Cast the provided value to an atom if appropriate. If the provided value is a string, convert it to an atom, otherwise return it as-is.

Executes protobuf message against server. Returns:

• {:ok, protobuf_message} on success .
• {:error, :not_authenticated} on wrong credentials.
• {:error, error_reason, protobuf_message} on failure.

EventStore uses ProtoBuf for taking requests and sending responses back. We are using exprotobuf to deal with them. List and specification of supported protobuf messages can be found in include/event_store.proto file.

Instead of wrapping each and every request in elixir function, we are using execute/2 function that takes server pid and request message:

{:ok, response} = Extreme.execute server, write_events()

where write_events can be helper function like:

alias Extreme.Messages, as: ExMsg

defp write_events(stream \ "people", events \ [%PersonCreated{name: "Pera Peric"}, %PersonChangedName{name: "Zika"}]) do
  proto_events = Enum.map(events, fn event -> 
    ExMsg.NewEvent.new(
      event_id: Extreme.Tools.gen_uuid(),
      event_type: to_string(event.__struct__),
      data_content_type: 0,
      metadata_content_type: 0,
      data: :erlang.term_to_binary(event),
      meta: ""
    ) end)
  ExMsg.WriteEvents.new(
    event_stream_id: stream, 
    expected_version: -2,
    events: proto_events,
    require_master: false
  )
end

This way you can fine tune your requests, i.e. choose your serialization. We are using erlang serialization in this case data: :erlang.term_to_binary(event), but you can do whatever suites you. For more information about protobuf messages EventStore uses, take a look at their documentation or for common use cases you can check test/extreme_test.exs file.

Reads events specified in read_events, sends them to subscriber and leaves subscriber subscribed per subscribe message.

subscriber is process that will keep receiving {:on_event, event} messages. read_events :: Extreme.Messages.ReadStreamEvents subscribe :: Extreme.Messages.SubscribeToStream

Returns {:ok, subscription} when subscription is success. If stream is hard deleted subscriber will receive message {:extreme, :error, :stream_hard_deleted, stream} If stream is soft deleted subscriber will receive message {:extreme, :warn, :stream_soft_deleted, stream}.

In case of soft deleted stream, new event will recreate stream and it will be sent to subscriber as described above Hard deleted streams can’t be recreated so suggestion is not to handle this message but rather crash when it happens

Examples:

defmodule MyApp.StreamSubscriber
  use GenServer

  def start_link(extreme, last_processed_event), 
    do: GenServer.start_link __MODULE__, {extreme, last_processed_event}

  def init({extreme, last_processed_event}) do
    stream = "people"
    state = %{ event_store: extreme, stream: stream, last_event: last_processed_event }
    GenServer.cast self, :subscribe
    {:ok, state}
  end

  def handle_cast(:subscribe, state) do
    # read only unprocessed events and stay subscribed
    {:ok, subscription} = Extreme.read_and_stay_subscribed state.event_store, self, state.stream, state.last_event + 1
    # we want to monitor when subscription is crashed so we can resubscribe
    ref = Process.monitor subscription
    {:noreply, %{state|subscription_ref: ref}}
  end

  def handle_info({:DOWN, ref, :process, _pid, _reason}, %{subscription_ref: ref} = state) do
    GenServer.cast self, :subscribe
    {:noreply, state}
  end
  def handle_info({:on_event, push}, state) do
    push.event.data
    |> :erlang.binary_to_term 
    |> process_event
    event_number = push.link.event_number
    :ok = update_last_event state.stream, event_number
    {:noreply, %{state|last_event: event_number}}
  end
  def handle_info(_msg, state), do: {:noreply, state}

  defp process_event(event), do: IO.puts("Do something with event: " <> inspect(event))

  defp update_last_event(_stream, _event_number), do: IO.puts("Persist last processed event_number for stream")
end

This way unprocessed events will be sent by Extreme, using {:on_event, push} message. After all persisted messages are sent, new messages will be sent the same way as they arrive to stream.

Since there’s a lot of boilerplate code here, you can use Extreme.Listener to reduce it and focus only on business part of code.

Starts connection to EventStore using connection_settings and optional opts.

Extreme can connect to single ES node or to cluster specified with node IPs and ports.

Example for connecting to single node:

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

• db_type - defaults to :node, thus it can be omitted
• host - check EXT IP setting of your EventStore
• port - check EXT TCP PORT setting of your EventStore
• reconnect_delay - in ms. Defaults to 1_000. If tcp connection fails this is how long it will wait for reconnection.
• max_attempts - Defaults to :infinity. Specifies how many times we’ll try to connect to EventStore

Example for connecting to cluster:

config :extreme, :event_store,
  db_type: :cluster,
  gossip_timeout: 300,
  nodes: [
    %{host: "10.10.10.29", port: 2113},
    %{host: "10.10.10.28", port: 2113},
    %{host: "10.10.10.30", port: 2113}
  ],
  username: "admin", 
  password: "changeit"

• gossip_timeout - in ms. Defaults to 1_000. We are iterating through nodes list, asking for cluster member details. This setting represents timeout for gossip response before we are asking next node from nodes list for cluster details.

• nodes - Mandatory for cluster connection. Represents list of nodes in the cluster as we know it

  • host - should be EXT IP setting of your EventStore node
  • port - should be EXT HTTP PORT setting of your EventStore node

Example of connection to cluster via DNS lookup

config :extreme, :event_store,
 db_type: :cluster_dns, 
 gossip_timeout: 300,
 host: "es-cluster.example.com", # accepts char list too, this whould be multy A record host enrty in your nameserver
 port: 2113, # the external gossip port
 username: "admin", 
 password: "changeit",
 max_attempts: :infinity 

When cluster mode is used, adapter goes thru nodes list and tries to gossip with node one after another until it gets response about nodes. Based on nodes information from that response it ranks their statuses and chooses the best candidate to connect to. For the way ranking is done, take a look at lib/cluster_connection.ex:

defp rank_state("Master"), do: 1
defp rank_state("PreMaster"), do: 2
defp rank_state("Slave"), do: 3
defp rank_state("Clone"), do: 4
defp rank_state("CatchingUp"), do: 5
defp rank_state("PreReplica"), do: 6
defp rank_state("Unknown"), do: 7
defp rank_state("Initializing"), do: 8

Note that above will work with same procedure with cluster_dns mode turned on, since internally it will get ip addresses to witch same connection procedure will be used.

Once client is disconnected from EventStore, supervisor should respawn it and connection starts over again.

Subscribe subscriber to stream using server.

subscriber is process that will keep receiving {:on_event, event} messages.

Returns {:ok, subscription} when subscription is success.

NOTE: If `stream` is hard deleted, `subscriber` will NOT receive any message!

Example:

def subscribe(server, stream \ "people"), do: Extreme.subscribe_to(server, self, stream)

def handle_info({:on_event, event}, state) do
  Logger.debug "New event added to stream 'people': " <> inspect(event)
  {:noreply, state}
end

As Extreme.read_and_stay_subscribed/7 has it’s abstraction in Extreme.Listener, there’s abstraction for this function as well in Extreme.FanoutListener behaviour.