Concurrent transactions

CONCURRENT TRANSACTIONS
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Technical Seminar - 2004
Presented By
Soumya Ranjan Biswal
Roll no-IT200118069
Under the guidance of
Mr. Anisur Rahman
Presented By:- Soumya Ranjan Biswal(IT200118069)

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Introduction
• A transaction is a unit of program execution that accesses and
possibly updates various data items in a database system.
• The transactions are required to have the ACID properties:
atomicity, consistency, isolation, and durability.
• Transaction processing system usually allows both single as
well as multiple transactions to run.
• Concurrent execution of transactions improves throughput of
transactions & system utilization & also reduces the waiting
time of transactions.

Transaction Concepts
• A transaction has the following properties:
Atomicity: All operations of the transaction are reflected
properly in the database, or none are.
Consistency: Execution of transaction in isolation preserves the
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consistency of the database.
Isolation: In this each transaction is unaware of other
transactions executing concurrently in the
system.
Durability: After a transaction completes properly, the changes
it has made to the database is retained even if there
are system failures.

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Transaction State
• A transaction from the beginning to the end, must be in one of
the following states:
Active –Transaction stays here while executing.
Partially Committed - After the final statement has
been executed.
Failed – If normal execution can no longer proceed.
Aborted – Transaction has been rolled back &
database restored to its prior state.
Committed – Transaction is successful.

Concurrent Execution
• Concurrent execution means running side by side or parallely
of transactions.
• Advantages of Concurrent execution are:
Improved throughput & Resource utilization – i.e. no.
of transactions executed increases in a given amount of
time & the processor is utilized properly.
Reduced Waiting time – The unpredictable delays in
running transactions as well as the average response time
is reduced.
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Serializability
• The transaction is a unit that preserves consistency, and serial
execution of transaction guarantees that consistency is
preserved.
• A schedule in a transaction captures the key action of
transaction that affects concurrent execution such as read &
write operation.
• So required is that any schedule produced by concurrent
processing set of transactions will have an effect equivalent to
a schedule produced when these transactions are run serially.
• A system that guarantees this property is said to ensure
serializability.

• There are 2 types of serializability:
- Conflict serializability
- View serializability
• Serializability of schedules generated by concurrent
transactions can be ensured through concurrency control
schemes.
• The schedules should have the following properties:
- they must be recoverable.
- they must be cascadeless.
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Transaction Definition in SQL
• It is a data manipulation language which specifies the set of
action that constitute a transaction.
• The SQL standard specifies that the transactions begin
explicitly.
• The transactions are ended by any one of the following SQL
statements:
- Commit work: commits the current transaction &
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begins a new one.
- Rollback work: It aborts the current transaction.

Concurrency control
• When transactions execute concurrently in the database , the
consistency of data may not be preserved . So in order to achieve
control , various Concurrency-control schemes are used.
• Concurrency-control schemes are also used to ensure
serializability. All these schemes either delay an operation or
abort the transaction that issued the operation.
• Most commonly used Concurrency-control schemes are:
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-locking protocols
-timestamp based protocols

Lock based protocols
• A locking protocol is a set of rules that state when a
transaction may lock and unlock each of the data items in the
database.
• Two-phase locking protocol: this protocol allows a
transaction to lock a new data item only if that transaction has
not yet unlocked any data item. This protocol ensures
serializability, but not deadlock freedom.
• Strict two-phase locking protocol: It permits release of
exclusive locks only at the end of transactions, in order to
ensure recoverability and cascadelessness of the resulting
schedules.
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Lock based protocols
• Rigorous two-phase locking protocol: This protocol releases
all locks only at the end of the transaction.
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Timestamp-based protocols
• A timestamp-based protocol ensures serializability by
selecting an ordering in advance between every pair of
transactions.
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• It works in the following manner:
-first a unique fixed timestamp is associated with each
transaction in the system .
-these timestamps of the transactions determine the
serializability order.
-if the timestamp of transaction Ti is smaller than the
timestamp of transaction Tm , then the scheme ensures that

Timestamp-based protocols
the produced schedule is equivalent to a serial schedule in
which transaction Ti appears before transaction Tm.
• It does so by rolling back a transaction whenever such an
order is violated.
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Conclusion
• This topic describes about the various concepts of transaction
processing. About what it is and how it works. Furthermore it
also gives a good layout about the concepts of Concurrent
transactions. And also how Concurrent execution of
transactions improves throughput of transactions and system
utilization reduces waiting time of transactions etc.Also
present in the report are the various concurrency-control
schemes which ensure a smooth running of the Concurrent
transactions as well as the consistency of the database to be
preserved

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THANK YOU!!!

Concurrent transactions

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