processing
DBMS
8/14/2019 Introduction to Transaction Processing 2
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Typical OLTP Environments
• Airline/ Railway Reservation Systems
• Banking Systems (ATM, EFT, ...)
• Trading and Brokerage Systems
• Hotel / Hospital Systems
• Standard Commercial Systems
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Types of Failures
• occurs several times a week
• recovery required in a fewminutes
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Types of Failures
• occurs intermittently
• recovery time depends on the nature of
failure
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Types of Failures
• occurs once or twice a year
• recovery required in a few hours
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Types of Failures
• occurs 10-100 times in a minute
• recovery required in transaction execution
time
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Motivating Scenario
Bank 1 Bank 2
0: (Acct.2565) bal += 2500
3: (Acct.165) bal -= 2500
Account #165 from bank 2 sends Rs. 2500/- to Account #2565
in bank 1.
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Motivating Scenario
Bank 1 Bank 2
0: (Acct.2565) bal += 2500
3: (Acct.165) bal -= 2500
Account #165 from bank 2 sends Rs. 2500/- to Account #2565
in bank 1.
B a n k
2 c r a
s h e s a t t
i m e =
2
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Motivating Scenario
Bank 1 Bank 2
0: (Acct.2565) bal += 2500
1: (Acct.165) bal -= 1500
3: (Acct.165) bal -= 2500
Account #165 from bank 2 sends Rs. 2500/- to Account #2565
in bank 1.
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Transactions
• A transaction is a logical unit of program
execution
• A combination of database updates which
have to be performed together
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What is a Transaction?
A logical unit of work.A logical unit of work.
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What is a Transaction?
A unit of work A unit of work
with respect towith respect toconcurrency and recovery.concurrency and recovery.
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What is a Transaction?
A sequence of operationsA sequence of operations
including database operationsincluding database operations
that is atomic with respect tothat is atomic with respect to
concurrency and recovery.concurrency and recovery.
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What is a Transaction?
An atomic execution unit that,An atomic execution unit that,
when applied to a consistentwhen applied to a consistent
database, generates adatabase, generates a
consistent but possiblyconsistent but possibly
different database.different database.
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What is a Transaction?
short sequence of operationsshort sequence of operationsith the database whichith the database which
epresents one meaningfulepresents one meaningful
ctivity in the user's environment.ctivity in the user's environment.
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ACID Property of Transactions
• Atomicity: Either all updates are performed or none
•Consistency: If the database state at the start of atransaction is consistent, it will be consistent at theend of the transaction
• Isolation: When multiple transactions are executedconcurrently, the net effect is as though eachtransaction has executed in isolation
• Durability: After a transaction completes(commits), its changes are persistent
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Atomicity
Consider the case of funds transfer from account A to account B.
A.bal -= amount;
B.bal += amount;
A.bal -= amount;
CRASH
…
…
RECOVERY
A.bal += amount;
Rollback
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Consistency
Consider the case of funds transfer from account A to account B.
A.bal -= amount;
B.bal += amount;
B.bal += amount;
A.bal -= amount (FAILS!! A’s balance is 0)
B.bal -= amount;
Rollback
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Isolation
Consider the case of funds transfer from account A to account B.
Transaction T1:
A.bal -= amount; (Let A’s balance become 0 after this…)B.bal += amount;
Transaction T2:
A.bal -= amount2;
Net effect should be either T1,T2 (in which case T2 fails) or
T2,T1 (in which case T1 fails)
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Durability
Consider the case of funds transfer from account A to account B.
Account A should have a balance of amount
Transaction T1:
A.bal -= amount;
B.bal += amount;
Commit
Account A should have a balance of 0.
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Transaction States
Active
Partially
Committed
Failed
Committed
Aborted
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ACD using Shadow Copy
DB
DBCopy of
DB
DB’
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ACD Using Shadow Copy
• Atomicity: Delete old DB on commit or
new DB on Failed transactions (all or
nothing)• Consistency: Delete new DB, if update
fails.
• Isolation: Not supported..
• Durability: One of the copies is persistent
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Serializability
On executing a set of concurrent transactions on a database the
net effect should be as though the transactions were executed
in some serial order.
Transaction T1:
read (A)
A = A – 50;write (A)
read (B)
B = B + 50;
write (B)
Transaction T2:
read (A)
t = A * 0.1;
A = A – t;write (A)
read (B)
B = B + t;
write (B)
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Serial Schedules
read (A)
A = A – 50;
write (A)
read (B)B = B + 50;
write (B)
read (A)
t = A * 0.1;A = A – t;
write (A)
read (B)
B = B + t;
write (B)
Equivalent to T1
followed by T2
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Serial Schedules
read (A)
A = A – 50;write (A)
read (B)
B = B + 50;
write (B)
read (A)
t = A * 0.1;
A = A – t;
write (A)read (B)
B = B + t;
write (B)
Equivalent to T2
followed by T1
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Serial Schedules
read (B)
B = B + t;write(B)
read (B)
B = B + 50;
write (B)
read (A)
t = A * 0.1;
A = A – t;
write (A)read (A)
A = A – 50;
write (A)
Equivalent to T1
followed by T2
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Conflict Serializability
Let instructions I and J belonging to transactions T1 and T2
be executed consecutively by the DBMS.
1. I and J can be swapped in their execution order if I and J
refer to different data elements
2. I and J can be swapped in their execution order iff I and J
refer to the same data element and both perform a read
operation only.
3. I and J are said to conflict if I and J belong to different
transactions and at least one of them is a write operation.
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Conflict Serializability
• If a schedule S can be transformed to another S’ by
swapping non conflicting instructions, then S and S’ are
said to be conflict equivalent .
• A schedule S is said to be conflict serializable if it is
conflict equivalent to some serial schedule.
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View Serializability
If S is a schedule, then S’ is a view equivalent schedule if
• For each data item Q, if a transaction Ti reads the initialvalue of Q in S, then it should read the initial value in S’
also• In schedule S, for each data item Q, if write(Q) of Tj
precedes read(Q) of Ti, it should be the same in S’
• The same transaction that performs the final write(Q) in S,
should perform it in S’.
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View Serializability
T1:
Read(Q)
Write (Q)
T2:
Write (Q)T3:
Write(Q)
A view equivalent schedule:
T1: Read(Q)T2: Write(Q)
T1: Write(Q)
T3: Write(Q)
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View Serializability
• Every conflict serializable schedule is alsoview serializable; however some view
serializable schedules are not conflictserializable
• Example in previous slide
• A schedule that is view serializable but notconflict serializable is characterized byblind writes.
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Recovery
• So far study of schedules that are acceptable fromthe viewpoint of consistency of database-
assuming no transaction failures.
• If transaction T fails- undo the effect of the
transaction to ensure atomicity.• Also, it is necessary to abort any other transaction
T1 that is dependent on T.
• To achieve the above two, restrictions on the typeof schedules permitted has to be laid down.
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Recoverable Schedules
• Consider the following schedule
T8 T9
read(A)
write(A)
read(A)
read(B)
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Recoverable Schedules
• Suppose T9 commits before T8• If T8 fails before it commits then T9 also
has to be aborted.
•However, T9 is already committed.• A situation where it is impossible to recover from the failure of T8
• This is an example of non recoverableschedule
• Database systems require recoverableschedules.
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Cascading Rollback
• Even if a schedule is recoverable, to recover from the
failure of a transaction, there is a need to rollback severaltransactions.
T T1 T2
read(A)
read(B)write(A)
read(A)
write(A)
read(A)if T fails, then it will lead to rolling back T1 and T2.
This is an example of cascading rollback.
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Cascadeless Schedule
• Cascading rollback is undesirable- leads toundoing a lot of work
• Restrict the schedules to those where
cascading rollbacks do not occur.• Such schedules are cascadeless schedule.
l f l
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Implementation of Isolation
• Concurrency-control schemes to ensure that even
when multiple transactions are executedconcurrently,only acceptable schedules aregenerated.
• An example, a transaction acquires a lock on an
entire database before it starts and then releasesthe lock after it finishes.
• No other transaction is allowed to access thedatabase.
• Therefore, only serializable schedules are produced.
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Implementation of isolation …
• The above discussed scheme leads to poor
performance.
• Goal of such schemes is to provide a highdegree of concurrency.
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Transaction definition in SQL
• SQL provides TCL(Transaction controllanguage)
• Commit- commits the current transaction
and begins a new one• Rollback- causes the current transactions to
abort
f l b l
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Testing for serializability
• When designing concurrency control schemes, wemust show that schedules generated by the schemeare serializable.
• A directed graph called precedence graph is used
for this purposeT1 ------- T2
• This indicates that all transactions of T1 areexecuted before T2
• T2 ------ T1• This indicates that all transactions of T2 are
executed before T1
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Testing for Serializability…
• If the precedence graph for a schedule s has a
cycle, then s is not conflict serializable.
• If the precedence graph for a schedule s does nothave a cycle, then s is conflict serializable.
• Serializability order of the transaction obtained
through topological sorting which determines a
linear order consistent with the partial order of the precedence graph