Which of the following is true concerning a global transaction?

Solution (By Examveda Team)

In the context of a global transaction in a distributed database system, several aspects determine how the transaction is handled and where the required data are located.

Option A: The required data are at one local site and the distributed DBMS routes requests as necessary.
This option describes a scenario where the required data for a global transaction are located at one local site. In such cases, the distributed DBMS can efficiently route requests to the local site without the need for extensive coordination with other sites. However, this scenario does not align with the typical characteristics of a global transaction, where data may be distributed across multiple sites.

Option B: The required data are located in at least one nonlocal site and the distributed DBMS routes requests as necessary.
This option accurately reflects the nature of a global transaction. In a global transaction, the required data may be distributed across multiple sites, including nonlocal sites. The distributed DBMS is responsible for identifying the locations of the required data and routing requests to the appropriate sites as necessary to ensure that the transaction can be completed successfully. This may involve coordinating with multiple sites and coordinating data access and updates across distributed resources.

Option C: The required data are at one local site and the distributed DBMS passes the request to only the local DBMS.
This option suggests that the required data for a global transaction are located at one local site, and the distributed DBMS simply passes the request to the local DBMS without involving other sites. However, in the context of a global transaction, data may be distributed across multiple sites, making it necessary for the distributed DBMS to coordinate data access and updates across distributed resources beyond the local site.

Option D: The required data are located in at least one nonlocal site and the distributed DBMS passes the request to only the local DBMS.
Similar to Option C, this option implies that the distributed DBMS passes the request to only the local DBMS, even if the required data are located in at least one nonlocal site. However, in a global transaction, the distributed DBMS needs to route requests to the appropriate sites where the required data reside, which may include nonlocal sites. Therefore, this option does not accurately describe the characteristics of a global transaction.

Therefore, in the context of a global transaction in a distributed database system, the correct option is The required data are located in at least one nonlocal site and the distributed DBMS routes requests as necessary (Option B).