Add global configuration to tweak defaults

- Only permit changing serializer for now, nothing else makes sense.
- In the future this would allow choosing PG backends, to make best fit
  for application constraints in which En57 is embedded.

Author: mostlyobvious

.mutant.yml

@@ -19,3 +19,4 @@ matcher:
     - En57::EventStore.for_sequel
     - En57::JsonSerializer#optional_big_decimal_type
     - En57::Migrator*
+    - En57::Repository#initialize

lib/en57.rb

@@ -11,7 +11,16 @@
 require_relative "en57/repository"
 require_relative "en57/migrator"
 require_relative "en57/event_store"
+require_relative "en57/configuration"
 
 module En57
   AppendConditionViolated = Class.new(StandardError)
+
+  def self.configuration = Configuration.instance
+
+  def self.configure
+    yield configuration
+  ensure
+    configuration.freeze
+  end
 end

lib/en57/active_record_adapter.rb

@@ -25,12 +25,7 @@ def with_serializable_transaction
 
   class EventStore
     def self.for_active_record(model = ActiveRecord::Base)
-      new(
-        Repository.new(
-          ActiveRecordAdapter.new(model.connection_pool),
-          JsonSerializer.new,
-        ),
-      )
+      new(Repository.new(ActiveRecordAdapter.new(model.connection_pool)))
     end
   end
 end

lib/en57/configuration.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+require "singleton"
+
+module En57
+  class Configuration
+    include Singleton
+
+    attr_accessor :serializer
+
+    def initialize
+      @serializer = JsonSerializer.new
+    end
+  end
+end

lib/en57/pg_adapter.rb

@@ -39,12 +39,7 @@ def with
 
   class EventStore
     def self.for_pg(connection_uri)
-      new(
-        Repository.new(
-          PgAdapter.for_connection(PG.connect(connection_uri)),
-          JsonSerializer.new,
-        ),
-      )
+      new(Repository.new(PgAdapter.for_connection(PG.connect(connection_uri))))
     end
   end
 
@@ -58,7 +53,6 @@ def self.for_pooled_pg(connection_uri, max_connections: 5)
                 PG.connect(connection_uri)
               end,
             ),
-            JsonSerializer.new,
           ),
         )
       end

lib/en57/repository.rb

@@ -4,7 +4,7 @@
 
 module En57
   class Repository
-    def initialize(adapter, serializer)
+    def initialize(adapter, serializer = En57.configuration.serializer)
       @adapter = adapter
       @serializer = serializer
       @record_encoder = PG::TextEncoder::Record.new

lib/en57/sequel_adapter.rb

@@ -24,7 +24,7 @@ def with_serializable_transaction
 
   class EventStore
     def self.for_sequel(database)
-      new(Repository.new(SequelAdapter.new(database), JsonSerializer.new))
+      new(Repository.new(SequelAdapter.new(database)))
     end
   end
 end

test/test_en57.rb

@@ -24,8 +24,36 @@ def test_requiring_en57_without_active_record_skips_active_record_adapter
       RUBY
     end
 
+    def test_configuration_default_serializer
+      assert_kind_of JsonSerializer, En57.configuration.serializer
+    end
+
+    def test_configure
+      with_empty_configuration do
+        serializer = Object.new
+        En57.configure { |c| c.serializer = serializer }
+
+        assert_equal serializer, En57.configuration.serializer
+      end
+    end
+
+    def test_configure_disallows_further_modifications_after_boot
+      with_empty_configuration do
+        serializer = Object.new
+        En57.configure { |c| c.serializer = serializer }
+
+        assert_raises(FrozenError) do
+          En57.configure { |c| c.serializer = serializer }
+        end
+      end
+    end
+
     private
 
+    def with_empty_configuration
+      En57.stub(:configuration, Class.new(Configuration).instance) { yield }
+    end
+
     def assert_ruby(script)
       reader, writer = IO.pipe
       pid =