When are voting and elections used 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.

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.

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.