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diff --git a/activerecord/lib/active_record/middleware/database_selector.rb b/activerecord/lib/active_record/middleware/database_selector.rb
new file mode 100644
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+++ b/activerecord/lib/active_record/middleware/database_selector.rb
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+# frozen_string_literal: true
+
+require "active_record/middleware/database_selector/resolver"
+
+module ActiveRecord
+  module Middleware
+    # The DatabaseSelector Middleware provides a framework for automatically
+    # swapping from the primary to the replica database connection. Rails
+    # provides a basic framework to determine when to swap and allows for
+    # applications to write custom strategy classes to override the default
+    # behavior.
+    #
+    # The resolver class defines when the application should switch (i.e. read
+    # from the primary if a write occurred less than 2 seconds ago) and an
+    # operations class that sets a value that helps the resolver class decide
+    # when to switch.
+    #
+    # Rails default middleware uses the request's session to set a timestamp
+    # that informs the application when to read from a primary or read from a
+    # replica.
+    #
+    # To use the DatabaseSelector in your application with default settings add
+    # the following options to your environment config:
+    #
+    #   config.active_record.database_selector = { delay: 2.seconds }
+    #   config.active_record.database_resolver = ActiveRecord::Middleware::DatabaseSelector::Resolver
+    #   config.active_record.database_operations = ActiveRecord::Middleware::DatabaseSelector::Resolver::Session
+    #
+    # New applications will include these lines commented out in the production.rb.
+    #
+    # The default behavior can be changed by setting the config options to a
+    # custom class:
+    #
+    #   config.active_record.database_selector = { delay: 2.seconds }
+    #   config.active_record.database_resolver = MyResolver
+    #   config.active_record.database_operations = MyResolver::MySession
+    class DatabaseSelector
+      def initialize(app, resolver_klass = Resolver, operations_klass = Resolver::Session)
+        @app = app
+        @resolver_klass = resolver_klass
+        @operations_klass = operations_klass
+      end
+
+      attr_reader :resolver_klass, :operations_klass
+
+      # Middleware that determines which database connection to use in a mutliple
+      # database application.
+      def call(env)
+        request = ActionDispatch::Request.new(env)
+
+        select_database(request) do
+          @app.call(env)
+        end
+      end
+
+      private
+
+        def select_database(request, &blk)
+          operations = operations_klass.build(request)
+          database_resolver = resolver_klass.call(operations)
+
+          if reading_request?(request)
+            database_resolver.read(&blk)
+          else
+            database_resolver.write(&blk)
+          end
+        end
+
+        def reading_request?(request)
+          request.get? || request.head?
+        end
+    end
+  end
+end