Second attempt which removes any kind proxying.

config/database.yml

@@ -1,13 +1,7 @@
 development:
   prepared_statements: false
-  adapter: postgresql_fallback
-  host: 172.17.0.2
-  port: 6432
+  adapter: postgresql
   database: discourse_development
-  username: tgxworld
-  password: test
-  replica_host: 172.17.0.3
-  replica_port: 6432
   min_messages: warning
   pool: 5
   timeout: 5000

lib/active_record/connection_adapters/postgresql_fallback_adapter.rb

@@ -14,7 +14,7 @@ class TaskObserver
         logger.info { "PG connection heartbeat: Master connection is not active.".freeze }
       else
         logger.error { "PG connection heartbeat failed with error: \"#{ex}\"" }
-      end
+     end
     end
   end
 
@@ -28,15 +28,21 @@ end
 module ActiveRecord
   module ConnectionHandling
     def postgresql_fallback_connection(config)
-      master_connection = postgresql_connection(config)
+      begin
+        connection = postgresql_connection(config)
+      rescue PG::ConnectionBad => e
+        connection = postgresql_connection(config.dup.merge({
+          "host" => config["replica_host"], "port" => config["replica_port"]
+        }))
 
-      replica_connection = postgresql_connection(config.dup.merge({
-        host: config[:replica_host], port: config[:replica_port]
-      }))
-      verify_replica(replica_connection)
+        verify_replica(connection)
 
-      klass = ConnectionAdapters::PostgreSQLFallbackAdapter.proxy_pass(master_connection.class)
-      klass.new(master_connection, replica_connection, logger, config)
+        Discourse.enable_readonly_mode if !Discourse.readonly_mode?
+
+        start_connection_heartbeart(connection, config)
+      end
+
+      connection
     end
 
     private
@@ -45,92 +51,24 @@ module ActiveRecord
       value = connection.raw_connection.exec("SELECT pg_is_in_recovery()").values[0][0]
       raise "Replica database server is not in recovery mode." if value == 'f'
     end
-  end
 
-  module ConnectionAdapters
-    class PostgreSQLFallbackAdapter < AbstractAdapter
-      ADAPTER_NAME = "PostgreSQLFallback".freeze
-      MAX_FAILURE = 5
-      HEARTBEAT_INTERVAL = 5
+    def interval
+      5
+    end
 
-      attr_reader :main_connection
+    def start_connection_heartbeart(existing_connection, config)
+      timer_task = Concurrent::TimerTask.new(execution_interval: interval) do |task|
+        connection = postgresql_connection(config)
 
-      def self.all_methods(klass)
-        methods = []
-
-        (klass.ancestors - AbstractAdapter.ancestors).each do |_klass|
-          %w(public protected private).map do |level|
-            methods << _klass.send("#{level}_instance_methods", false)
-          end
-        end
-
-        methods.flatten.uniq.sort
-      end
-
-      def self.proxy_pass(klass)
-        Class.new(self) do
-          (self.all_methods(klass) - self.all_methods(self)).each do |method|
-            self.class_eval <<-EOF
-              def #{method}(*args, &block)
-                proxy_method(:#{method}, *args, &block)
-              end
-            EOF
-          end
+        if connection.active?
+          existing_connection.disconnect!
+          Discourse.disable_readonly_mode if Discourse.readonly_mode?
+          task.shutdown
         end
       end
 
-      def initialize(master_connection, replica_connection, logger, config)
-        super(nil, logger, config)
-
-        @master_connection = master_connection
-        @main_connection = @master_connection
-        @replica_connection = replica_connection
-        @failure_count = 0
-        load!
-      end
-
-      def proxy_method(method, *args, &block)
-        @main_connection.send(method, *args, &block)
-      rescue ActiveRecord::StatementInvalid => e
-        if e.message.include?("PG::UnableToSend") && @main_connection == @master_connection
-          @failure_count += 1
-
-          if @failure_count == MAX_FAILURE
-            Discourse.enable_readonly_mode if !Discourse.readonly_mode?
-            @main_connection = @replica_connection
-            load!
-            connection_heartbeart(@master_connection)
-            @failure_count = 0
-          else
-            proxy_method(method, *args, &block)
-          end
-        end
-
-        raise e
-      end
-
-      private
-
-      def load!
-        @visitor = @main_connection.visitor
-        @connection = @main_connection.raw_connection
-      end
-
-      def connection_heartbeart(connection, interval = HEARTBEAT_INTERVAL)
-        timer_task = Concurrent::TimerTask.new(execution_interval: interval) do |task|
-          connection.reconnect!
-
-          if connection.active?
-            @main_connection = connection
-            load!
-            Discourse.disable_readonly_mode if Discourse.readonly_mode?
-            task.shutdown
-          end
-        end
-
-        timer_task.add_observer(TaskObserver.new)
-        timer_task.execute
-      end
+      timer_task.add_observer(TaskObserver.new)
+      timer_task.execute
     end
   end
 end

spec/components/active_record/connection_adapters/postgresql_fallback_adapter_spec.rb

@@ -1,55 +1,67 @@
 require 'rails_helper'
 require_dependency 'active_record/connection_adapters/postgresql_fallback_adapter'
 
-describe ActiveRecord::ConnectionAdapters::PostgreSQLFallbackAdapter do
-  let(:master_connection) { ActiveRecord::Base.connection }
-  let(:replica_connection) { master_connection.dup }
-  let(:adapter) { described_class.new(master_connection, replica_connection, nil, nil) }
-
-  before :each do
-    ActiveRecord::Base.clear_all_connections!
+describe ActiveRecord::ConnectionHandling do
+  let(:config) do
+    ActiveRecord::Base.configurations["test"].merge({
+      "adapter" => "postgresql_fallback",
+      "replica_host" => "localhost",
+      "replica_port" => "6432"
+    })
   end
 
-  describe "proxy_method" do
-    context "when master connection is not active" do
+  after do
+    ActiveRecord::Base.clear_all_connections!
+    Discourse.disable_readonly_mode
+  end
+
+  describe "#postgresql_fallback_connection" do
+    it 'should return a PostgreSQL adapter' do
+      expect(ActiveRecord::Base.postgresql_fallback_connection(config))
+        .to be_an_instance_of(ActiveRecord::ConnectionAdapters::PostgreSQLAdapter)
+    end
+
+    context 'when master server is down' do
       before do
-        replica_connection.stubs(:send)
-        master_connection.stubs(:send).raises(ActiveRecord::StatementInvalid.new('PG::UnableToSend'))
-        master_connection.stubs(:reconnect!)
-        master_connection.stubs(:active?).returns(false)
+        @replica_connection = mock('replica_connection')
 
-        @old_const = described_class::HEARTBEAT_INTERVAL
-        described_class.const_set("HEARTBEAT_INTERVAL", 0.1)
+        ActiveRecord::Base.expects(:postgresql_connection).with(config).raises(PG::ConnectionBad)
+
+        ActiveRecord::Base.expects(:postgresql_connection).with(config.merge({
+          "host" => "localhost", "port" => "6432"
+        })).returns(@replica_connection)
+
+        ActiveRecord::Base.expects(:verify_replica).with(@replica_connection)
+
+        @replica_connection.expects(:disconnect!)
+
+        ActiveRecord::Base.stubs(:interval).returns(0.1)
+
+        Concurrent::TimerTask.any_instance.expects(:shutdown)
       end
 
-      after do
-        Discourse.disable_readonly_mode
-        described_class.const_set("HEARTBEAT_INTERVAL", @old_const)
-      end
+      it 'should failover to a replica server' do
+        ActiveRecord::Base.postgresql_fallback_connection(config)
 
-      it "should set site to readonly mode and carry out failover and switch back procedures" do
-        expect(adapter.main_connection).to eq(master_connection)
-        adapter.proxy_method('some method')
         expect(Discourse.readonly_mode?).to eq(true)
-        expect(adapter.main_connection).to eq(replica_connection)
 
-        master_connection.stubs(:active?).returns(true)
+        ActiveRecord::Base.unstub(:postgresql_connection)
         sleep 0.15
 
         expect(Discourse.readonly_mode?).to eq(false)
-        expect(adapter.main_connection).to eq(master_connection)
+
+        expect(ActiveRecord::Base.connection)
+          .to be_an_instance_of(ActiveRecord::ConnectionAdapters::PostgreSQLAdapter)
       end
     end
 
-    it 'should raise errors not related to the database connection' do
-      master_connection.stubs(:send).raises(StandardError.new)
-      expect { adapter.proxy_method('some method') }.to raise_error(StandardError)
-    end
+    context 'when both master and replica server is down' do
+      it 'should raise the right error' do
+        ActiveRecord::Base.expects(:postgresql_connection).raises(PG::ConnectionBad).twice
 
-    it 'should proxy methods successfully' do
-      expect(adapter.proxy_method(:execute, 'SELECT 1').values[0][0]).to eq("1")
-      expect(adapter.proxy_method(:active?)).to eq(true)
-      expect(adapter.proxy_method(:raw_connection)).to eq(master_connection.raw_connection)
+        expect { ActiveRecord::Base.postgresql_fallback_connection(config) }
+          .to raise_error(PG::ConnectionBad)
+      end
     end
   end
 end