Question

a transaction, in the context of a database, is a logical unit that is independently executed for data retrieval or updates.

3.1 provide an explanation of database transactions including their properties.

3.2 Elaborate on the purpose of a transaction log.

Answer 1

3.1 Transaction in dbms follows ACID properties:

1. Atomicity-

• This property ensures that either the transaction occurs completely or it does not occur at all.
• In other words, it ensures that no transaction occurs partially.

2. Consistency-

• This property ensures that integrity constraints are maintained.
• In other words, it ensures that the database remains consistent before and after the transaction.

3. Isolation-

• This property ensures that multiple transactions can occur simultaneously without causing any inconsistency.
• During execution, each transaction feels as if it is getting executed alone in the system.
• A transaction does not realize that there are other transactions as well getting executed parallely.
• Changes made by a transaction becomes visible to other transactions only after they are written in the memory.

4. Durability-

• This property ensures that all the changes made by a transaction after its successful execution are written successfully to the disk.
• It also ensures that these changes exist permanently and are never lost even if there occurs a failure of any kind.

3.2 Purpose of a transaction log:

The transaction log contains enough information to undo all changes made to the data file as part of any individual transaction. The log records the start of a transaction, all the changes considered to be a part of it, and then the final commit or rollback of the transaction. Each database has at least one physical transaction log and one data file that is exclusive to the database for which it was created.

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