Question

Compare the primary site method with the primary copy method for distributed concurrency control. How does the use of backup sites affect each?

Short answer

Primary Site Method assigns a specific site for reading and writing of a data item with no redundancy, while Primary Copy Method selects a copy of data item as primary, offering redundancy with multiple copies. Backup sites improve the capacity of both methods, as they increase data availability and reliability by being promoted to primary sites in case of primary site failure.

Step-by-step solution

Understanding Primary Site Method

The primary site method assigns one of the sites as primary site for a given data item. This primary site is responsible for processing all requests for reading or writing the data item. In case of system failures, this method might have disadvantages as it does not offer redundancy. In other words, if the primary site goes down, the data item becomes unavailable.

Understanding Primary Copy Method

In the primary copy method, one of the copies of a data item is chosen as the primary copy and its corresponding site, the primary site. All other copies are backup copies. As in the Primary Site Method, only the primary copy can be used for writing, although multiple copies can be read. However, as multiple copies of the item are maintained, it provides data redundancy.

Influence of Backup Sites on Primary Site Method

In the Primary Site Method, backup sites can provide data availability if the primary site fails. This would involve making a backup site the primary site in case of original primary site failure. This strategy has no effect on data consistency, but it improves the system's availability and reliability.

Influence of Backup Sites on Primary Copy Method

For Primary Copy Method, backup sites are essential as they hold the backup copies of data items. If the primary site fails, one of the backup sites can be promoted to act as the primary site. In this method, having backup sites improves data availability and also offers data redundancy.

Key concepts

Primary Site Method

In the realm of distributed concurrency control, the primary site method designates a singular location as the key authority for a specific data item. Picture this as having a team leader in a project; this site, akin to the leader, handles all the commands for reading or modifying the data. Now, imagine if the leader is suddenly unavailable—this situation reflects a significant drawback of the primary site method. Without a backup leader, or in this case, redundancy, the project comes to a halt. Similarly, if the primary site experiences downtime, accessibility to the data item is severed, causing potential issues in system continuity and reliability. To mitigate this, backup sites can play a monumental role. They can step up as a provisional primary site when the original is out of commission, thus safeguarding data availability and maintaining system operations.

Primary Copy Method

Conversely, the primary copy method enhances the distributed system's robustness by incorporating a more resilient approach to data management. Here, among the multitude of data copies spread across various sites, one is elected as the 'primary copy'. This mirrors the idea of having a main document as the source of truth, with multiple photocopies available for reference. While this ‘main document’ is the sole version that can undergo alterations, the 'photocopies' are available for read operations. This nifty organizational approach not only makes data more resilient to site failures but also promotes data redundancy. When a problem arises at the primary site, a backup site that has a duplicate copy can rise to the occasion, effectively taking over the primary role. This transition ensures that the system's performance remains unaffected and downtime is minimized, showcasing a marked improvement in data availability.

Data Redundancy

Data redundancy, although it might sound like unnecessary repetition, is a strategic ace in the deck for distributed databases. Imagine each piece of data having a twin. This means if one is lost, there's always another one ready to take its place, virtually eliminating the chances of data loss. In distributed systems, redundancy is a precautionary tapestry weaved to anticipate site failures without compromising data integrity. It is the foundation upon which the primary copy method is built. By maintaining additional copies of data items across different sites, redundancy not only ensures that data is recoverable but also enhances data availability. Crucial in situations where the primary site is experiencing issues, backup copies in other sites ensure that the system remains operational—a safeguard against single points of failure.

System Availability

When we talk about system availability in distributed databases, we're referring to the system’s aptitude to continuously provide service, even when faced with internal adversities like hardware or software failure. It's the equivalent of a store remaining open during renovation; services remain accessible to users despite ongoing maintenance or issues. In the context of distributed concurrency control methods, high system availability is the Holy Grail. Both the primary site method and primary copy method aim to minimize downtime with their distinct approaches. Backup sites are the unsung heroes in this narrative. They can assume the primary role with minimal disruption when the leading site fails, ensuring that data remains accessible, transactions continue to process, and users barely notice any hindrance. Overall, the system availability ties intimately with data redundancy strategies to create a robust and reliable distributed database environment.