Repository: commanded/commanded (elixir)

Issue:
Allow event handlers and process managers to implement an `init/1` callback function
Similar to the [`Commanded.Application.init/1`](https://hexdocs.pm/commanded/Commanded.Application.html#c:init/1) callback function which allows an application to be dynamically configured at runtime. Allow the implementation of an `init/1` callback function for `Commanded.Event.Handler` and `Commanded.ProcessManagers.ProcessManager` modules which would be called with the merged compile-time and runtime config and must return the (optionally) updated config.

#### Example

```elixir
defmodule ExampleHandler do
  use Commanded.Event.Handler, application: ExampleApp
 
  def init(config)
    {tenant, config} = Keyword.pop!(config, :tenant)
    
    # Set a tenant-specific name
    config = Keyword.put(config, :name, "ExampleHandler." <> tenant)
 
    {:ok, config}
  end
end  
```

Usage:

```elixir
{:ok, _pid} = ExampleHandler.start_link(tenant: "tenant1")
```

Or supervised as: `{ExampleHandler, tenant: "tenant1"}`

Pull request:
Add `init/1` callback function to event handlers and process managers
Allow event handlers and process managers to define an `init/1` callback function. This is called at runtime with the merged compile-time and runtime config and must return the (optionally) updated config.

#### Example

```elixir
defmodule ExampleHandler do
  use Commanded.Event.Handler, application: ExampleApp
 
  def init(config)
    {tenant, config} = Keyword.pop!(config, :tenant)
    
    # Set a tenant-specific name which will create a unique subscription per tenant.
    # This allows multiple instances of this handler to be started and run independently.
    config = Keyword.put(config, :name, "ExampleHandler." <> tenant)
 
    {:ok, config}
  end
end  
```

Usage:

```elixir
{:ok, _pid} = ExampleHandler.start_link(tenant: "tenant1")
```

Or supervised as: `{ExampleHandler, tenant: "tenant1"}`

Closes #392.

Patch:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0dcfb7b..16ec6e9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## Next release
+
+### Enhancements
+
+- Add `init/1` callback function to event handlers and process managers ([#393](https://github.com/commanded/commanded/pull/393)).
+
 ## v1.1.1
 
 ### Enhancements
diff --git a/lib/commanded/event/handler.ex b/lib/commanded/event/handler.ex
index 0669834..dcf7d54 100644
--- a/lib/commanded/event/handler.ex
+++ b/lib/commanded/event/handler.ex
@@ -28,8 +28,8 @@ defmodule Commanded.Event.Handler do
 
   The name you specify is used when subscribing to the event store. Therefore
   you *should not* change the name once the handler has been deployed. A new
-  subscription will be created when you change the name, and you event handler
-  will receive already handled events.
+  subscription will be created if you change the name and the event handler will
+  receive already handled events.
 
   You can use the module name of your event handler using the `__MODULE__`
   special form:
@@ -89,12 +89,30 @@ defmodule Commanded.Event.Handler do
 
   This will ensure the handler only receives events appended to that stream.
 
+  ## Runtime event handler configuration
+
+  Runtime options can be provided to the event handler's `start_link/1` function
+  or its child spec. The `c:init/1` callback function can also be used to define
+  runtime configuration.
+
+  ### Example
+
+  Provide runtime configuration to `start_link/1`:
+
+      {:ok, _pid} = ExampleHandler.start_link(application: ExampleApp, name: "ExampleHandler")
+
+  Or when supervised:
+
+      Supervisor.start_link([
+        {ExampleHandler, application: ExampleApp, name: "ExampleHandler"}
+      ], strategy: :one_for_one)
+
   ## `c:init/0` callback
 
   You can define an `c:init/0` function in your handler to be called once it has
   started and successfully subscribed to the event store.
 
-  This callback function must return `:ok`, any other return value will
+  This callback function must return `:ok`. Any other return value will
   terminate the event handler with an error.
 
       defmodule ExampleHandler do
@@ -102,8 +120,8 @@ defmodule Commanded.Event.Handler do
           application: ExampleApp,
           name: "ExampleHandler"
 
+        # Optional initialisation
         def init do
-          # optional initialisation
           :ok
         end
 
@@ -113,6 +131,36 @@ defmodule Commanded.Event.Handler do
         end
       end
 
+    ## `c:init/1` callback
+
+    An `c:init/1` function can be defined in your handler which is used to
+    provide runtime configuration. This callback function must return
+    `{:ok, config}` with the updated config.
+
+    ### Example
+
+    The `c:init/1` function is used to define the handler's application and name
+    based upon a value provided at runtime:
+
+        defmodule ExampleHandler do
+          use Commanded.Event.Handler
+
+          def init(config) do
+            {tenant, config} = Keyword.pop!(config, :tenant)
+
+            config =
+              config
+              |> Keyword.put(:application, Module.concat([ExampleApp, tenant]))
+              |> Keyword.put(:name, Module.concat([__MODULE__, tenant]))
+
+            {:ok, config}
+          end
+        end
+
+    Usage:
+
+        {:ok, _pid} = ExampleHandler.start_link(tenant: :tenant1)
+
   ## `c:error/3` callback
 
   You can define an `c:error/3` callback function to handle any errors returned
@@ -253,6 +301,14 @@ defmodule Commanded.Event.Handler do
   """
   @callback init() :: :ok | {:stop, reason :: any()}
 
+  @doc """
+  Optional callback function called to configure the handler before it starts.
+
+  It is passed the merged compile-time and runtime config, and must return the
+  updated config.
+  """
+  @callback init(config :: Keyword.t()) :: {:ok, Keyword.t()}
+
   @doc """
   Event handler behaviour to handle a domain event and its metadata.
 
@@ -305,22 +361,16 @@ defmodule Commanded.Event.Handler do
   defmacro __using__(opts) do
     quote location: :keep do
       @before_compile unquote(__MODULE__)
-
       @behaviour Handler
-
-      {application, name} = Handler.compile_config(__MODULE__, unquote(opts))
-
       @opts unquote(opts)
-      @application application
-      @name name
 
       @doc false
       def start_link(opts \\ []) do
-        application = Keyword.get(opts, :application, @application)
-        module_opts = Keyword.drop(@opts, [:application, :name])
-        opts = Handler.start_opts(__MODULE__, module_opts, opts)
+        opts = Keyword.merge(@opts, opts)
 
-        Handler.start_link(application, @name, __MODULE__, opts)
+        {application, name, config} = Handler.parse_config!(__MODULE__, opts)
+
+        Handler.start_link(application, name, __MODULE__, config)
       end
 
       @doc """
@@ -335,11 +385,13 @@ defmodule Commanded.Event.Handler do
 
       """
       def child_spec(opts) do
-        application = Keyword.get(opts, :application, @application)
+        opts = Keyword.merge(@opts, opts)
+
+        {application, name, config} = Handler.parse_config!(__MODULE__, opts)
 
         default = %{
-          id: {__MODULE__, application, @name},
-          start: {__MODULE__, :start_link, [opts]},
+          id: {__MODULE__, application, name},
+          start: {Handler, :start_link, [application, name, __MODULE__, config]},
           restart: :permanent,
           type: :worker
         }
@@ -348,60 +400,53 @@ defmodule Commanded.Event.Handler do
       end
 
       @doc false
-      def __name__, do: @name
+      def init, do: :ok
 
       @doc false
-      def init, do: :ok
+      def init(config), do: {:ok, config}
 
       @doc false
       def before_reset, do: :ok
 
-      defoverridable init: 0, before_reset: 0
+      defoverridable init: 0, init: 1, before_reset: 0
     end
   end
 
   @doc false
-  def compile_config(module, opts) do
-    application = Keyword.get(opts, :application)
-
-    unless application do
-      raise ArgumentError, inspect(module) <> " expects :application option"
-    end
+  def parse_config!(module, config) do
+    {:ok, config} = module.init(config)
 
-    name = parse_name(module, Keyword.get(opts, :name))
+    {_valid, invalid} =
+      Keyword.split(config, [:application, :consistency, :name, :start_from, :subscribe_to])
 
-    {application, name}
-  end
+    if Enum.any?(invalid) do
+      raise ArgumentError,
+            inspect(module) <> " specifies invalid options: " <> inspect(Keyword.keys(invalid))
+    end
 
-  @doc false
-  def parse_name(module, name) when name in [nil, ""] do
-    raise ArgumentError, inspect(module) <> " expects :name option"
-  end
+    application = Keyword.get(config, :application)
 
-  def parse_name(_module, name) when is_binary(name), do: name
-  def parse_name(_module, name), do: inspect(name)
+    unless application do
+      raise ArgumentError, inspect(module) <> " expects :application option"
+    end
 
-  @doc false
-  def start_opts(module, module_opts, local_opts, additional_allowed_opts \\ []) do
-    {valid, invalid} =
-      module_opts
-      |> Keyword.merge(local_opts)
-      |> Keyword.split(
-        [:application, :consistency, :start_from, :subscribe_to] ++ additional_allowed_opts
-      )
+    name = parse_name(Keyword.get(config, :name))
 
-    if Enum.any?(invalid) do
-      raise ArgumentError,
-            inspect(module) <> " specifies invalid options: " <> inspect(Keyword.keys(invalid))
+    unless name do
+      raise ArgumentError, inspect(module) <> " expects :name option"
     end
 
-    valid
+    {application, name, config}
   end
 
-  # Include default `handle/2` and `error/3` callback functions in module
+  @doc false
+  def parse_name(name) when name in [nil, ""], do: nil
+  def parse_name(name) when is_binary(name), do: name
+  def parse_name(name), do: inspect(name)
 
   @doc false
   defmacro __before_compile__(_env) do
+    # Include default `handle/2` and `error/3` callback functions in module
     quote generated: true do
       @doc false
       def handle(_event, _metadata), do: :ok
@@ -445,7 +490,7 @@ defmodule Commanded.Event.Handler do
 
     with {:ok, pid} <- Registration.start_link(application, name, __MODULE__, handler) do
       # Register the started event handler as a subscription with the given consistency
-      :ok = Subscriptions.register(application, handler_name, pid, consistency)
+      :ok = Subscriptions.register(application, handler_name, handler_module, pid, consistency)
 
       {:ok, pid}
     end
@@ -473,19 +518,6 @@ defmodule Commanded.Event.Handler do
     {:reply, last_seen_event, state}
   end
 
-  @doc false
-  @impl GenServer
-  def handle_call(:config, _from, %Handler{} = state) do
-    %Handler{
-      consistency: consistency,
-      subscription: %Subscription{subscribe_from: subscribe_from, subscribe_to: subscribe_to}
-    } = state
-
-    config = [consistency: consistency, start_from: subscribe_from, subscribe_to: subscribe_to]
-
-    {:reply, config, state}
-  end
-
   @doc false
   @impl GenServer
   def handle_info(:reset, %Handler{} = state) do
diff --git a/lib/commanded/process_managers/process_manager.ex b/lib/commanded/process_managers/process_manager.ex
index e7059b5..dd960a3 100644
--- a/lib/commanded/process_managers/process_manager.ex
+++ b/lib/commanded/process_managers/process_manager.ex
@@ -50,6 +50,36 @@ defmodule Commanded.ProcessManagers.ProcessManager do
 
       {:ok, process_manager} = ExampleProcessManager.start_link()
 
+  ## `c:init/1` callback
+
+  An `c:init/1` function can be defined in your process manager which is used to
+  provide runtime configuration. This callback function must return
+  `{:ok, config}` with the updated config.
+
+  ### Example
+
+  The `c:init/1` function is used to define the process manager's application
+  and name based upon a value provided at runtime:
+
+      defmodule ExampleProcessManager do
+        use Commanded.ProcessManagers.ProcessManager
+
+        def init(config) do
+          {tenant, config} = Keyword.pop!(config, :tenant)
+
+          config =
+            config
+            |> Keyword.put(:application, Module.concat([ExampleApp, tenant]))
+            |> Keyword.put(:name, Module.concat([__MODULE__, tenant]))
+
+          {:ok, config}
+        end
+      end
+
+  Usage:
+
+      {:ok, _pid} = ExampleProcessManager.start_link(tenant: :tenant1)
+
   ## Error handling
 
   You can define an `c:error/3` callback function to handle any errors or
@@ -199,6 +229,15 @@ defmodule Commanded.ProcessManagers.ProcessManager do
   @type process_uuid :: String.t() | [String.t()]
   @type consistency :: :eventual | :strong
 
+  @doc """
+  Optional callback function called to configure the process manager before it
+  starts.
+
+  It is passed the merged compile-time and runtime config, and must return the
+  updated config.
+  """
+  @callback init(config :: Keyword.t()) :: {:ok, Keyword.t()}
+
   @doc """
   Is the process manager interested in the given command?
 
@@ -326,7 +365,6 @@ defmodule Commanded.ProcessManagers.ProcessManager do
               | {:skip, :continue_pending}
               | {:stop, reason :: term()}
 
-  alias Commanded.Event.Handler
   alias Commanded.ProcessManagers.ProcessManager
   alias Commanded.ProcessManagers.ProcessRouter
 
@@ -334,21 +372,15 @@ defmodule Commanded.ProcessManagers.ProcessManager do
   defmacro __using__(opts) do
     quote location: :keep do
       @before_compile unquote(__MODULE__)
-
       @behaviour ProcessManager
-
-      {application, name} = ProcessManager.compile_config(__MODULE__, unquote(opts))
-
       @opts unquote(opts)
-      @application application
-      @name name
 
       def start_link(opts \\ []) do
-        application = Keyword.get(opts, :application, @application)
-        module_opts = Keyword.drop(@opts, [:application, :name])
-        opts = Handler.start_opts(__MODULE__, module_opts, opts, [:event_timeout, :idle_timeout])
+        opts = Keyword.merge(@opts, opts)
 
-        ProcessRouter.start_link(application, @name, __MODULE__, opts)
+        {application, name, config} = ProcessManager.parse_config!(__MODULE__, opts)
+
+        ProcessRouter.start_link(application, name, __MODULE__, config)
       end
 
       @doc """
@@ -363,11 +395,13 @@ defmodule Commanded.ProcessManagers.ProcessManager do
 
       """
       def child_spec(opts) do
-        application = Keyword.get(opts, :application, @application)
+        opts = Keyword.merge(@opts, opts)
+
+        {application, name, config} = ProcessManager.parse_config!(__MODULE__, opts)
 
         default = %{
-          id: {__MODULE__, application, @name},
-          start: {__MODULE__, :start_link, [opts]},
+          id: {__MODULE__, application, name},
+          start: {ProcessRouter, :start_link, [application, name, __MODULE__, config]},
           restart: :permanent,
           type: :worker
         }
@@ -376,13 +410,15 @@ defmodule Commanded.ProcessManagers.ProcessManager do
       end
 
       @doc false
-      def __name__, do: @name
+      def init(config), do: {:ok, config}
+
+      defoverridable init: 1
     end
   end
 
   @doc false
   defmacro __before_compile__(_env) do
-    # include default fallback functions at end, with lowest precedence
+    # Include default fallback functions at end, with lowest precedence
     quote generated: true do
       @doc false
       def interested?(_event), do: false
@@ -425,15 +461,42 @@ defmodule Commanded.ProcessManagers.ProcessManager do
   """
   defdelegate identity, to: Commanded.ProcessManagers.ProcessManagerInstance
 
-  def compile_config(module, opts) do
-    application = Keyword.get(opts, :application)
+  def parse_config!(module, config) do
+    {:ok, config} = module.init(config)
+
+    {_valid, invalid} =
+      Keyword.split(config, [
+        :application,
+        :consistency,
+        :name,
+        :start_from,
+        :subscribe_to,
+        :event_timeout,
+        :idle_timeout
+      ])
+
+    if Enum.any?(invalid) do
+      raise ArgumentError,
+            inspect(module) <> " specifies invalid options: " <> inspect(Keyword.keys(invalid))
+    end
+
+    application = Keyword.get(config, :application)
 
     unless application do
       raise ArgumentError, inspect(module) <> " expects :application option"
     end
 
-    name = Handler.parse_name(module, Keyword.get(opts, :name))
+    name = parse_name(Keyword.get(config, :name))
 
-    {application, name}
+    unless name do
+      raise ArgumentError, inspect(module) <> " expects :name option"
+    end
+
+    {application, name, config}
   end
+
+  @doc false
+  def parse_name(name) when name in [nil, ""], do: nil
+  def parse_name(name) when is_binary(name), do: name
+  def parse_name(name), do: inspect(name)
 end
diff --git a/lib/commanded/process_managers/process_router.ex b/lib/commanded/process_managers/process_router.ex
index 344bb5e..0bad09e 100644
--- a/lib/commanded/process_managers/process_router.ex
+++ b/lib/commanded/process_managers/process_router.ex
@@ -60,7 +60,7 @@ defmodule Commanded.ProcessManagers.ProcessRouter do
 
     with {:ok, pid} <- Registration.start_link(application, name, __MODULE__, state) do
       # Register the process manager as a subscription with the given consistency.
-      :ok = Subscriptions.register(application, process_name, pid, consistency)
+      :ok = Subscriptions.register(application, process_name, process_module, pid, consistency)
 
       {:ok, pid}
     end
diff --git a/lib/commanded/subscriptions.ex b/lib/commanded/subscriptions.ex
index 558795c..638cf5a 100644
--- a/lib/commanded/subscriptions.ex
+++ b/lib/commanded/subscriptions.ex
@@ -22,7 +22,9 @@ defmodule Commanded.Subscriptions do
     GenServer.start_link(__MODULE__, subscriptions_opts, start_opts)
   end
 
-  defdelegate register(application, name, pid \\ self(), consistency), to: Subscriptions.Registry
+  defdelegate register(application, name, module, pid \\ self(), consistency),
+    to: Subscriptions.Registry
+
   defdelegate all(application), to: Subscriptions.Registry
 
   @doc """
@@ -205,15 +207,20 @@ defmodule Commanded.Subscriptions do
     %Subscriptions{application: application} = state
 
     Subscriptions.Registry.all(application)
-    |> Enum.reject(fn {_name, pid} -> MapSet.member?(exclude, pid) end)
-    |> Enum.filter(fn {name, _pid} ->
+    |> Enum.reject(fn {_name, _module, pid} -> MapSet.member?(exclude, pid) end)
+    |> Enum.filter(fn {name, module, _pid} ->
       # Optionally filter subscriptions to those provided by the `consistency` option
       case consistency do
-        :strong -> true
-        consistency when is_list(consistency) -> Enum.member?(consistency, name)
+        :strong ->
+          true
+
+        consistency when is_list(consistency) ->
+          Enum.member?(consistency, module) or Enum.member?(consistency, name)
       end
     end)
-    |> Enum.all?(fn {name, _pid} -> handled_by?(name, stream_uuid, stream_version, state) end)
+    |> Enum.all?(fn {name, _module, _pid} ->
+      handled_by?(name, stream_uuid, stream_version, state)
+    end)
   end
 
   # Has the named subscription handled the event for the given stream and version
@@ -287,18 +294,9 @@ defmodule Commanded.Subscriptions do
 
   defp parse_consistency(consistency) when is_list(consistency) do
     Enum.map(consistency, fn
-      name when is_binary(name) ->
-        name
-
-      module when is_atom(module) ->
-        if function_exported?(module, :__name__, 0) do
-          apply(module, :__name__, [])
-        else
-          raise "Expected module #{module} to contain a public `__name__/0` function"
-        end
-
-      invalid ->
-        raise "Invalid consistency: #{inspect(invalid)}"
+      name when is_binary(name) -> name
+      module when is_atom(module) -> module
+      invalid -> raise "Invalid consistency: #{inspect(invalid)}"
     end)
   end
 
diff --git a/lib/commanded/subscriptions/registry.ex b/lib/commanded/subscriptions/registry.ex
index 777d0af..0414a91 100644
--- a/lib/commanded/subscriptions/registry.ex
+++ b/lib/commanded/subscriptions/registry.ex
@@ -16,16 +16,16 @@ defmodule Commanded.Subscriptions.Registry do
   @doc """
   Register an event store subscription with the given consistency guarantee.
   """
-  def register(application, name, pid, consistency)
+  def register(application, name, module, pid, consistency)
 
   # Ignore subscriptions with `:eventual` consistency
-  def register(_application, _name, _pid, :eventual), do: :ok
+  def register(_application, _name, _module, _pid, :eventual), do: :ok
 
   # Register subscriptions with `:strong` consistency
-  def register(application, name, pid, :strong) do
+  def register(application, name, module, pid, :strong) do
     table_name = table_name(application)
 
-    true = :ets.insert(table_name, {name, pid})
+    true = :ets.insert(table_name, {name, module, pid})
 
     :ok
   end
diff --git a/lib/mix/tasks/commanded.reset.ex b/lib/mix/tasks/commanded.reset.ex
index 3325844..a241eea 100644
--- a/lib/mix/tasks/commanded.reset.ex
+++ b/lib/mix/tasks/commanded.reset.ex
@@ -4,7 +4,7 @@ defmodule Mix.Tasks.Commanded.Reset do
 
   ## Usage
 
-      mix commanded.reset --app <app> --handler <handler>
+      mix commanded.reset --app <app> --handler <handler_name>
 
   ## Examples
 
@@ -14,7 +14,7 @@ defmodule Mix.Tasks.Commanded.Reset do
   ## Command line options
 
     * `-a`, `--app` - the Commanded application
-    * `-h`, `--handler` - the event handler to reset
+    * `-h`, `--handler` - the name of the event handler to reset
     * `-q`, `--quiet` - do not log output
 
   """
@@ -49,12 +49,11 @@ defmodule Mix.Tasks.Commanded.Reset do
     end
 
     app = String.to_atom("Elixir." <> app)
-    handler = String.to_atom("Elixir." <> handler)
 
     reset(app, handler, parsed)
   end
 
-  defp reset(app, handler, opts) do
+  defp reset(app, handler_name, opts) do
     case app.start_link() do
       {:ok, _pid} ->
         :ok
@@ -67,16 +66,14 @@ defmodule Mix.Tasks.Commanded.Reset do
     end
 
     quiet = Keyword.get(opts, :quiet, false)
-
-    handler_name = handler.__name__()
     registry_name = Commanded.Event.Handler.name(app, handler_name)
 
     case Commanded.Registration.whereis_name(app, registry_name) do
       :undefined ->
-        unless quiet, do: Mix.shell().info("No process found for #{inspect(handler)}")
+        unless quiet, do: Mix.shell().info("No process found for #{inspect(handler_name)}")
 
       pid ->
-        unless quiet, do: Mix.shell().info("Resetting #{inspect(handler)}")
+        unless quiet, do: Mix.shell().info("Resetting #{inspect(handler_name)}")
         send(pid, :reset)
     end
   end
diff --git a/mix.lock b/mix.lock
index 0db78cd..dfb5d60 100644
--- a/mix.lock
+++ b/mix.lock
@@ -2,18 +2,18 @@
   "backoff": {:hex, :backoff, "1.1.6", "83b72ed2108ba1ee8f7d1c22e0b4a00cfe3593a67dbc792799e8cce9f42f796b", [:rebar3], [], "hexpm", "cf0cfff8995fb20562f822e5cc47d8ccf664c5ecdc26a684cbe85c225f9d7c39"},
   "benchfella": {:hex, :benchfella, "0.3.5", "b2122c234117b3f91ed7b43b6e915e19e1ab216971154acd0a80ce0e9b8c05f5", [:mix], [], "hexpm", "23f27cbc482cbac03fc8926441eb60a5e111759c17642bac005c3225f5eb809d"},
   "dialyxir": {:hex, :dialyxir, "1.0.0", "6a1fa629f7881a9f5aaf3a78f094b2a51a0357c843871b8bc98824e7342d00a5", [:mix], [{:erlex, ">= 0.2.6", [hex: :erlex, repo: "hexpm", optional: false]}], "hexpm", "aeb06588145fac14ca08d8061a142d52753dbc2cf7f0d00fc1013f53f8654654"},
-  "earmark": {:hex, :earmark, "1.4.4", "4821b8d05cda507189d51f2caeef370cf1e18ca5d7dfb7d31e9cafe6688106a4", [:mix], [], "hexpm", "1f93aba7340574847c0f609da787f0d79efcab51b044bb6e242cae5aca9d264d"},
+  "earmark_parser": {:hex, :earmark_parser, "1.4.10", "6603d7a603b9c18d3d20db69921527f82ef09990885ed7525003c7fe7dc86c56", [:mix], [], "hexpm", "8e2d5370b732385db2c9b22215c3f59c84ac7dda7ed7e544d7c459496ae519c0"},
   "elixir_uuid": {:hex, :elixir_uuid, "1.2.1", "dce506597acb7e6b0daeaff52ff6a9043f5919a4c3315abb4143f0b00378c097", [:mix], [], "hexpm", "f7eba2ea6c3555cea09706492716b0d87397b88946e6380898c2889d68585752"},
   "erlex": {:hex, :erlex, "0.2.6", "c7987d15e899c7a2f34f5420d2a2ea0d659682c06ac607572df55a43753aa12e", [:mix], [], "hexpm", "2ed2e25711feb44d52b17d2780eabf998452f6efda104877a3881c2f8c0c0c75"},
-  "ex_doc": {:hex, :ex_doc, "0.22.1", "9bb6d51508778193a4ea90fa16eac47f8b67934f33f8271d5e1edec2dc0eee4c", [:mix], [{:earmark, "~> 1.4.0", [hex: :earmark, repo: "hexpm", optional: false]}, {:makeup_elixir, "~> 0.14", [hex: :makeup_elixir, repo: "hexpm", optional: false]}], "hexpm", "d957de1b75cb9f78d3ee17820733dc4460114d8b1e11f7ee4fd6546e69b1db60"},
+  "ex_doc": {:hex, :ex_doc, "0.22.2", "03a2a58bdd2ba0d83d004507c4ee113b9c521956938298eba16e55cc4aba4a6c", [:mix], [{:earmark_parser, "~> 1.4.0", [hex: :earmark_parser, repo: "hexpm", optional: false]}, {:makeup_elixir, "~> 0.14", [hex: :makeup_elixir, repo: "hexpm", optional: false]}], "hexpm", "cf60e1b3e2efe317095b6bb79651f83a2c1b3edcb4d319c421d7fcda8b3aff26"},
   "file_system": {:hex, :file_system, "0.2.8", "f632bd287927a1eed2b718f22af727c5aeaccc9a98d8c2bd7bff709e851dc986", [:mix], [], "hexpm", "97a3b6f8d63ef53bd0113070102db2ce05352ecf0d25390eb8d747c2bde98bca"},
   "global_flags": {:hex, :global_flags, "1.0.0", "ee6b864979a1fb38d1fbc67838565644baf632212bce864adca21042df036433", [:rebar3], [], "hexpm", "85d944cecd0f8f96b20ce70b5b16ebccedfcd25e744376b131e89ce61ba93176"},
   "jason": {:hex, :jason, "1.2.1", "12b22825e22f468c02eb3e4b9985f3d0cb8dc40b9bd704730efa11abd2708c44", [:mix], [{:decimal, "~> 1.0", [hex: :decimal, repo: "hexpm", optional: true]}], "hexpm", "b659b8571deedf60f79c5a608e15414085fa141344e2716fbd6988a084b5f993"},
   "local_cluster": {:hex, :local_cluster, "1.1.0", "a2a0e3e965aa1549939108066bfa537ce89f0107917f5b0260153e2fdb304116", [:mix], [{:global_flags, "~> 1.0", [hex: :global_flags, repo: "hexpm", optional: false]}], "hexpm", "fef6476083cf6f4c0526bb682de7ff75cd8b4bd4b7e20b3be60c1dab05f28ca7"},
-  "makeup": {:hex, :makeup, "1.0.1", "82f332e461dc6c79dbd82fbe2a9c10d48ed07146f0a478286e590c83c52010b5", [:mix], [{:nimble_parsec, "~> 0.5.0", [hex: :nimble_parsec, repo: "hexpm", optional: false]}], "hexpm", "49736fe5b66a08d8575bf5321d716bac5da20c8e6b97714fec2bcd6febcfa1f8"},
-  "makeup_elixir": {:hex, :makeup_elixir, "0.14.0", "cf8b7c66ad1cff4c14679698d532f0b5d45a3968ffbcbfd590339cb57742f1ae", [:mix], [{:makeup, "~> 1.0", [hex: :makeup, repo: "hexpm", optional: false]}], "hexpm", "d4b316c7222a85bbaa2fd7c6e90e37e953257ad196dc229505137c5e505e9eff"},
+  "makeup": {:hex, :makeup, "1.0.3", "e339e2f766d12e7260e6672dd4047405963c5ec99661abdc432e6ec67d29ef95", [:mix], [{:nimble_parsec, "~> 0.5", [hex: :nimble_parsec, repo: "hexpm", optional: false]}], "hexpm", "2e9b4996d11832947731f7608fed7ad2f9443011b3b479ae288011265cdd3dad"},
+  "makeup_elixir": {:hex, :makeup_elixir, "0.14.1", "4f0e96847c63c17841d42c08107405a005a2680eb9c7ccadfd757bd31dabccfb", [:mix], [{:makeup, "~> 1.0", [hex: :makeup, repo: "hexpm", optional: false]}], "hexpm", "f2438b1a80eaec9ede832b5c41cd4f373b38fd7aa33e3b22d9db79e640cbde11"},
   "mix_test_watch": {:hex, :mix_test_watch, "1.0.2", "34900184cbbbc6b6ed616ed3a8ea9b791f9fd2088419352a6d3200525637f785", [:mix], [{:file_system, "~> 0.2.1 or ~> 0.3", [hex: :file_system, repo: "hexpm", optional: false]}], "hexpm", "47ac558d8b06f684773972c6d04fcc15590abdb97aeb7666da19fcbfdc441a07"},
   "mox": {:hex, :mox, "0.5.2", "55a0a5ba9ccc671518d068c8dddd20eeb436909ea79d1799e2209df7eaa98b6c", [:mix], [], "hexpm", "df4310628cd628ee181df93f50ddfd07be3e5ecc30232d3b6aadf30bdfe6092b"},
-  "nimble_parsec": {:hex, :nimble_parsec, "0.5.3", "def21c10a9ed70ce22754fdeea0810dafd53c2db3219a0cd54cf5526377af1c6", [:mix], [], "hexpm", "589b5af56f4afca65217a1f3eb3fee7e79b09c40c742fddc1c312b3ac0b3399f"},
+  "nimble_parsec": {:hex, :nimble_parsec, "0.6.0", "32111b3bf39137144abd7ba1cce0914533b2d16ef35e8abc5ec8be6122944263", [:mix], [], "hexpm", "27eac315a94909d4dc68bc07a4a83e06c8379237c5ea528a9acff4ca1c873c52"},
   "phoenix_pubsub": {:hex, :phoenix_pubsub, "2.0.0", "a1ae76717bb168cdeb10ec9d92d1480fec99e3080f011402c0a2d68d47395ffb", [:mix], [], "hexpm", "c52d948c4f261577b9c6fa804be91884b381a7f8f18450c5045975435350f771"},
 }


Test patch:
diff --git a/test/event/event_handler_macro_test.exs b/test/event/event_handler_macro_test.exs
index b9856b9..3f370f2 100644
--- a/test/event/event_handler_macro_test.exs
+++ b/test/event/event_handler_macro_test.exs
@@ -7,13 +7,6 @@ defmodule Commanded.Event.EventHandlerMacroTest do
   alias Commanded.ExampleDomain.BankAccount.AccountBalanceHandler
   alias Commanded.Helpers.{EventFactory, Wait}
 
-  describe "event handler macro" do
-    test "should provide `__name__/0` function" do
-      assert AccountBalanceHandler.__name__() ==
-               "Commanded.ExampleDomain.BankAccount.AccountBalanceHandler"
-    end
-  end
-
   describe "event handler" do
     setup do
       start_supervised!(BankApp)
diff --git a/test/event/handle_event_test.exs b/test/event/handle_event_test.exs
index f202298..d1cbb50 100644
--- a/test/event/handle_event_test.exs
+++ b/test/event/handle_event_test.exs
@@ -234,47 +234,48 @@ defmodule Commanded.Event.HandleEventTest do
 
   describe "event handler name" do
     test "should parse string" do
-      assert Commanded.Event.Handler.parse_name(__MODULE__, "foo") == "foo"
+      assert Commanded.Event.Handler.parse_name("foo") == "foo"
     end
 
     test "should parse atom to string" do
-      assert Commanded.Event.Handler.parse_name(__MODULE__, :foo) == ":foo"
+      assert Commanded.Event.Handler.parse_name(:foo) == ":foo"
     end
 
     test "should parse tuple to string" do
-      assert Commanded.Event.Handler.parse_name(__MODULE__, {:foo, :bar}) == "{:foo, :bar}"
+      assert Commanded.Event.Handler.parse_name({:foo, :bar}) == "{:foo, :bar}"
     end
 
-    test "should error when parsing empty string" do
-      assert_raise ArgumentError, fn ->
-        Commanded.Event.Handler.parse_name(__MODULE__, "")
-      end
+    test "should parse empty string to `nil`" do
+      assert Commanded.Event.Handler.parse_name("") == nil
     end
 
-    test "should error when parsing `nil`" do
-      assert_raise ArgumentError, fn ->
-        Commanded.Event.Handler.parse_name(__MODULE__, nil)
-      end
+    test "should parse `nil` to `nil`" do
+      assert Commanded.Event.Handler.parse_name(nil) == nil
     end
   end
 
+  defmodule NoAppEventHandler do
+    use Commanded.Event.Handler, name: __MODULE__
+  end
+
   test "should ensure an application is provided" do
-    assert_raise ArgumentError, "NoAppEventHandler expects :application option", fn ->
-      Code.eval_string("""
-        defmodule NoAppEventHandler do
-          use Commanded.Event.Handler, name: __MODULE__
-        end
-      """)
+    expected_error =
+      "Commanded.Event.HandleEventTest.NoAppEventHandler expects :application option"
+
+    assert_raise ArgumentError, expected_error, fn ->
+      NoAppEventHandler.start_link()
     end
   end
 
+  defmodule UnnamedEventHandler do
+    use Commanded.Event.Handler, application: Commanded.DefaultApp
+  end
+
   test "should ensure an event handler name is provided" do
-    assert_raise ArgumentError, "UnnamedEventHandler expects :name option", fn ->
-      Code.eval_string("""
-        defmodule UnnamedEventHandler do
-          use Commanded.Event.Handler, application: Commanded.DefaultApp
-        end
-      """)
+    expected_error = "Commanded.Event.HandleEventTest.UnnamedEventHandler expects :name option"
+
+    assert_raise ArgumentError, expected_error, fn ->
+      UnnamedEventHandler.start_link()
     end
   end
 
@@ -286,48 +287,6 @@ defmodule Commanded.Event.HandleEventTest do
     """)
   end
 
-  describe "Mix task must be able to reset" do
-    setup do
-      start_supervised!(BankApp)
-
-      :ok
-    end
-
-    test "Can reset an event handler" do
-      stream_uuid = UUID.uuid4()
-      initial_events = [%BankAccountOpened{account_number: "ACC123", initial_balance: 1_000}]
-
-      :ok = EventStore.append_to_stream(BankApp, stream_uuid, 0, to_event_data(initial_events))
-
-      start_supervised!(BankAccountHandler)
-
-      Wait.until(fn ->
-        assert BankAccountHandler.current_accounts() == ["ACC123"]
-      end)
-
-      :ok = BankAccountHandler.change_prefix("PREF_")
-
-      handler_name = BankAccountHandler.__name__()
-      registry_name = Commanded.Event.Handler.name(BankApp, handler_name)
-
-      pid = Commanded.Registration.whereis_name(BankApp, registry_name)
-
-      assert :undefined != pid
-
-      Mix.Tasks.Commanded.Reset.run([
-        "--app",
-        "Commanded.ExampleDomain.BankApp",
-        "--handler",
-        "Commanded.ExampleDomain.BankAccount.BankAccountHandler",
-        "--quiet"
-      ])
-
-      Wait.until(fn ->
-        assert BankAccountHandler.current_accounts() == ["PREF_ACC123"]
-      end)
-    end
-  end
-
   defp to_event_data(events) do
     Commanded.Event.Mapper.map_to_event_data(events,
       causation_id: UUID.uuid4(),
diff --git a/test/event/handler_config_test.exs b/test/event/handler_config_test.exs
index 5ff8be1..3f1adb5 100644
--- a/test/event/handler_config_test.exs
+++ b/test/event/handler_config_test.exs
@@ -11,24 +11,36 @@ defmodule Commanded.Event.HandlerConfigTest do
 
   setup do
     start_supervised!(DefaultApp)
-
     :ok
   end
 
   test "should default to `:eventual` consistency and start from `:origin`" do
     {:ok, handler} = DefaultConfigHandler.start_link()
 
-    assert_config(handler, consistency: :eventual, start_from: :origin, subscribe_to: :all)
+    assert_config(handler,
+      application: Commanded.DefaultApp,
+      name: "Commanded.Event.HandlerConfigTest.DefaultConfigHandler",
+      consistency: :eventual,
+      start_from: :origin,
+      subscribe_to: :all
+    )
   end
 
   describe "global config change to default consistency" do
-    test "should default to `:eventual` consistency and start from `:origin`" do
-      Mix.Config.persist(commanded: [default_consistency: :strong])
+    test "should use overridden default consistency" do
+      Application.put_env(:commanded, :default_consistency, :strong)
+
       {:ok, handler} = DefaultConfigHandler.start_link()
 
-      assert_config(handler, consistency: :strong, start_from: :origin, subscribe_to: :all)
+      assert_config(handler,
+        application: Commanded.DefaultApp,
+        name: "Commanded.Event.HandlerConfigTest.DefaultConfigHandler",
+        consistency: :strong,
+        start_from: :origin,
+        subscribe_to: :all
+      )
 
-      Mix.Config.persist(commanded: [default_consistency: :eventual])
+      Application.put_env(:commanded, :default_consistency, :eventual)
     end
   end
 
@@ -42,10 +54,16 @@ defmodule Commanded.Event.HandlerConfigTest do
   end
 
   describe "config provided to `start_link/1`" do
-    test "should use config defined in handler" do
+    test "should use default config defined in handler when no opts passed to `start_link`" do
       {:ok, handler} = ExampleHandler.start_link()
 
-      assert_config(handler, consistency: :strong, start_from: :current, subscribe_to: "stream1")
+      assert_config(handler,
+        application: Commanded.DefaultApp,
+        name: "Commanded.Event.HandlerConfigTest.ExampleHandler",
+        consistency: :strong,
+        start_from: :current,
+        subscribe_to: "stream1"
+      )
     end
 
     test "should use overriden config when provided" do
@@ -56,43 +74,86 @@ defmodule Commanded.Event.HandlerConfigTest do
           subscribe_to: "stream2"
         )
 
-      assert_config(handler, consistency: :eventual, start_from: :origin, subscribe_to: "stream2")
+      assert_config(handler,
+        application: Commanded.DefaultApp,
+        name: "Commanded.Event.HandlerConfigTest.ExampleHandler",
+        consistency: :eventual,
+        start_from: :origin,
+        subscribe_to: "stream2"
+      )
     end
 
-    test "should use default config when not provided" do
+    test "should merge default config with provided opts" do
       {:ok, handler} = ExampleHandler.start_link(start_from: :origin)
 
-      assert_config(handler, consistency: :strong, start_from: :origin, subscribe_to: "stream1")
+      assert_config(handler,
+        application: Commanded.DefaultApp,
+        name: "Commanded.Event.HandlerConfigTest.ExampleHandler",
+        consistency: :strong,
+        start_from: :origin,
+        subscribe_to: "stream1"
+      )
     end
-  end
 
-  describe "config provided to `child_spec`" do
-    test "should use config defined in handler" do
-      {:ok, sup} =
-        Supervisor.start_link(
-          [
-            ExampleHandler
-          ],
-          strategy: :one_for_one
-        )
+    test "allow name to be provided" do
+      {:ok, handler} = ExampleHandler.start_link(name: "handler_name")
+
+      assert_config(handler,
+        application: Commanded.DefaultApp,
+        name: "handler_name",
+        consistency: :strong,
+        start_from: :current,
+        subscribe_to: "stream1"
+      )
+    end
+
+    test "allow application to be provided" do
+      start_supervised!({DefaultApp, name: :dynamic_app})
 
-      [{_, handler, _, _}] = Supervisor.which_children(sup)
+      {:ok, handler} = ExampleHandler.start_link(application: :dynamic_app)
 
-      assert_config(handler, consistency: :strong, start_from: :current, subscribe_to: "stream1")
+      assert_config(handler,
+        application: :dynamic_app,
+        name: "Commanded.Event.HandlerConfigTest.ExampleHandler",
+        consistency: :strong,
+        start_from: :current,
+        subscribe_to: "stream1"
+      )
+    end
+  end
+
+  describe "config provided to `child_spec`" do
+    test "should use default config defined in handler when no overrides provided" do
+      handler = start_supervised!(ExampleHandler)
+
+      assert_config(handler,
+        application: Commanded.DefaultApp,
+        name: "Commanded.Event.HandlerConfigTest.ExampleHandler",
+        consistency: :strong,
+        start_from: :current,
+        subscribe_to: "stream1"
+      )
     end
 
     test "should use overriden config when provided" do
-      {:ok, sup} =
-        Supervisor.start_link(
-          [
-            {ExampleHandler, [consistency: :eventual, start_from: :origin]}
-          ],
-          strategy: :one_for_one
+      start_supervised!({DefaultApp, name: :dynamic_app})
+
+      handler =
+        start_supervised!(
+          {ExampleHandler,
+           application: :dynamic_app,
+           name: "handler_name",
+           consistency: :eventual,
+           start_from: :origin}
         )
 
-      [{_, handler, _, _}] = Supervisor.which_children(sup)
-
-      assert_config(handler, consistency: :eventual, start_from: :origin, subscribe_to: "stream1")
+      assert_config(handler,
+        application: :dynamic_app,
+        name: "handler_name",
+        consistency: :eventual,
+        start_from: :origin,
+        subscribe_to: "stream1"
+      )
     end
   end
 
@@ -103,21 +164,70 @@ defmodule Commanded.Event.HandlerConfigTest do
       invalid_config_option: "invalid"
   end
 
-  test "should validate provided config" do
+  test "should validate provided config at runtime" do
     expected_error =
       "Commanded.Event.HandlerConfigTest.InvalidConfiguredHandler specifies invalid options: [:invalid_config_option]"
 
     assert_raise ArgumentError, expected_error, fn ->
-      Code.eval_string("""
-        alias Commanded.Event.HandlerConfigTest.InvalidConfiguredHandler
+      InvalidConfiguredHandler.start_link()
+    end
+  end
+
+  defmodule RuntimeConfiguredHandler do
+    use Commanded.Event.Handler
+  end
+
+  describe "runtime configured handler" do
+    test "should require application" do
+      expected_error =
+        "Commanded.Event.HandlerConfigTest.RuntimeConfiguredHandler expects :application option"
+
+      assert_raise ArgumentError, expected_error, fn ->
+        RuntimeConfiguredHandler.start_link()
+      end
+    end
+
+    test "should require name" do
+      expected_error =
+        "Commanded.Event.HandlerConfigTest.RuntimeConfiguredHandler expects :name option"
+
+      assert_raise ArgumentError, expected_error, fn ->
+        RuntimeConfiguredHandler.start_link(application: DefaultApp)
+      end
+    end
+
+    test "started with runtime opts" do
+      assert {:ok, _pid} =
+               RuntimeConfiguredHandler.start_link(name: "handler1", application: DefaultApp)
+
+      assert {:ok, _pid} =
+               RuntimeConfiguredHandler.start_link(name: "handler2", application: DefaultApp)
+    end
 
-        {:ok, _pid} = InvalidConfiguredHandler.start_link()
-      """)
+    test "start supervised with runtime opts" do
+      start_supervised!({RuntimeConfiguredHandler, name: "handler1", application: DefaultApp})
+      start_supervised!({RuntimeConfiguredHandler, name: "handler2", application: DefaultApp})
     end
   end
 
   defp assert_config(handler, expected_config) do
-    actual_config = GenServer.call(handler, :config)
+    alias Commanded.Event.Handler
+    alias Commanded.EventStore.Subscription
+
+    %Handler{
+      application: application,
+      handler_name: name,
+      consistency: consistency,
+      subscription: %Subscription{subscribe_from: subscribe_from, subscribe_to: subscribe_to}
+    } = :sys.get_state(handler)
+
+    actual_config = [
+      application: application,
+      name: name,
+      consistency: consistency,
+      start_from: subscribe_from,
+      subscribe_to: subscribe_to
+    ]
 
     assert actual_config == expected_config
   end
diff --git a/test/event/handler_init_test.exs b/test/event/handler_init_test.exs
index 5a58e06..8039035 100644
--- a/test/event/handler_init_test.exs
+++ b/test/event/handler_init_test.exs
@@ -3,36 +3,49 @@ defmodule Commanded.Event.HandlerInitTest do
 
   import Mox
 
-  alias Commanded.Event.InitHandler
+  alias Commanded.DefaultApp
+  alias Commanded.Event.{InitHandler, RuntimeConfigHandler}
   alias Commanded.EventStore.Adapters.Mock, as: MockEventStore
 
-  setup do
-    reply_to = self()
+  describe "event handler `init/1` callback" do
+    setup do
+      for tenant <- [:tenant1, :tenant2, :tenant3] do
+        start_supervised!({DefaultApp, name: Module.concat([DefaultApp, tenant])})
+      end
 
-    subscribe_to = fn _event_store, :all, handler_name, handler, _subscribe_from ->
-      assert is_binary(handler_name)
+      :ok
+    end
+
+    test "should be called at runtime" do
+      {:ok, _handler1} = RuntimeConfigHandler.start_link(tenant: :tenant1, reply_to: self())
+      {:ok, _handler2} = RuntimeConfigHandler.start_link(tenant: :tenant2, reply_to: self())
+      {:ok, _handler3} = RuntimeConfigHandler.start_link(tenant: :tenant3, reply_to: self())
 
-      {:ok, handler}
+      assert_receive {:init, :tenant1}
+      assert_receive {:init, :tenant2}
+      assert_receive {:init, :tenant3}
     end
+  end
 
-    expect(MockEventStore, :subscribe_to, subscribe_to)
+  describe "event handler `init/0` callback" do
+    setup do
+      reply_to = self()
 
-    {:ok, agent} = Agent.start_link(fn -> reply_to end, name: InitHandler)
+      subscribe_to = fn _event_store, :all, handler_name, handler, _subscribe_from ->
+        assert is_binary(handler_name)
 
-    handler = start_supervised!(InitHandler)
+        {:ok, handler}
+      end
 
-    [agent: agent, handler: handler]
-  end
+      expect(MockEventStore, :subscribe_to, subscribe_to)
 
-  describe "event handler `init/0` callback" do
-    test "should be called", %{handler: handler} do
-      send_subscribed(handler)
+      {:ok, _agent} = Agent.start_link(fn -> reply_to end, name: InitHandler)
 
-      assert_receive {:init, ^handler}
+      handler = start_supervised!(InitHandler)
+
+      [handler: handler]
     end
-  end
 
-  describe "event handler `init/0` callback after subscribed" do
     test "should be called after subscription subscribed", %{handler: handler} do
       refute_receive {:init, ^handler}
 
diff --git a/test/event/support/init/runtime_config_handler.ex b/test/event/support/init/runtime_config_handler.ex
new file mode 100644
index 0000000..0bcb966
--- /dev/null
+++ b/test/event/support/init/runtime_config_handler.ex
@@ -0,0 +1,19 @@
+defmodule Commanded.Event.RuntimeConfigHandler do
+  use Commanded.Event.Handler
+
+  alias Commanded.DefaultApp
+
+  def init(config) do
+    {reply_to, config} = Keyword.pop!(config, :reply_to)
+    {tenant, config} = Keyword.pop!(config, :tenant)
+
+    config =
+      config
+      |> Keyword.put(:application, Module.concat([DefaultApp, tenant]))
+      |> Keyword.put(:name, Module.concat([__MODULE__, tenant]))
+
+    send(reply_to, {:init, tenant})
+
+    {:ok, config}
+  end
+end
diff --git a/test/mix/tasks/reset_event_handler_test.exs b/test/mix/tasks/reset_event_handler_test.exs
new file mode 100644
index 0000000..deab4a6
--- /dev/null
+++ b/test/mix/tasks/reset_event_handler_test.exs
@@ -0,0 +1,58 @@
+defmodule Commanded.Mix.Tasks.ResetEventHandlerTest do
+  use ExUnit.Case
+
+  alias Commanded.EventStore
+  alias Commanded.ExampleDomain.BankApp
+  alias Commanded.ExampleDomain.BankAccount.BankAccountHandler
+  alias Commanded.ExampleDomain.BankAccount.Events.BankAccountOpened
+  alias Commanded.Helpers.Wait
+
+  setup do
+    start_supervised!(BankApp)
+    :ok
+  end
+
+  describe "mix `commanded.reset` task" do
+    test "can reset an event handler" do
+      stream_uuid = UUID.uuid4()
+      initial_events = [%BankAccountOpened{account_number: "ACC123", initial_balance: 1_000}]
+
+      :ok = EventStore.append_to_stream(BankApp, stream_uuid, 0, to_event_data(initial_events))
+
+      start_supervised!(BankAccountHandler)
+
+      Wait.until(fn ->
+        assert BankAccountHandler.current_accounts() == ["ACC123"]
+      end)
+
+      :ok = BankAccountHandler.change_prefix("PREF_")
+
+      handler_name = "Commanded.ExampleDomain.BankAccount.BankAccountHandler"
+      registry_name = Commanded.Event.Handler.name(BankApp, handler_name)
+
+      pid = Commanded.Registration.whereis_name(BankApp, registry_name)
+
+      assert :undefined != pid
+
+      Mix.Tasks.Commanded.Reset.run([
+        "--app",
+        "Commanded.ExampleDomain.BankApp",
+        "--handler",
+        handler_name,
+        "--quiet"
+      ])
+
+      Wait.until(fn ->
+        assert BankAccountHandler.current_accounts() == ["PREF_ACC123"]
+      end)
+    end
+  end
+
+  defp to_event_data(events) do
+    Commanded.Event.Mapper.map_to_event_data(events,
+      causation_id: UUID.uuid4(),
+      correlation_id: UUID.uuid4(),
+      metadata: %{}
+    )
+  end
+end
diff --git a/test/process_managers/process_manager_init_test.exs b/test/process_managers/process_manager_init_test.exs
new file mode 100644
index 0000000..8a5f508
--- /dev/null
+++ b/test/process_managers/process_manager_init_test.exs
@@ -0,0 +1,26 @@
+defmodule Commanded.ProcessManager.ProcessManagerInitTest do
+  use ExUnit.Case
+
+  alias Commanded.DefaultApp
+  alias Commanded.ProcessManagers.RuntimeConfigProcessManager
+
+  describe "process manager `init/1` callback" do
+    setup do
+      for tenant <- [:tenant1, :tenant2, :tenant3] do
+        start_supervised!({DefaultApp, name: Module.concat([DefaultApp, tenant])})
+      end
+
+      :ok
+    end
+
+    test "should be called at runtime" do
+      {:ok, _pm1} = RuntimeConfigProcessManager.start_link(tenant: :tenant1, reply_to: self())
+      {:ok, _pm2} = RuntimeConfigProcessManager.start_link(tenant: :tenant2, reply_to: self())
+      {:ok, _pm3} = RuntimeConfigProcessManager.start_link(tenant: :tenant3, reply_to: self())
+
+      assert_receive {:init, :tenant1}
+      assert_receive {:init, :tenant2}
+      assert_receive {:init, :tenant3}
+    end
+  end
+end
diff --git a/test/process_managers/process_manager_instance_test.exs b/test/process_managers/process_manager_instance_test.exs
index 7b87b14..1367b1b 100644
--- a/test/process_managers/process_manager_instance_test.exs
+++ b/test/process_managers/process_manager_instance_test.exs
@@ -132,39 +132,42 @@ defmodule Commanded.ProcessManagers.ProcessManagerInstanceTest do
                "Commanded.ExampleDomain.TransferMoneyProcessManager received unexpected message: :unexpected_message"
     end
 
-    test "should provide `__name__/0` function" do
-      assert TransferMoneyProcessManager.__name__() ==
-               "Commanded.ExampleDomain.TransferMoneyProcessManager"
+    defmodule NoAppProcessManager do
+      use Commanded.ProcessManagers.ProcessManager, name: __MODULE__
     end
 
     test "should ensure a process manager application is provided" do
-      assert_raise ArgumentError, "NoAppProcessManager expects :application option", fn ->
-        Code.eval_string("""
-          defmodule NoAppProcessManager do
-            use Commanded.ProcessManagers.ProcessManager, name: __MODULE__
-          end
-        """)
+      expected_error =
+        "Commanded.ProcessManagers.ProcessManagerInstanceTest.NoAppProcessManager expects :application option"
+
+      assert_raise ArgumentError, expected_error, fn ->
+        NoAppProcessManager.start_link()
       end
     end
 
+    defmodule UnnamedProcessManager do
+      use Commanded.ProcessManagers.ProcessManager, application: Commanded.DefaultApp
+    end
+
     test "should ensure a process manager name is provided" do
-      assert_raise ArgumentError, "UnnamedProcessManager expects :name option", fn ->
-        Code.eval_string("""
-          defmodule UnnamedProcessManager do
-            use Commanded.ProcessManagers.ProcessManager, application: Commanded.DefaultApp
-          end
-        """)
+      expected_error =
+        "Commanded.ProcessManagers.ProcessManagerInstanceTest.UnnamedProcessManager expects :name option"
+
+      assert_raise ArgumentError, expected_error, fn ->
+        UnnamedProcessManager.start_link()
       end
     end
 
+    defmodule MyProcessManager do
+      use Commanded.ProcessManagers.ProcessManager,
+        application: Commanded.DefaultApp,
+        name: __MODULE__
+    end
+
     test "should allow using process manager module as name" do
-      Code.eval_string("""
-        defmodule MyProcessManager do
-          use Commanded.ProcessManagers.ProcessManager,
-            application: Commanded.DefaultApp,
-            name: __MODULE__
-        end
-      """)
+      start_supervised!(Commanded.DefaultApp)
+
+      assert {:ok, _pid} = MyProcessManager.start_link()
     end
   end
 
diff --git a/test/process_managers/support/init/runtime_config_process_manager.ex b/test/process_managers/support/init/runtime_config_process_manager.ex
new file mode 100644
index 0000000..601b360
--- /dev/null
+++ b/test/process_managers/support/init/runtime_config_process_manager.ex
@@ -0,0 +1,24 @@
+defmodule Commanded.ProcessManagers.RuntimeConfigProcessManager do
+  @moduledoc false
+
+  use Commanded.ProcessManagers.ProcessManager
+
+  alias Commanded.DefaultApp
+
+  @derive Jason.Encoder
+  defstruct [:process_uuid]
+
+  def init(config) do
+    {reply_to, config} = Keyword.pop!(config, :reply_to)
+    {tenant, config} = Keyword.pop!(config, :tenant)
+
+    config =
+      config
+      |> Keyword.put(:application, Module.concat([DefaultApp, tenant]))
+      |> Keyword.put(:name, Module.concat([__MODULE__, tenant]))
+
+    send(reply_to, {:init, tenant})
+
+    {:ok, config}
+  end
+end
diff --git a/test/subscriptions/distributed_subscriptions_test.exs b/test/subscriptions/distributed_subscriptions_test.exs
index 105aeca..1764ae6 100644
--- a/test/subscriptions/distributed_subscriptions_test.exs
+++ b/test/subscriptions/distributed_subscriptions_test.exs
@@ -18,8 +18,7 @@ defmodule Commanded.DistributedSubscriptionsTest do
       DistributedSubscribers.start_subscribers(nodes)
       DistributedSubscribers.query_subscriptions(nodes)
 
-      expected_subscribers =
-        DistributedSubscribers.all() |> Enum.map(& &1.__name__()) |> Enum.sort()
+      expected_subscribers = DistributedSubscribers.all() |> Enum.map(&inspect/1) |> Enum.sort()
 
       for node <- nodes do
         assert_receive {:subscriptions, ^node, ^expected_subscribers}
diff --git a/test/subscriptions/subscriptions_test.exs b/test/subscriptions/subscriptions_test.exs
index 847a4e1..6aefd44 100644
--- a/test/subscriptions/subscriptions_test.exs
+++ b/test/subscriptions/subscriptions_test.exs
@@ -12,23 +12,26 @@ defmodule Commanded.SubscriptionsTest do
     end
 
     test "should be registered" do
-      :ok = Subscriptions.register(DefaultApp, "handler1", :strong)
-      :ok = Subscriptions.register(DefaultApp, "handler2", :eventual)
-      :ok = Subscriptions.register(DefaultApp, "handler3", :strong)
-
-      assert Subscriptions.all(DefaultApp) == [{"handler3", self()}, {"handler1", self()}]
+      :ok = Subscriptions.register(DefaultApp, "handler1", Handler1, :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler2", Handler2, :eventual)
+      :ok = Subscriptions.register(DefaultApp, "handler3", Handler3, :strong)
+
+      assert Subscriptions.all(DefaultApp) == [
+               {"handler3", Handler3, self()},
+               {"handler1", Handler1, self()}
+             ]
     end
 
     test "should not remove PID when process terminates" do
       pid =
         spawn_link(fn ->
-          :ok = Subscriptions.register(DefaultApp, "handler1", :strong)
+          :ok = Subscriptions.register(DefaultApp, "handler1", Handler1, :strong)
         end)
 
       ref = Process.monitor(pid)
       assert_receive {:DOWN, ^ref, :process, _, :normal}
 
-      assert Subscriptions.all(DefaultApp) == [{"handler1", pid}]
+      assert Subscriptions.all(DefaultApp) == [{"handler1", Handler1, pid}]
     end
 
     test "should replace registered subscription PID" do
@@ -36,27 +39,27 @@ defmodule Commanded.SubscriptionsTest do
 
       pid1 =
         spawn_link(fn ->
-          :ok = Subscriptions.register(DefaultApp, "handler1", :strong)
+          :ok = Subscriptions.register(DefaultApp, "handler1", Handler1, :strong)
 
           send(reply_to, {:handler, self()})
         end)
 
       assert_receive {:handler, ^pid1}
-      assert Subscriptions.all(DefaultApp) == [{"handler1", pid1}]
+      assert Subscriptions.all(DefaultApp) == [{"handler1", Handler1, pid1}]
 
       pid2 =
         spawn_link(fn ->
-          :ok = Subscriptions.register(DefaultApp, "handler1", :strong)
+          :ok = Subscriptions.register(DefaultApp, "handler1", Handler1, :strong)
 
           send(reply_to, {:handler, self()})
         end)
 
       assert_receive {:handler, ^pid2}
-      assert Subscriptions.all(DefaultApp) == [{"handler1", pid2}]
+      assert Subscriptions.all(DefaultApp) == [{"handler1", Handler1, pid2}]
     end
 
     test "should ack event" do
-      :ok = Subscriptions.register(DefaultApp, "handler1", :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler1", Handler1, :strong)
 
       :ok =
         Subscriptions.ack_event(DefaultApp, "handler1", :strong, %RecordedEvent{
@@ -69,8 +72,8 @@ defmodule Commanded.SubscriptionsTest do
     end
 
     test "should require all subscriptions to ack event" do
-      :ok = Subscriptions.register(DefaultApp, "handler1", :strong)
-      :ok = Subscriptions.register(DefaultApp, "handler2", :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler1", Handler1, :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler2", Handler2, :strong)
 
       :ok =
         Subscriptions.ack_event(DefaultApp, "handler1", :strong, %RecordedEvent{
@@ -100,7 +103,7 @@ defmodule Commanded.SubscriptionsTest do
     end
 
     test "should ignore current process as handler" do
-      :ok = Subscriptions.register(DefaultApp, "handler1", :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler1", Handler1, :strong)
 
       # current process should not block handler
       assert Subscriptions.handled?(DefaultApp, "stream1", 1, exclude: [self()])
@@ -118,7 +121,7 @@ defmodule Commanded.SubscriptionsTest do
     end
 
     test "should immediately succeed when waited event has already been ack'd" do
-      :ok = Subscriptions.register(DefaultApp, "handler", :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler", Handler, :strong)
 
       :ok =
         Subscriptions.ack_event(DefaultApp, "handler", :strong, %RecordedEvent{
@@ -136,13 +139,13 @@ defmodule Commanded.SubscriptionsTest do
     end
 
     test "should immediately succeed when excluding handler process" do
-      :ok = Subscriptions.register(DefaultApp, "handler", :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler", Handler, :strong)
 
       assert :ok == Subscriptions.wait_for(DefaultApp, "stream1", 2, exclude: [self()])
     end
 
     test "should succeed when waited event is ack'd" do
-      :ok = Subscriptions.register(DefaultApp, "handler", :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler", Handler, :strong)
 
       wait_task =
         Task.async(fn ->
@@ -165,7 +168,7 @@ defmodule Commanded.SubscriptionsTest do
     end
 
     test "should ignore events before requested" do
-      :ok = Subscriptions.register(DefaultApp, "handler", :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler", Handler, :strong)
 
       :ok =
         Subscriptions.ack_event(DefaultApp, "handler", :strong, %RecordedEvent{event_number: 1})
@@ -174,9 +177,9 @@ defmodule Commanded.SubscriptionsTest do
     end
 
     test "should wait for all subscriptions to ack event" do
-      :ok = Subscriptions.register(DefaultApp, "handler1", :strong)
-      :ok = Subscriptions.register(DefaultApp, "handler2", :strong)
-      :ok = Subscriptions.register(DefaultApp, "handler3", :eventual)
+      :ok = Subscriptions.register(DefaultApp, "handler1", Handler1, :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler2", Handler2, :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler3", Handler3, :eventual)
 
       refute Subscriptions.handled?(DefaultApp, "stream1", 2)
 
@@ -210,7 +213,7 @@ defmodule Commanded.SubscriptionsTest do
     end
 
     test "should allow subscriptions to skip events when ack" do
-      :ok = Subscriptions.register(DefaultApp, "handler", :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler", Handler, :strong)
 
       refute Subscriptions.handled?(DefaultApp, "stream1", 2)
 
@@ -223,9 +226,9 @@ defmodule Commanded.SubscriptionsTest do
       assert Subscriptions.handled?(DefaultApp, "stream1", 2)
     end
 
-    test "should allow per-handler consistency" do
-      :ok = Subscriptions.register(DefaultApp, "handler1", :strong)
-      :ok = Subscriptions.register(DefaultApp, "handler2", :strong)
+    test "should allow per-handler consistency by name" do
+      :ok = Subscriptions.register(DefaultApp, "handler1", Handler1, :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler2", Handler2, :strong)
 
       :ok =
         Subscriptions.ack_event(DefaultApp, "handler1", :strong, %RecordedEvent{
@@ -237,11 +240,29 @@ defmodule Commanded.SubscriptionsTest do
       assert :ok == Subscriptions.wait_for(DefaultApp, "stream1", 2, consistency: ["handler1"])
     end
 
+    test "should allow per-handler consistency by module" do
+      :ok = Subscriptions.register(DefaultApp, "handler1", Handler1, :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler2", Handler2, :strong)
+
+      :ok =
+        Subscriptions.ack_event(DefaultApp, "handler1", :strong, %RecordedEvent{
+          stream_id: "stream1",
+          stream_version: 2
+        })
+
+      refute Subscriptions.handled?(DefaultApp, "stream1", 2)
+      refute Subscriptions.handled?(DefaultApp, "stream1", 2, consistency: ["handler2"])
+      refute Subscriptions.handled?(DefaultApp, "stream1", 2, consistency: [Handler2])
+      assert :ok == Subscriptions.wait_for(DefaultApp, "stream1", 2, consistency: [Handler1])
+      assert Subscriptions.handled?(DefaultApp, "stream1", 2, consistency: ["handler1"])
+      assert Subscriptions.handled?(DefaultApp, "stream1", 2, consistency: [Handler1])
+    end
+
     test "should wait for each configured handler consistency" do
-      :ok = Subscriptions.register(DefaultApp, "handler1", :strong)
-      :ok = Subscriptions.register(DefaultApp, "handler2", :strong)
-      :ok = Subscriptions.register(DefaultApp, "handler3", :strong)
-      :ok = Subscriptions.register(DefaultApp, "handler3", :eventual)
+      :ok = Subscriptions.register(DefaultApp, "handler1", Handler1, :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler2", Handler2, :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler3", Handler3, :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler3", Handler4, :eventual)
 
       :ok =
         Subscriptions.ack_event(DefaultApp, "handler1", :strong, %RecordedEvent{
@@ -293,8 +314,8 @@ defmodule Commanded.SubscriptionsTest do
     end
 
     test "should be associated with an application" do
-      :ok = Subscriptions.register(:app1, "handler1", :strong)
-      :ok = Subscriptions.register(:app2, "handler1", :strong)
+      :ok = Subscriptions.register(:app1, "handler1", Handler1, :strong)
+      :ok = Subscriptions.register(:app2, "handler1", Handler1, :strong)
 
       :ok =
         Subscriptions.ack_event(:app1, "handler1", :strong, %RecordedEvent{
@@ -323,7 +344,7 @@ defmodule Commanded.SubscriptionsTest do
     end
 
     test "should expire stale acks" do
-      :ok = Subscriptions.register(DefaultApp, "handler1", :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler1", Handler1, :strong)
 
       :ok =
         Subscriptions.ack_event(DefaultApp, "handler1", :strong, %RecordedEvent{
@@ -340,7 +361,7 @@ defmodule Commanded.SubscriptionsTest do
     end
 
     test "should not expire fresh acks" do
-      :ok = Subscriptions.register(DefaultApp, "handler1", :strong)
+      :ok = Subscriptions.register(DefaultApp, "handler1", Handler1, :strong)
 
       :ok =
         Subscriptions.ack_event(DefaultApp, "handler1", :strong, %RecordedEvent{
diff --git a/test/subscriptions/support/distributed_subscribers.ex b/test/subscriptions/support/distributed_subscribers.ex
index d52fa9a..e50c7c8 100644
--- a/test/subscriptions/support/distributed_subscribers.ex
+++ b/test/subscriptions/support/distributed_subscribers.ex
@@ -61,7 +61,7 @@ defmodule Commanded.Subscriptions.DistributedSubscribers do
       Node.spawn_link(node, fn ->
         subscriptions =
           Subscriptions.all(DistributedApp)
-          |> Enum.map(fn {name, _pid} -> name end)
+          |> Enum.map(fn {name, _module, _pid} -> name end)
           |> Enum.sort()
 
         send(reply_to, {:subscriptions, node, subscriptions})