Feature: Read replica support

.env

@@ -1,6 +1,7 @@
 PG_DATABASE_URL=postgres://granite:password@localhost:5432/granite_db
 MYSQL_DATABASE_URL=mysql://granite:password@localhost:3306/granite_db
 SQLITE_DATABASE_URL=sqlite3:./granite.db
+SQLITE_REPLICA_URL=sqlite3:./granite_replica.db
 CURRENT_ADAPTER=pg
 PG_VERSION=15.2
 MYSQL_VERSION=5.7

spec/adapter/adapters_spec.cr

@@ -13,20 +13,26 @@ end
 describe Granite::Connections do
   describe "registration" do
     it "should allow connections to be be saved and looked up" do
-      Granite::Connections.registered_connections.size.should eq 3
+      Granite::Connections.registered_connections.size.should eq 4
 
       if connection = Granite::Connections["mysql"]
-        connection.url.should eq ENV["MYSQL_DATABASE_URL"]
+        connection[:writer].url.should eq ENV["MYSQL_DATABASE_URL"]
       else
         connection.should_not be_falsey
       end
       if connection = Granite::Connections["pg"]
-        connection.url.should eq ENV["PG_DATABASE_URL"]
+        connection[:writer].url.should eq ENV["PG_DATABASE_URL"]
       else
         connection.should_not be_falsey
       end
       if connection = Granite::Connections["sqlite"]
-        connection.url.should eq ENV["SQLITE_DATABASE_URL"]
+        connection[:writer].url.should eq ENV["SQLITE_DATABASE_URL"]
+      else
+        connection.should_not be_falsey
+      end
+      if connection = Granite::Connections["sqlite_replica"]
+        connection[:writer].url.should eq ENV["SQLITE_DATABASE_URL"]
+        connection[:reader].url.should eq ENV["SQLITE_REPLICA_URL"]
       else
         connection.should_not be_falsey
       end

spec/granite/connection_switching_spec.cr

@@ -0,0 +1,17 @@
+require "spec"
+
+describe "Granite::Base track time since last write" do
+  it "should ensure that time since last write to model is saved" do
+    # This is a good way to test last write time
+    # It is unlikely that the test suite takes more than an hour to run
+    (Chat.time_since_last_write < 1.hours).should be_true
+
+    # Update the time since hte last write and make sure it updated to the time within the currnet second
+    Chat.update_last_write_time
+
+    # Time since last write should be less than a second since we just updated it (see line above)
+    unless (Chat.time_since_last_write < 1.second)
+      raise "ERROR!!!!!"
+    end
+  end
+end

spec/spec_helper.cr

@@ -6,6 +6,10 @@ Granite::Connections << Granite::Adapter::Mysql.new(name: "mysql", url: ENV["MYS
 Granite::Connections << Granite::Adapter::Pg.new(name: "pg", url: ENV["PG_DATABASE_URL"])
 Granite::Connections << Granite::Adapter::Sqlite.new(name: "sqlite", url: ENV["SQLITE_DATABASE_URL"])
 
+# Connections with replicas
+# TODO: Experiment to find a better API.
+Granite::Connections.<<(name: "sqlite_replica", writer: ENV["SQLITE_DATABASE_URL"], reader: ENV["SQLITE_REPLICA_URL"], adapter_type: Granite::Adapter::Sqlite)
+
 require "spec"
 require "../src/granite"
 require "../src/adapter/**"

src/granite/base.cr

@@ -39,6 +39,39 @@ abstract class Granite::Base
   extend Integrators
   extend Select
 
+  @@last_write_time = Time.monotonic
+
+  def self.last_write_time
+    @@last_write_time
+  end
+
+  # This is done this way because callbacks don't work on class mthods
+  def self.update_last_write_time
+    @@last_write_time = Time.monotonic
+  end
+
+  def update_last_write_time
+    self.class.update_last_write_time
+  end
+
+  def self.time_since_last_write
+    Time.monotonic - @@last_write_time
+  end
+
+  def time_since_last_write
+    self.class.time_since_last_write
+  end
+
+  def self.switch_to_reader_adapter
+    if time_since_last_write > 2.seconds
+      @@adapter = @@reader_adapter
+    end
+  end
+
+  def self.switch_to_writer_adapter
+    @@adapter = @@writer_adapter
+  end
+
   macro inherited
     protected class_getter select_container : Container = Container.new(table_name: table_name, fields: fields)
 
@@ -70,5 +103,10 @@ abstract class Granite::Base
 
     disable_granite_docs? def initialize
     end
+
+    after_save :update_last_write_time
+    after_update :update_last_write_time
+    after_create :update_last_write_time
+    after_destroy :update_last_write_time
   end
 end

src/granite/connections.cr

@@ -1,16 +1,23 @@
 module Granite
   class Connections
-    class_getter registered_connections = [] of Granite::Adapter::Base
+    class_getter registered_connections = [] of {writer: Granite::Adapter::Base, reader: Granite::Adapter::Base}
 
     # Registers the given *adapter*.  Raises if an adapter with the same name has already been registered.
     def self.<<(adapter : Granite::Adapter::Base) : Nil
-      raise "Adapter with name '#{adapter.name}' has already been registered." if @@registered_connections.any? { |conn| conn.name == adapter.name }
-      @@registered_connections << adapter
+      raise "Adapter with name '#{adapter.name}' has already been registered." if @@registered_connections.any? { |conn| conn[:writer].name == adapter.name }
+      @@registered_connections << {writer: adapter, reader: adapter}
+    end
+
+    # TODO: Find cleaner type restriction method
+    def self.<<(*, name : String, reader : String, writer : String, adapter_type : Granite::Adapter::Base.class) : Nil
+      reader_adapter = adapter_type.new(name: name, url: reader)
+      writer_adapter = adapter_type.new(name: name, url: writer)
+      @@registered_connections << {writer: writer_adapter, reader: reader_adapter}
     end
 
     # Returns a registered connection with the given *name*, otherwise `nil`.
-    def self.[](name : String) : Granite::Adapter::Base?
-      registered_connections.find { |conn| conn.name == name }
+    def self.[](name : String) : {writer: Granite::Adapter::Base, reader: Granite::Adapter::Base}?
+      registered_connections.find { |conn| conn[:writer].name == name }
     end
   end
 end

src/granite/table.cr

@@ -11,7 +11,7 @@ end
 module Granite::Tables
   module ClassMethods
     def adapter : Granite::Adapter::Base
-      Granite::Connections.registered_connections.first? || raise "No connections have been registered."
+      Granite::Connections.registered_connections.first?.not_nil![:writer] || raise "No connections have been registered."
     end
 
     def primary_name
@@ -57,6 +57,9 @@ module Granite::Tables
 
   # specify the database connection you will be using for this model.
   macro connection(name)
-    class_getter adapter : Granite::Adapter::Base = Granite::Connections[{{(name.is_a?(StringLiteral) ? name : name.id.stringify)}}] || raise "No registered connection with the name '{{name.id}}'"
+    class_getter writer_adapter : Granite::Adapter::Base = Granite::Connections[{{(name.is_a?(StringLiteral) ? name : name.id.stringify)}}].not_nil![:writer]
+    class_getter reader_adapter : Granite::Adapter::Base = Granite::Connections[{{(name.is_a?(StringLiteral) ? name : name.id.stringify)}}].not_nil![:reader]
+    class_getter adapter : Granite::Adapter::Base = @@writer_adapter
+
   end
 end