Choosing an event store

You must decide which event store to use with Commanded. You have a choice between two existing event store adapters:

• PostgreSQL-based Elixir EventStore.

• Greg Young’s Event Store.

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.

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PostgreSQL-based Elixir EventStore

EventStore is an open-source event store using PostgreSQL for persistence, implemented in Elixir.

1. Add commanded_eventstore_adapter to your list of dependencies in mix.exs:

   def deps do
     [{:commanded_eventstore_adapter, "~> 0.1"}]
   end

2. Include :eventstore in the extra applications list in mix.exs:

   def application do
     [
       extra_applications: [
         :logger,
         :eventstore,
       ],
       # ...
     ]
   end

3. Configure Commanded to use the EventStore adapter:

   config :commanded,
     event_store_adapter: Commanded.EventStore.Adapters.EventStore

4. Configure the eventstore in each environment’s mix config file (e.g. config/dev.exs), specifying usage of the Commanded JSON serializer:

   config :eventstore, EventStore.Storage,
     serializer: Commanded.Serialization.JsonSerializer,
     username: "postgres",
     password: "postgres",
     database: "eventstore_dev",
     hostname: "localhost",
     pool_size: 10

5. Create the eventstore database and tables using the mix task:

   $ mix do event_store.create, event_store.init

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Greg Young’s Event Store

Event Store is 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 the 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.

1. Add commanded_extreme_adapter to your list of dependencies in mix.exs:

   def deps do
     [{:commanded_extreme_adapter, "~> 0.1"}]
   end

2. Configure Commanded to use the event store adapter:

   config :commanded,
     event_store_adapter: Commanded.EventStore.Adapters.Extreme

3. Configure the extreme library connection with your Event Store connection details:

   config :extreme, :event_store,
     db_type: :node,
     host: "localhost",
     port: 1113,
     username: "admin",
     password: "changeit",
     reconnect_delay: 2_000,
     max_attempts: :infinity

4. Configure the commanded_extreme_adapter to use the Commanded JSON serializer and specify a stream prefix to be used by all Commanded event streams:

   config :commanded_extreme_adapter,
     serializer: Commanded.Serialization.JsonSerializer,
     stream_prefix: "commandeddev"

   The stream prefix is used for category projections and event handler subscriptions. It must not contain a dash character (“-“).

Running 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.

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Writing your own event store provider

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.