Event handlers are pure and transparent functions that apply events to create application state.
Pure and transparent functions
Pure and transparent means that there are no side-effects and it is deterministic with the same input. Something functional programming languages like Elixir excel at!
They should not be confused with command handlers
Command handlers create events out of commands and validate commands.
Reference
Hoffman, Kevin. Real-World Event Sourcing: Distribute, Evolve, and Scale Your Elixir Applications. The Pragmatic Bookshelf, 2025. The Pragmatic Programmers.
Highlights or timestamps
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