Elixir Adapter for EventStore
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README.md

Extreme

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Erlang/Elixir TCP client for Event Store.

This version is tested with EventStore 3.9.3 - 4.1.1, Elixir 1.5, 1.6, 1.7 and Erlang/OTP 19.3, 20.3 and 21.0

INSTALL

Add Extreme as a dependency in your mix.exs file.

def deps do
  [{:extreme, "~> 0.13.2"}]
end

In order to deploy your app using exrm you should also update your application list to include :extreme:

def application do
  [applications: [:extreme]]
end

Extreme includes all its dependencies so you don't have to name them separately.

After you are done, run mix deps.get in your shell to fetch and compile Extreme and its dependencies.

EventStore v4 and later note

Starting from EventStore version 4.0 there are some upgrades to communication protocol. Event number size is changed to 64bits and there is new messages IdentifyClient and ClientIdentified. Since we would like to keep backward compatibility with older v3 protocol, we introduced new configuration for :extreme application, where you have to set :protocol_version equal to 4 if you want to use new protocol, default is 3. Below is exact line you have to add in you application config file in order to activate new protocol:

config :extreme, :protocol_version, 4

USAGE

The best way to understand how adapter should be used is by investigating test/extreme_test.exs file, but we'll try to explain some details in here as well.

Extreme is implemented using GenServer and is OTP compatible. If client is disconnected from server we are not trying to reconnect, instead you should rely on your supervisor. For example:

defmodule MyApp.Supervisor do
  use Supervisor

  def start_link do
    Supervisor.start_link __MODULE__, :ok
  end

  @event_store MyApp.EventStore

  def init(:ok) do
    event_store_settings = Application.get_env :my_app, :event_store

    children = [
      worker(Extreme, [event_store_settings, [name: @event_store]]),
      # ... other workers / supervisors
    ]
    supervise children, strategy: :one_for_one
  end
end

You can manually start adapter as well (as you can see in test file):

{:ok, server} = Application.get_env(:extreme, :event_store) |> Extreme.start_link

From now on, server pid is used for further communication. Since we are relying on supervisor to reconnect, it is wise to name server as we did in example above.

MODES

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,
  connection_name: :my_app,
  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.
  • connection_name - Optional param introduced in EventStore 4. Connection can be identified by this name on ES UI
  • 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,
  mode: :read,
  nodes: [
    %{host: "10.10.10.29", port: 2113},
    %{host: "10.10.10.28", port: 2113},
    %{host: "10.10.10.30", port: 2113}
  ],
  connection_name: :my_app,
  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
  • mode - Defaults to :write where Master node is prefered over Slave, otherwise prefer Slave over Master

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
 connection_name: :my_app,
 username: "admin",
 password: "changeit",
 mode: :write,
 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 :write mode (default) Master node is prefered over Slave, but for :read mode it is opposite. For the way ranking is done, take a look at lib/cluster_connection.ex:

defp rank_state("Master", :write),    do: 1
defp rank_state("Master", _),         do: 2
defp rank_state("PreMaster", :write), do: 2
defp rank_state("PreMaster", _),      do: 3
defp rank_state("Slave", :write),     do: 3
defp rank_state("Slave", _),          do: 1

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

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

Communication

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.Msg, 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),
      metadata: ""
    ) 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.

Subscriptions

Extreme.subscribe_to/3 function is used to get notified on new events on particular stream. This way subscriber, in next example self, will get message {:on_event, push_message} when new event is added to stream people.

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

Extreme.read_and_stay_subscribed/7 reads all events that follow a specified event number, and subscribes to future events.

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(:caught_up, state) do
    Logger.debug "We are up to date!"
    {:noreply, state}
  end
  def handle_info(_msg, state), do: {:noreply, state}

  defp process_event(event), do: IO.puts("Do something with #{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, :caught_up message is sent and then new messages will be sent the same way as they arrive to stream.

If you subscribe to non existing stream you'll receive message {:extreme, severity, problem, stream} where severity can be either :error (for subscription on hard deleted stream) or :warn (for subscription on non existing or soft deleted stream). Problem is explanation of problem (i.e. :stream_hard_deleted). So in your receiver you can either have catch all handle_info(_message, _state) or you can handle such message:

def handle_info({:extreme, _, problem, stream}=message, state) do
  Logger.warn "Stream #{stream} issue: #{to_string problem}"
  {:noreply, state}
end

Extreme.Listener

Since it is common on read side of system to read events and denormalize them, there is Extreme.Listener macro that hides noise from listener:

defmodule MyApp.MyListener do
  use Extreme.Listener
  import MyApp.MyProcessor

  # returns last processed event by MyListener on stream_name, -1 if none has been processed so far
  defp get_last_event(stream_name), do: DB.get_last_event MyListener, stream_name

  defp process_push(push, stream_name) do
    #for indexed stream we need to follow push.link.event_number, otherwise push.event.event_number
    event_number = push.link.event_number
    DB.in_transaction fn ->
      Logger.info "Do some processing of event #{inspect push.event.event_type}"
      :ok = push.event.data
             |> :erlang.binary_to_term
             |> process_event(push.event.event_type)
      DB.ack_event(MyListener, stream_name, event_number)  
    end
    {:ok, event_number}
  end

  # This override is optional
  defp caught_up, do: Logger.debug("We are up to date. YEEEY!!!")
end

defmodule MyApp.MyProcessor do
  def process_event(data, "Elixir.MyApp.Events.PersonCreated") do
    Logger.debug "Doing something with #{inspect data}"
    :ok
  end
  def process_event(_, _), do: :ok # Just acknowledge events we are not interested in
end

Listener can be started manually but it is most common to place it in supervisor AFTER specifing Extreme:

defmodule MyApp.Supervisor do
  use Supervisor

  def start_link, do: Supervisor.start_link __MODULE__, :ok

  @event_store MyApp.EventStore

  def init(:ok) do
    event_store_settings = Application.get_env :my_app, :event_store

    children = [
      worker(Extreme, [event_store_settings, [name: @event_store]]),
      worker(MyApp.MyListener, [@event_store, "my_indexed_stream", [name: MyListener]]),
      # ... other workers / supervisors
    ]
    supervise children, strategy: :one_for_one
  end
end

Subscription can be paused:

{:ok, last_event_number} = MyApp.MyListener.pause MyListener

and resumed

:ok = MyApp.MyListener.resume MyListener

Extreme.FanoutListener

It's not uncommon situation to listen live events and propagate them (for example on web sockets). For that situation there is Extreme.FanoutListener macro that hides noise from listener:

defmodule MyApp.MyFanoutListener do
  use Extreme.FanoutListener
  import MyApp.MyPusher

  defp process_push(push) do
    Logger.info "Forward to web socket event #{inspect push.event.event_type}"
    :ok = push.event.data
           |> :erlang.binary_to_term
           |> process_event(push.event.event_type)
  end
end

defmodule MyApp.MyPusher do
  def process_event(data, "Elixir.MyApp.Events.PersonCreated") do
    Logger.debug "Transform and push event with data: #{inspect data}"
    :ok
  end
  def process_event(_, _), do: :ok # Just acknowledge events we are not interested in
end

Listener can be started manually but it is most common to place it in supervisor AFTER specifing Extreme:

defmodule MyApp.Supervisor do
  use Supervisor

  def start_link, do: Supervisor.start_link __MODULE__, :ok

  @event_store MyApp.EventStore

  def init(:ok) do
    event_store_settings = Application.get_env :my_app, :event_store

    children = [
      worker(Extreme, [event_store_settings, [name: @event_store]]),
      worker(MyApp.MyFanoutListener, [@event_store, "my_indexed_stream", [name: MyFanoutListener]]),
      # ... other workers / supervisors
    ]
    supervise children, strategy: :one_for_one
  end
end

Persistent subscriptions

The Event Store provides an alternate event subscription model, from version 3.2.0, known as competing consumers. Instead of the client holding the state of the subscription, the server remembers it.

Create a persistent subscription

The first step in using persistent subscriptions is to create a new subscription. This can be done using the Event Store admin website or in your application code, as shown below. You must provide a unique subscription group name and the stream to receive events from.

alias Extreme.Msg, as: ExMsg

{:ok, _} = Extreme.execute(server, ExMsg.CreatePersistentSubscription.new(
  subscription_group_name: "person-subscription",
  event_stream_id: "people",
  resolve_link_tos: false,
  start_from: 0,
  message_timeout_milliseconds: 10_000,
  record_statistics: false,
  live_buffer_size: 500,
  read_batch_size: 20,
  buffer_size: 500,
  max_retry_count: 10,
  prefer_round_robin: true,
  checkpoint_after_time: 1_000,
  checkpoint_max_count: 500,
  checkpoint_min_count: 1,
  subscriber_max_count: 1
))

Connect to a persistent subscription

Extreme.connect_to_persistent_subscription/5 function is used subscribe to an existing persistent subscription. The subscriber, in this example self, will receive message {:on_event, push_message} when each new event is added to stream people.

{:ok, subscription} = Extreme.connect_to_persistent_subscription(server, self(), group, stream, buffer_size)

Receive & acknowledge events

You must acknowledge receipt, and successful processing, of each received event. The Event Store will remember the last acknowledged event. The subscription will resume from this position should the subscriber process terminate and reconnect. This simplifies the client logic - the code you must write.

Extreme.PersistentSubscription.ack/3 function is used to acknowledge receipt of an event.

receive do
  {:on_event, event, correlation_id} ->
    Logger.debug "New event added to stream 'people': #{inspect event}"
    :ok = Extreme.PersistentSubscription.ack(subscription, event, correlation_id)
end

You must track the subscription PID returned from the Extreme.connect_to_persistent_subscription/5 function as part of the process state when using a GenServer subscriber.

def handle_info({:on_event, event, correlation_id}, %{subscription: subscription} = state) do
  Logger.debug "New event added to stream 'people': #{inspect event}"
  :ok = Extreme.PersistentSubscription.ack(subscription, event, correlation_id)
  {:noreply, state}
end

Events can also be not acknowledged. They can be not acknowledged with a nack_action of :Park, :Retry, :Skip, or :Stop.

def handle_info({:on_event, event, correlation_id}, %{subscription: subscription} = state) do
  Logger.debug "New event added to stream 'people': #{inspect event}"
  if needs_to_retry do
    :ok = Extreme.PersistentSubscription.nack(subscription, event, correlation_id, :Retry)
  else
    :ok = Extreme.PersistentSubscription.ack(subscription, event, correlation_id)
  end
  {:noreply, state}
end

Contributing

  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request

Licensed under The MIT License.