Cluster

EventStore supports running on a cluster of nodes. It uses the Swarm library for process distribution.

Running on a cluster

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

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

2. Fetch the dependencies:

     $ mix deps.get

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

     config :eventstore,
       registry: :distributed

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

     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: 2,
       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.

Automatic cluster formation

Swarm can be used with libcluster, a library that provides a mechanism for automatically forming clusters of Erlang nodes, with either static or dynamic node membership.

You will need to include libcluster as an additional dependency:

defp deps do
  [{:libcluster, "~> 2.2"}]
end

Then configure the cluster topology in the environment config (e.g. config/config.exs). An example is shown below using the standard Erlang epmd daemon strategy:

config :libcluster,
  topologies: [
    example: [
      strategy: Cluster.Strategy.Epmd,
      config: [hosts: [:"node1@127.0.0.1", :"node2@127.0.0.1", :"node3@127.0.0.1"]],
    ]
  ]

Please refer to the libcluster docs for more detail.

Starting a cluster

1. Run an Erlang Port Mapper Daemon (epmd):

     $ epmd -d

2. Start an iex console per node:

     $ MIX_ENV=distributed iex --name node1@127.0.0.1 -S mix

     $ MIX_ENV=distributed iex --name node2@127.0.0.1 -S mix

     $ MIX_ENV=distributed iex --name node3@127.0.0.1 -S mix

The cluster will be automatically formed as soon as the nodes start.

Static cluster topology and formation

Instead of using libcluster you can configure the :kernel application to wait for cluster formation before starting your application during node start up. This approach is useful when you have a static cluster topology that can be defined in config.

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.

Each node requires its own individual configuration, listing the other nodes in the cluster:

# node1 config
config :kernel,
  sync_nodes_optional: [:"node2@192.168.1.1", :"node3@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 file for development purposes.

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

% node1.sys.config
[{kernel,
  [
    {sync_nodes_optional, ['node2@127.0.0.1', 'node3@127.0.0.1']},
    {sync_nodes_timeout, 30000}
  ]}
].

Starting a cluster

1. Run an Erlang Port Mapper Daemon (epmd):

     $ epmd -d

2. Start an iex console per node:

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

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

     $ 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.

Usage

Append events to a stream

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

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

Read all events

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

Subscribe to all Streams

{: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:

Swarm.Registry.registered()

Discover which node a stream process is running on:

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