Real-World Event Sourcing

Handling Errors by Modeling Failure

Let’s take this example to the next step. The code you’ve written so far is actually fairly brittle. For example, the code can easily be broken with a divide by zero error:

iex(4)> evt2 = EventSourcedCalculator.V2.handle_command(%{value: -1},
...(4)>  %{cmd: :div, value: 0})
%{event_type: :value_divided, value: 0}
iex(5)> state2 = EventSourcedCalculator.V2.handle_event(%{value: 9}, evt2)
** (ArithmeticError) bad argument in arithmetic expression
    es_calc_v2.exs:31: EventSourcedCalculator.V2.handle_event/2

Modeling errors and failure conditions is an enormous topic covered in detail later in the book, so for now, you’ll only handle a couple of easily managed cases. Create a third version ...