A review of consensus protocols

The consensus problem is a fundamental problem in multi-agent systems which requires a group of processes (or agents) to reliably and timely agree on a single data value. Although extensively discussed in the context of distributed computing it's not exclusive to this field, also being present in our society in a variety of situations such as in democratic elections, the legislative process, jury trial proceedings, and so forth. It's solved through the employment of a consensus protocol governing how processes (agents) interact with one another. It may seem redundant but, to solve the consensus problem, first all processes agree to follow the same consensus protocol. Some of these processes may fail or be unreliable in other ways (such as in a conflict of interest situation) so consensus protocols must be fault tolerant or resilient.