diff --git a/features/examples/topology.change-history.feature b/features/examples/topology.change-history.feature
new file mode 100644
index 000000000..9f1f33225
--- /dev/null
+++ b/features/examples/topology.change-history.feature
@@ -0,0 +1,38 @@
+Feature: Topology Ruby API example
+  
+  The change-history.rb example ([trema]/src/examples/topology/change-history.rb)
+  is a sample controller which uses Topology Ruby API.
+
+  @slow_process
+  Scenario: Run the Ruby example
+    Given a file named "change-history.conf" with:
+      """
+      1.upto( 3 ).each do | sw |
+        vswitch { dpid sw }
+        1.upto( sw - 1 ).each do | peer |
+          link "%#x" % sw, "%#x" % peer
+        end
+      end
+      
+      run {
+        path "../../objects/topology/topology"
+      }
+      
+      run {
+        path "../../objects/examples/dumper/dumper"
+      }
+      
+      event :port_status => "topology", :packet_in => "filter", :state_notify => "topology"
+      filter :lldp => "topology", :packet_in => "dumper"
+      """
+    When I run `../../trema run ../../src/examples/topology/change-history.rb -c change-history.conf -d`
+    Then wait until "topology" is up
+    Then *** sleep 32 ***
+
+    Then the file "change-history.dot" should match /.*graph \[label="Gen \d+\\n\(0x3 -> 0x1\) up"\];.*/
+    Then the file "change-history.dot" should match /.*graph \[label="Gen \d+\\n\(0x3 -> 0x2\) up"\];.*/
+    Then the file "change-history.dot" should match /.*graph \[label="Gen \d+\\n\(0x2 -> 0x1\) up"\];.*/
+    Then the file "change-history.dot" should match /.*graph \[label="Gen \d+\\n\(0x2 -> 0x3\) up"\];.*/
+    Then the file "change-history.dot" should match /.*graph \[label="Gen \d+\\n\(0x1 -> 0x2\) up"\];.*/
+    Then the file "change-history.dot" should match /.*graph \[label="Gen \d+\\n\(0x1 -> 0x3\) up"\];.*/
+    
\ No newline at end of file
diff --git a/features/examples/topology.show_switch_status.feature b/features/examples/topology.show_switch_status.feature
index b1ae61122..26e5be95b 100644
--- a/features/examples/topology.show_switch_status.feature
+++ b/features/examples/topology.show_switch_status.feature
@@ -1,6 +1,7 @@
 Feature: show_switch_status example.
   
   show_switch_status is a simple usage example of topology C API.
+  show-switch-status.rb is a simple usage example of topology Ruby API.
   
   show_switch_status command will query for all the switch and port information 
   that the topology daemon hold and print them to standard output.
@@ -44,3 +45,39 @@ Feature: show_switch_status example.
       """
     And the output should match /  dpid : 0xe0, port : 1\(.+\), status : up, external : (true|false)/
     And the output should match /  dpid : 0xe0, port : 2\(.+\), status : up, external : (true|false)/
+
+  @slow_process
+  Scenario: [Ruby API] Show switch and port information obtained from topology.
+    Given a file named "show_switch_status.conf" with:
+      """
+      vswitch("topology1") { datapath_id "0xe0" }
+      vhost ("host1") {
+        ip "192.168.0.1"
+        netmask "255.255.0.0"
+        mac "00:00:00:01:00:01"
+      }
+      
+      vhost ("host2") {
+        ip "192.168.0.2"
+        netmask "255.255.0.0"
+        mac "00:00:00:01:00:02"
+      }
+      
+      link "topology1", "host1"
+      link "topology1", "host2"
+      """
+    And I run `trema run ../repeater_hub/repeater-hub.rb -c show_switch_status.conf -d`
+    And I run `trema run "../../../objects/topology/topology -d"`
+    And *** sleep 4 ***
+    When I run `trema run ./show-switch-status.rb`
+    Then the output should contain:
+      """
+      Switch status
+        dpid : 0xe0, status : up
+      """
+    And the output should contain:
+      """
+      Port status
+      """
+    And the output should match /  dpid : 0xe0, port : 1\(.+\), status : up, external : (true|false)/
+    And the output should match /  dpid : 0xe0, port : 2\(.+\), status : up, external : (true|false)/
diff --git a/features/examples/topology.show_topology.feature b/features/examples/topology.show_topology.feature
index 4791bbb29..77277d9b4 100644
--- a/features/examples/topology.show_topology.feature
+++ b/features/examples/topology.show_topology.feature
@@ -1,6 +1,7 @@
 Feature: show_topology example.
   
   show_topology is a simple usage example of topology C API.
+  show-topology.rb is a simple usage example of topology Ruby API.
   
   show_topology command will query for all the link information that 
   the topology daemon hold and print them in trema network DSL style.
@@ -157,3 +158,73 @@ Feature: show_topology example.
       """
       link "0x4", "0x1"
       """
+
+  @slow_process
+  Scenario: [Ruby API] Show discovered link topology. (Directly start topology daemon)
+    Given a file named "show_topology.conf" with:
+      """
+      vswitch("topology1") { datapath_id "0x1" }
+      vswitch("topology2") { datapath_id "0x2" }
+      vswitch("topology3") { datapath_id "0x3" }
+      vswitch("topology4") { datapath_id "0x4" }
+      
+      link "topology1", "topology2"
+      link "topology1", "topology3"
+      link "topology1", "topology4"
+      link "topology2", "topology3"
+      link "topology2", "topology4"
+      link "topology3", "topology4"
+      """
+    And I run `trema run ../repeater_hub/repeater-hub.rb -c show_topology.conf -d`
+    And I run `trema run "../../../objects/topology/topology -d --always_run_discovery"`
+    And *** sleep 4 ***
+    When I run `trema run ./show-topology.rb`
+    Then the output should contain:
+      """
+      vswitch {
+        datapath_id "0x2"
+      }
+      """
+    And the output should contain:
+      """
+      vswitch {
+        datapath_id "0x3"
+      }
+      """
+    And the output should contain:
+      """
+      vswitch {
+        datapath_id "0x1"
+      }
+      """
+    And the output should contain:
+      """
+      vswitch {
+        datapath_id "0x4"
+      }
+      """
+    And the output should contain:
+      """
+      link "0x2", "0x1"
+      """
+    And the output should contain:
+      """
+      link "0x3", "0x2"
+      """
+    And the output should contain:
+      """
+      link "0x3", "0x1"
+      """
+    And the output should contain:
+      """
+      link "0x4", "0x2"
+      """
+    And the output should contain:
+      """
+      link "0x4", "0x3"
+      """
+    And the output should contain:
+      """
+      link "0x4", "0x1"
+      """
+
diff --git a/src/examples/topology/README.md b/src/examples/topology/README.md
index 94a9e6c04..6a7482558 100644
--- a/src/examples/topology/README.md
+++ b/src/examples/topology/README.md
@@ -6,7 +6,7 @@ This directory includes sample application using libtopology.
 - `show_topology` is a command line application, which retrieves 
   link information from Topology daemon and print them in trema network DSL style.
 
-- `show_swith_status` is a command line application, which retrieves 
+- `show_switch_status` is a command line application, which retrieves 
   switch and port information from Topology daemon and print them to stdout.
 
 - `enable_discovery` is a command line application, which enables
@@ -27,8 +27,8 @@ This directory includes sample application using libtopology.
         +----------+                         packet out(LLDP)       +-----------+
 
 
-How to run
-----------
+How to run (show_topology/show_switch_status)
+---------------------------------------------
 
 1. Change to trema directory and build trema, if you haven't done so already. 
 
@@ -51,7 +51,7 @@ How to run
 
         $ ./trema run ./objects/examples/topology/enable_discovery
 
-4. Run show_topology/show_switch_status from trema run 
+4. (C version) Run show_topology/show_switch_status from trema run 
 
         $ ./trema run objects/examples/topology/show_topology
         $ ./trema run objects/examples/topology/show_switch_status
@@ -61,6 +61,63 @@ How to run
         $ env TREMA_HOME=`pwd` src/examples/topology/show_topology
         $ env TREMA_HOME=`pwd` src/examples/topology/show_switch_status
 
+4. (Ruby version) Run show-topology.rb/show-switch-status.rb from trema run 
+
+        $ ./trema run src/examples/topology/show-topology.rb
+        $ ./trema run src/examples/topology/show-switch-status.rb
+
+
+How to run (change-history.rb)
+------------------------------
+
+1. Change to trema directory and build trema, if you haven't done so already. 
+
+        $ cd $TREMA_HOME
+        $ ./build.rb
+
+2. Start topology daemon.
+
+        $ ./trema run -c src/examples/topology/change-history.conf &
+
+3. Run change-history.rb from trema run 
+
+        $ ./trema run src/examples/topology/change-history.rb
+
+4. Controller will output will be written to change-history.dot in current directory.
+
+        (In another terminal)
+        $ tail -F change-history.dot
+        digraph {
+          subgraph cluster0 {
+            graph [label="Gen 0\ninitial"];
+            "0x1_0" [label="0x1", color="green"];
+            "0x2_0" [label="0x2", color="green"];
+            "0x3_0" [label="0x3", color="green"];
+            "0x1_0" -> "0x2_0" [color="green"];
+            "0x1_0" -> "0x3_0" [color="green"];
+            "0x2_0" -> "0x3_0" [color="green"];
+            "0x3_0" -> "0x2_0" [color="green"];
+            "0x3_0" -> "0x1_0" [color="green"];
+            "0x2_0" -> "0x1_0" [color="green"];
+
+5. See how topology change is notified to dot file.
+   Example: stop switch with dpid 0x3
+
+        (In another terminal)
+        $ ./trema kill 0x3
+
+   Example: restart switch with dpid 0x3
+
+        $ ./trema up 0x3
+
+6. Graphically see topology change history.
+
+        $ dot -Tsvg -O change-history.dot
+        $ firefox change-history.dot.svg
+
+   You may need to install graphviz package to use dot command.
+
+        $ sudo apt-get install graphviz
 
 License & Terms
 ---------------
diff --git a/src/examples/topology/change-history.conf b/src/examples/topology/change-history.conf
new file mode 100644
index 000000000..e838c90f1
--- /dev/null
+++ b/src/examples/topology/change-history.conf
@@ -0,0 +1,20 @@
+
+1.upto( 3 ).each do | sw |
+  vswitch { dpid sw }
+  1.upto( sw - 1 ).each do | peer |
+    link "%#x" % sw, "%#x" % peer
+  end
+end
+
+run {
+  path "./objects/topology/topology"
+#  options "-l debug"
+}
+
+run {
+  path "./objects/examples/dumper/dumper"
+}
+
+event :port_status => "topology", :packet_in => "filter", :state_notify => "topology"
+filter :lldp => "topology", :packet_in => "dumper"
+
diff --git a/src/examples/topology/change-history.rb b/src/examples/topology/change-history.rb
new file mode 100644
index 000000000..cd56a8c5b
--- /dev/null
+++ b/src/examples/topology/change-history.rb
@@ -0,0 +1,161 @@
+require "trema/topology"
+require "trema/topology/topology_cache"
+
+
+# Controller which keeps printing Switch and Link change event as graphviz dot file.
+# It will shutdown itself if there was no Switch or Link event for 60 seconds.
+#
+class ChangeHistory < Controller
+  include Topology
+
+  # TODO Add option to specify idle check interval.
+  periodic_timer_event :shutdown_on_idle, 60
+
+  oneshot_timer_event :on_start, 0
+  def on_start
+    enable_topology_discovery
+    send_rebuild_cache_request
+
+    # TODO Add option to specify file from command line.
+    @dotfile = File.open( "change-history.dot", "w" )
+
+    # digraph {
+    puts_dotfile "digraph {"
+    at_exit do
+      # } for digraph
+      puts_dotfile "}"
+    end
+  end
+
+
+  def cache_ready g
+    @generation = 0
+
+    # Initially color everything as updated (green)
+    g.switches.each do | _, sw |
+      sw[:color] = "green"
+    end
+    g.links.each do | _, lnk |
+      lnk[:color] = "green"
+    end
+
+    puts_dotfile to_dot_subgraph( g, @generation, "initial" )
+  end
+
+
+  def switch_status_updated sw_attr
+    if not instance_variable_defined?(:@generation) then
+      return nil
+    end
+
+    @generation += 1
+    sw_attr[:generation] = @generation
+
+    g = get_cache
+    remove_old_colors( g, @generation )
+
+    if sw_attr[:up] then
+      sw_attr[:color] = "green"
+      update_cache_by_switch_hash sw_attr
+      sublabel = "0x#{sw_attr[:dpid].to_s(16)} up"
+      puts_dotfile to_dot_subgraph( g, @generation, sublabel )
+    else
+      g.switches[ sw_attr[:dpid] ][:color] = "red" if g.switches[ sw_attr[:dpid] ]
+      sublabel = "0x#{sw_attr[:dpid].to_s(16)} down"
+      puts_dotfile to_dot_subgraph( g, @generation, sublabel )
+      # update to new state. (switch instance will be removed)
+      update_cache_by_switch_hash sw_attr
+    end
+
+  end
+
+
+  def link_status_updated link_attr
+    if not instance_variable_defined?(:@generation) then
+      return nil
+    end
+    link = Link[link_attr]
+
+    @generation += 1
+    link[:generation] = @generation
+
+    g = get_cache
+    remove_old_colors( g, @generation )
+
+    if link.up? then
+      link[:color] = "green"
+      update_cache_by_link_hash link
+      sublabel = "(0x#{link.from_dpid.to_s(16)} -> 0x#{link.to_dpid.to_s(16)}) up"
+      puts_dotfile to_dot_subgraph( g, @generation, sublabel )
+    else
+      g.links[ link.key ][:color] = "red" if g.links[ link.key ]
+      sublabel = "(0x#{link.from_dpid.to_s(16)} -> 0x#{link.to_dpid.to_s(16)}) down"
+      puts_dotfile to_dot_subgraph( g, @generation, sublabel )
+      # update to new state. (link instance will be removed)
+      update_cache_by_link_hash link
+    end
+  end
+
+
+  def shutdown_on_idle
+    @last_generation ||= -1
+    if @last_generation == @generation then
+      shutdown!
+    else
+      @last_generation = @generation
+    end
+  end
+
+
+  def remove_old_colors( g, generation )
+    g.switches.each do | _, sw |
+      sw[:generation] ||= 0
+      sw.delete(:color) if sw[:generation] != generation
+    end
+    g.links.each do | _, lnk |
+      lnk[:generation] ||= 0
+      lnk.delete(:color) if lnk[:generation] != generation
+    end
+  end
+
+
+  def to_dot_subgraph( g, generation, sublabel="" )
+    s = "  subgraph cluster#{generation} {\n"
+    s << %Q(    graph [label="Gen #{generation}\\n#{sublabel}"];\n)
+    # // switches
+    g.switches.each do | _, sw |
+      dot_attrs = %Q(label="0x#{sw.dpid.to_s(16)}")
+      if sw.member?(:color) then
+        dot_attrs << ", " unless dot_attrs.empty?
+        dot_attrs << %Q(color="#{sw[:color]}")
+      end
+      s << %Q(    "0x#{sw.dpid.to_s(16)}_#{generation}" [#{dot_attrs}];\n)
+    end
+    # // links
+    g.links.each do | _, lnk |
+      dot_attrs = ""
+      if lnk.member?(:color) then
+        dot_attrs << ", " unless dot_attrs.empty?
+        dot_attrs << %Q(color="#{lnk[:color]}")
+      end
+      s << %Q(    "0x#{lnk.from_dpid.to_s(16)}_#{generation}" -> "0x#{lnk.to_dpid.to_s(16)}_#{generation}" [#{dot_attrs}];\n)
+    end
+    # } for subgraph
+    s << "  }\n"
+    return s
+  end
+
+
+  def puts_dotfile s
+    begin
+      @dotfile.puts s
+      @dotfile.flush
+    rescue
+      error "Failed to write dotfile (#{$!.inspect})\n\t#{$!.backtrace.join("\n\t") }"
+      STDERR.puts "Failed to write dotfile."
+      shutdown!
+    end
+  end
+end
+
+
diff --git a/src/examples/topology/show-switch-status.rb b/src/examples/topology/show-switch-status.rb
new file mode 100644
index 000000000..cd9d2b61e
--- /dev/null
+++ b/src/examples/topology/show-switch-status.rb
@@ -0,0 +1,53 @@
+require "trema/topology"
+
+
+class ShowSwitchStatus < Controller
+  include Topology
+
+
+  oneshot_timer_event :timed_out, 15
+  oneshot_timer_event :on_start, 0
+  def on_start
+    send_all_switch_status_request
+  end
+
+  def all_switch_status_reply sw_status
+    puts "Switch status"
+    sw_status.each do | sw_hash |
+      sw = Topology::Switch[ sw_hash ]
+
+      status_str = "unknown"
+      if sw.up? then
+        status_str = "up"
+      else
+        status_str = "down"
+      end
+      puts "  dpid : 0x#{sw.dpid.to_s(16)}, status : #{status_str}"
+    end
+
+    send_all_port_status_request
+  end
+
+  def all_port_status_reply port_status
+    puts "Port status"
+    port_status.each do | port_hash |
+      port = Port[ port_hash ]
+      status_str = "unknown"
+      if port.up? then
+        status_str = "up"
+      else
+        status_str = "down"
+      end
+      
+      puts "  dpid : 0x#{port.dpid.to_s(16)}, port : #{port.portno.to_s}(#{port.name}), status : #{status_str}, external : #{port.external?.to_s}"
+    end
+
+    shutdown!
+  end
+
+
+  def timed_out
+    error "timed out."
+    shutdown!
+  end
+end
diff --git a/src/examples/topology/show-topology.rb b/src/examples/topology/show-topology.rb
new file mode 100644
index 000000000..da0220aaa
--- /dev/null
+++ b/src/examples/topology/show-topology.rb
@@ -0,0 +1,53 @@
+require "trema/topology"
+
+
+class ShowTopology < Controller
+  include Topology
+
+
+  oneshot_timer_event :timed_out, 10
+  oneshot_timer_event :on_start, 0
+  def on_start
+    send_all_link_status_request
+  end
+
+
+  def all_link_status_reply link_status
+    dpids = Hash.new
+    links = Hash.new
+
+    debug "topology: entries #{link_status.size}"
+
+    link_status.each do | link_hash |
+      link = Link[link_hash]
+      if link.up? then
+        dpids[link.from_dpid] = nil
+        dpids[link.to_dpid] = nil
+
+        dpid_pair = [link.from_dpid, link.to_dpid]
+
+        links[ [dpid_pair.max, dpid_pair.min] ] = nil
+      else
+        debug "link down"
+      end
+    end
+
+    dpids.keys.each do | dpid |
+      puts "vswitch {"
+      puts %Q(  datapath_id "0x#{dpid.to_s(16)}")
+      puts "}\n\n"
+    end
+
+    links.keys.each do | dpid0, dpid1 |
+      puts %Q(link "0x#{dpid0.to_s(16)}", "0x#{dpid1.to_s(16)}")
+    end
+
+    shutdown!
+  end
+
+
+  def timed_out
+    error "timed out."
+    shutdown!
+  end
+end