Eventium

Composable, type-safe event sourcing and CQRS for Haskell — event stores with optimistic concurrency, pure projections, command handlers, process managers, event subscriptions, and pluggable storage backends (in-memory, SQLite, PostgreSQL).

eventium-core      -- storage-agnostic core
eventium-sqlite    -- + a backend (or -memory / -postgresql)

Get started → · Docs · Examples · Hackage · GitHub

Why Eventium

• Everything is a plain value. Projections, command handlers, process managers, stores, and codecs are records you pass around — not typeclasses resolved by the compiler. They compose by function application.
• Domain logic is pure. Command handlers and process managers are ordinary functions — state and input in, values out. Test your whole domain with a list of events and an assertion. No database, no mocking.
• The backend is a deployment decision. The same domain code runs against an in-memory STM store in tests and PostgreSQL in production. Swap it at the application boundary.
• Safety is in the types. Optimistic concurrency, domain rejections, compensation logic, and schema evolution are all visible in the API rather than hidden behind defaults.

Start reading

• Getting Started — install and build a runnable counter in ~40 lines.
• Event Sourcing & CQRS — the mental model.
• Examples — three complete apps, from a counter to a full CQRS bank.