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eulerfx / EventMachines.md
Last active July 15, 2024 13:01
The relationship between state machines and event sourcing

A state machine is defined as follows:

  • Input - a set of inputs
  • Output - a set of outputs
  • State - a set of states
  • S0 ∈ S - an initial state
  • T : Input * State -> Output * State - a transition function

If you model your services (aggregates, projections, process managers, sagas, whatever) as state machines, one issue to address is management of State. There must be a mechanism to provide State to the state machine, and to persist resulting State for subsequent retrieval. One way to address this is by storing State is a key-value store. Another way is to use a SQL database. Yet another way is event sourcing. The benefit of even sourcing is that you never need to store State itself. Instead, you rely on the Output of a service to reconstitute state. In order to do that, the state machine transition function needs to be factored into two functions as follows: