You must decide which event store to use with Commanded. You have a choice between two existing event stores:
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PostgreSQL-based Elixir EventStore (adapter).
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Greg Young's Event Store (adapter).
There is also an in-memory event store adapter for test use only.
Want to use a different event store? Then you will need to write your own event store provider.
Use EventStore Adapter to persist events to a PostgreSQL database. As the name implies, this is an adapter for EventStore, which is open-source and implemented in Elixir.
Use Extreme Adapter to persist events to Event Store: an open-source, functional database with Complex Event Processing in JavaScript. It can run as a cluster of nodes containing the same data, which remains available for writes provided at least half the nodes are alive and connected.
The quickest way to get started with Event Store is by using their official Event Store Docker container.
The Commanded adapter uses the Extreme Elixir TCP client to connect to the Event Store.
You must run the Event Store with all projections enabled and standard projections started.
Use the --run-projections=all --start-standard-projections=true flags when running the Event Store executable.
To use an alternative event store with Commanded you will need to implement the Commanded.EventStore behaviour. This defines the contract to be implemented by an adapter module to allow an event store to be used with Commanded. Tests to verify an adapter conforms to the behaviour are provided in test/event_store_adapter.
You can use one of the existing adapters (commanded_eventstore_adapter or commanded_extreme_adapter) to understand what is required.