eventstore

# Cluster

EventStore supports running on a cluster of nodes. It uses the [Swarm](https://hex.pm/packages/swarm) library for process distribution.

## Running on a cluster

  1. Add `:swarm` as a dependency in your `mix.exs` file:

      ```elixir
      defp deps do
        [
          {:swarm, "~> 3.0"},
        ]
      end
      ```

  2. Fetch the dependencies:

      ```console
      $ mix deps.get
      ```

  3. Configure the EventStore to use the `:distributed` registry in the environment config (e.g. `config/config.exs`):

      ```elixir
      config :eventstore,
        registry: :distributed
      ```

  4. Swarm must be configured to use the `Swarm.Distribution.StaticQuorumRing` distribution strategy:

      ```elixir
      config :swarm,
        nodes: [:"node1@127.0.0.1", :"node2@127.0.0.1", :"node3@127.0.0.1"],
        node_blacklist: [~r/^primary@.+$/],
        distribution_strategy: Swarm.Distribution.StaticQuorumRing,
        static_quorum_size: 3,
        sync_nodes_timeout: 0,
        debug: false
      ```

    This is to ensure consistency during a network partition. The `static_quorum_size` setting defines the minimum number of nodes that must be connected in the cluster to allow process registration and distribution. If there are fewer nodes currently available than the quorum size, any calls to the `EventStore` will block until enough nodes have started.

  5. Compile the EventStore to include the code used for process distribution:

      ```console
      $ mix deps.compile eventstore --force
      ```

## Cluster formation on node start

You can configure the `:kernel` application to wait for cluster formation before starting your application during node start up. The `sync_nodes_optional` configuration specifies which nodes to attempt to connect to within the `sync_nodes_timeout` window, defined in milliseconds, before continuing with startup. There is also a `sync_nodes_mandatory` setting which can be used to enforce all nodes are connected within the timeout window or else the node terminates.

```elixir
config :kernel,
  sync_nodes_optional: [:"node1@192.168.1.1", :"node2@192.168.1.2"],
  sync_nodes_timeout: 30_000
```

The `sync_nodes_timeout` can be configured as `:infinity` to wait indefinitely for all nodes to
connect. All involved nodes must have the same value for `sync_nodes_timeout`.

This approach will only work for Elixir releases. You will need to use [Erlang's `sys.config`](http://erlang.org/doc/man/config.html) file for development purposes.

The Erlang equivalent of the `:kernerl` mix config, as above, is:

```erlang
[{kernel,
  [
    {sync_nodes_optional, ['node1@127.0.0.1', 'node2@127.0.0.1']},
    {sync_nodes_timeout, 30000}
  ]}
].
```

## Starting a cluster

  1. Run an [Erlang Port Mapper Daemon](http://erlang.org/doc/man/epmd.html) (epmd):

      ```console
      $ epmd -d
      ```

  2. Start an `iex` console per node:

      ```console
      $ MIX_ENV=distributed iex --name node1@127.0.0.1 --erl "-config cluster/node1.sys.config" -S mix
      ```

      ```console
      $ MIX_ENV=distributed iex --name node2@127.0.0.1 --erl "-config cluster/node2.sys.config" -S mix
      ```

      ```console
      $ MIX_ENV=distributed iex --name node3@127.0.0.1 --erl "-config cluster/node3.sys.config" -S mix
      ```

The node specific `<node>.sys.config` files ensure the cluster is formed before starting the `:eventstore` application, assuming this occurs within the 30 seconds timeout.

Once the cluster has formed, you can use the EventStore API from any node. Stream processes will be distributed amongst the cluster and moved around on node up/down.

### Append events to a stream

```elixir
stream_uuid = UUID.uuid4()
events = EventStore.EventFactory.create_events(3)

:ok = EventStore.append_to_stream(stream_uuid, 0, events)
```

### Read all events

```elixir
recorded_events = EventStore.stream_all_forward() |> Enum.to_list()
```

### Subscribe to all Streams

```elixir
{:ok, subscription} = EventStore.subscribe_to_all_streams("example-subscription", self(), start_from: :origin)

receive do
  {:events, events} ->
    IO.puts "Received events: #{inspect events}"
    EventStore.ack(subscription, events)

  reply ->
    IO.puts reply
end
```

## Cluster diagnostics

Peek into the Swarm process registry:

```elixir
Swarm.Registry.registered()
```

Discover which node a stream process is running on:

```elixir
stream_uuid |> EventStore.Streams.Stream.name() |> Swarm.whereis_name() |> node()
```