# Upgrade guide v0.17.x to v1.0

In v0.17.x and earlier EventStore was a singleton application with all functionality exposed by the `EventStore` module. To [support multiple event stores](https://github.com/commanded/eventstore/pull/168) in a single Elixir application you must now define your own event store module using the `EventStore` macro. You can define as many event stores as you like, each one is isolated and can be configured to use its own Postgres database.

### Getting started

First, you must define your own event store module using the `EventStore` macro:

```elixir
# lib/my_app/event_store.ex
defmodule MyApp.EventStore do
  use EventStore, otp_app: :my_app

  # Optional `init/1` function to modify config at runtime.
  def init(config) do
    {:ok, config}
  end
end
```

You can name your event store module however you like and optionally implement an `init/1` callback function.

Secondly, configure the `MyApp.EventStore` module (in `config/config.exs` or each environment's config):

```elixir
config :my_app, MyApp.EventStore,
  serializer: EventStore.JsonSerializer,
  username: "postgres",
  password: "postgres",
  database: "myapp_eventstore",
  hostname: "localhost",
  pool_size: 10
```

**Note:** To use an EventStore with Commanded you should configure the event
store to use Commanded's JSON serializer which provides additional support for
JSON decoding:

```elixir
config :my_app, MyApp.EventStore, serializer: Commanded.Serialization.JsonSerializer
```

Finally, the event store module must be included within your application's supervision tree, inside the `start/2` function:

```elixir
# lib/my_app/application.ex
defmodule MyApp.Application do
  use Application

  def start(_type, _args) do
    children = [
      MyApp.EventStore
    ]

    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
  end
end
```

Optionally, you can configure the event store modules in `config/config.exs` to allow running the event store mix tasks without providing the event store module as a command line argument:

```elixir
config :my_app, event_stores: [MyApp.EventStore]
```

The above configuration allows you to run `mix event_store.init` instead of `mix event_store.init -e MyApp.EventStore` (as an example).

### Usage

Use your event store module exactly as you would have previously used the `EventStore` itself.

```elixir
:ok = MyApp.EventStore.append_to_stream(stream_uuid, expected_version, events)
```

For ease of upgrading you can alias your own event store module as `EventStore` allowing you to use it without making any further code changes:

```elixir
alias MyApp.EventStore, as: EventStore

:ok = EventStore.append_to_stream(stream_uuid, expected_version, events)
```