Implementing a fault-tolerant service using state machine replication requires us to be able to view our service in terms of state machines. Let's understand the characteristics of a state machine.

Core components of a state machine

A state machine consists of two main components: state variables and commands.

State variables

State variables of a state machine specify its state. These variables are any instances of data structures that provide details about the configuration of a server or information stored by them. They are like normal variables because they contain values.

Commands

Commands of a state machine are programs, functions, or code that alter the state of the state machine. These commands can perform the following operations:

  1. Update values of state variables

  2. Give an output

A vital characteristic of these commands is that they are deterministic. We can assume that the program or code that implements these commands is deterministic. Another important characteristic of these commands is that their execution is atomic with other commands. These assumptions are necessary so that we can compare the result or output of the state machines with each other.

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