The consensus module on a server receives commands from clients and adds them to its log. It communicates with the consensus modules on other servers to ensure that every log eventually contains the same requests in the same order, even if some servers fail.

Since the state machines are non-deterministic, each computes the same state and the same sequence of outputs.

Keeping the replicated log consistent is the job of the client algorithm.

Once commands are properly replicated, each server’s state machine processes them in log order, and the outputs are returned to clients. As a result, the servers appear to form a single, highly reliable state machine.

Replicated state machines are typically implemented using a replicated log. Each server stores a log containing a series of commands, which its state machine executes in order. Each log contains the same commands in the same order, so each state machine processes the same sequence of commands.

Replicated state machines are used to solve a variety of fault tolerance problems in distributed systems.

Keeping the replicated log consistent is the job of the consensus algorithm. The consensus module on a server receives commands from clients and adds them to its log.

It communicates with the consensus modules on other servers to ensure that every log eventually contains the same requests in the same order, even if some servers fail.