# EventStore

CQRS Event Store implemented in Elixir. Uses [PostgreSQL]( as the underlying storage. Requires version 9.5 or newer. 

License is MIT.

## Getting started

EventStore is [available in Hex](, the package can be installed as follows:

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

        def deps do
          [{:eventstore, "~> 0.0.1"}]

  2. Ensure eventstore is started before your application:

        def application do
          [applications: [:eventstore]]

  3. Add an `eventstore` config entry containing the PostgreSQL connection details to each environment's mix config file (e.g. `config/dev.exs`).

    config :eventstore, EventStore.Storage,
      username: "postgres",
      password: "postgres",
      database: "eventstore_dev",
      hostname: "localhost"

  4. Create the EventStore database using the `mix` task

    mix event_store.create

    This will create the database and tables.

## Sample usage

# start the Storage process
{:ok, storage} = EventStore.Storage.start_link

### Writing to a stream

# create a unique identity for the stream
stream_uuid = UUID.uuid4()

# a new stream will be created when the expected version is zero
expected_version = 0

# list of events to persist
events = [
  	headers: %{user: ""},
    payload: %ExampleEvent{key: "value"}

# append events to stream
{:ok, events} = EventStore.append_to_stream(storage, stream_uuid, expected_version, events)

### Reading from a stream

# read all events from the stream, starting at the beginning (as from version is 0)
{:ok, recorded_events} = EventStore.read_stream_forward(storage, uuid, 0)

### Subscribe to all streams

Subscriptions are in progress, the usage will be further refined using a supervision tree.

#### Transient subscriptions

Events are received in batches after being persisted. Only events published while the subscription is active will be recevied.

# using an example subscriber
defmodule Subscriber do
  use GenServer

  def start_link do
    GenServer.start_link(__MODULE__, [])

  def received_events(server) do, :received_events)

  def init(events) do
    {:ok, %{events: events}}

  def handle_info({:events, stream_uuid, stream_version, events}, state) do
    {:noreply, %{state | events: events ++}}

  def handle_call(:received_events, _from, state) do
    result = |> Enum.reverse
    {:reply, result, state}

# create subscriptions supervisor
{:ok, supervisor} = Subscriptions.Supervisor.start_link(storage)
{:ok, subscriptions} = Subscriptions.start_link(supervisor)

# create subscriber and subscribe to all streams
{:ok, subscriber} = Subscriber.start_link
{:ok, subscription} = EventStore.subscribe_to_all_streams(subscriptions, "example_subscription", subscriber)

#### Persistent subscriptions (not yet implemented)

These will ensure at least once delivery of every persisted event. Each subscription may be independently paused, then later resume from where it stopped.

## Benchmarking performance

Run the benchmark suite using mix with the `bench` environment, as configured in `config/bench.exs`. Logging is disabled for benchmarking. 

MIX_ENV=bench mix do es.reset, app.start, bench

Example output:

## AppendEventsBench
append events, single writer         200   9738.10 µs/op
## ReadEventsBench
read events, single reader           500   3151.80 µs/op