Summary of Transactions

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Summary

• A transaction is a unit of program execution that accesses and possibly updates various data items. Understanding the concept of a transaction is critical for understanding and implementing updates of data in a database, in such a way that concurrent executions and failures of various forms do not result in the database becoming inconsistent.

• Transactions are required to have the ACID properties: atomicity, consistency, isolation, and durability.

Atomicity ensures that either all the effects of a transaction are reflected in the database, or none are; a failure cannot leave the database in a state where a transaction is partially executed.

Consistency ensures that, if the database is initially consistent, the execution of the transaction (by itself) leaves the database in a consistent state.

Isolation ensures that concurrently executing transactions are isolated from one another, so that each has the impression that no other transaction is executing concurrently with it.

Durability ensures that, once a transaction has been committed, that transaction’s updates do not get lost, even if there is a system failure.

• Concurrent execution of transactions improves throughput of transactions and system utilization, and also reduces waiting time of transactions.

• When several transactions execute concurrently in the database, the consistency of data may no longer be preserved. It is therefore necessary for the system to control the interaction among the concurrent transactions.

Since a transaction is a unit that preserves consistency, a serial execution of transactions guarantees that consistency is preserved.

A schedule captures the key actions of transactions that affect concurrent execution, such as read and write operations, while abstracting away internal details of the execution of the transaction.

We require that any schedule produced by concurrent processing of a set of transactions will have an effect equivalent to a schedule produced when these transactions are run serially in some order.

A system that guarantees this property is said to ensure serializability.

There are several different notions of equivalence leading to the concepts of conflict serializability and view serializability.

• Serializability of schedules generated by concurrently executing transactions can be ensured through one of a variety of mechanisms called concurrency- control schemes.

• Schedules must be recoverable, to make sure that if transaction a sees the effects of transaction b, and b then aborts, then a also gets aborted.

• Schedules should preferably be cascadeless, so that the abort of a transaction does not result in cascading aborts of other transactions. Cascadelessness is ensured by allowing transactions to only read committed data.

• The concurrency-control – management component of the database is responsible for handling the concurrency-control schemes. Chapter 16 describes concurrency-control schemes.

• The recovery-management component of a database is responsible for ensuring the atomicity and durability properties of transactions.

The shadow copy scheme is used for ensuring atomicity and durability in text editors; however, it has extremely high overheads when used for database systems, and, moreover, it does not support concurrent execution. Chapter 17 covers better schemes.

• We can test a given schedule for conflict serializability by constructing a precedence graph for the schedule, and by searching for absence of cycles in the graph. However, there are more efficient concurrency control schemes for ensuring serializability.

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