Question

When are voting and elections used in distributed databases?

Short answer

Voting and elections are used in distributed databases to maintain system consistency and correctness even in case of system failures. Voting is used to agree on the final outcome of a transaction among multiple sites. Elections are used to select a new coordinator when the existing one fails or seems to have failed.

Step-by-step solution

Understanding Distributed Databases

Distributed Databases are databases in which storage devices are not all attached to a common processor. It may be stored in multiple computers located in the same physical location, or dispersed over a network of interconnected systems. Distributed databases are usually non-relational databases.

Understanding Voting Process in Distributed Databases

When a transaction is executed in a distributed database, it may access data at multiple sites. Therefore, for the correctness and consistency of the database, it is crucial that either all the sites participating in the execution should agree on the final outcome of the transaction (commit), or all should agree that transaction should be rolled back (abort). Any disagreement on this issue may lead to an inconsistent state of the database. This characteristic is satisfied by a voting mechanism, where every site votes for a commit or abort.

Understanding Elections in Distributed Databases

Election algorithms are used in distributed computing systems to resolve conflicts for resource usage or to provide a consistent view of the system, even after failures. In distributed databases, when a coordinator site fails, or when it seems to have failed, an election procedure is started. The site with the smallest site identifier (a unique value for each site) among the available sites is normally chosen to be the new coordinator.

Key concepts

Voting in Distributed Databases

In distributed databases, where data is spread across multiple locations, ensuring a synchronous operation is a crucial challenge. Voting plays a pivotal role in maintaining unity and order during database transactions. Imagine a scenario where a database transaction, such as an account update or a record change, is in progress. Now consider this transaction accesses data that's scattered across different sites. Each site holds a piece of the database's jigsaw puzzle.

To ensure that all these pieces fit together correctly after the transaction, each site votes whether to 'commit' to the transaction (signifying agreement) or to 'abort' (signifying disagreement). If all sites agree to commit, then the changes made by the transaction are saved; if any site votes to abort, the whole transaction is rolled back, as if it never happened. This process guarantees that each site has a consistent view of the data. Notably, a strong voting system prevents the database from ending up in an inconsistent state, which is essential for the reliability of distributed systems.

Election Algorithms

Distributed systems often face the issue of a central coordinator's failure, which can cause disarray within the system. This is where election algorithms come in, serving as the democratic process of digital entities. When the designated leader or coordinator of a distributed database system fails or appears to fail, it's time for the system to elect a new leader.

An election algorithm ensures that a new, reliable coordinator is selected among the available sites. The algorithm may be based on several criteria, such as the lowest identifier number, which ensures a deterministic and unambiguous selection process. Once a new coordinator is elected, it takes charge of managing the transactions and maintaining consistency across the distributed database, thereby reducing downtime and maintaining seamless operation even in the event of failures.

Transaction Consistency

Maintaining transaction consistency in a distributed database is akin to conducting a symphony orchestra - every player must be in harmony. Consistency refers to a transaction's adherence to the established rules and constraints of the database across all the sites involved.

In distributed databases, transactions may involve multiple steps spread across different nodes. It's critical that either every step is completed successively (commit) or none are (abort), in order to maintain the integrity of the data. This is where the concept of two-phase commit protocol comes into play, which is a type of voting method for achieving consistency. The first phase is where a preliminary vote is taken, followed by a second phase where, based on the votes collected, a global decision to either commit or abort the transaction is made. This protocol ensures that every site reaches a consensus and thus, keeps the distributed database consistent and reliable.