instrument.rb

  class Dot
    class Node
      attr_reader :children, :name, :started
      attr_accessor :finished

      def initialize name, fields = [], started = Time.now
        @name     = name
        @fields   = fields
        @children = []
        @started  = started
        @finished = nil
      end

      def label
        fields = @fields.map { |k,v|
          "<tr><td>#{k.to_s}</td><td>#{v.to_s}</td></tr>"
        }.join "\n"

        <<-eolabel
#{object_id}[
  shape="plaintext";
  label=<
<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
  <TR><TD colspan="2">#{name} (#{delta_ms} ms)</TD></TR>
  #{fields}
</TABLE>>];
        eolabel
      end

      def delta_ms
        if finished
          1000.0 * (finished - started)
        else
          "..."
        end
      end

      def to_dot
        stack = [self]
        edges = []
        until stack.empty?
          node = stack.pop
          node.children.each do |child|
            edges << [node, child]
            stack << child
          end
        end

        nodes = edges.flatten.uniq
        e = edges.map { |l,r| "#{l.object_id} -> #{r.object_id}" }.join ";\n"
        <<-eodot
digraph G {
  #{nodes.map(&:label).join(";\n")};
  #{e};
}
        eodot
      end
    end

    def initialize
      @root  = Node.new 'ROOT'
      @stack = [@root]
      trap(:INFO) {
        File.open('/tmp/out.dot', 'w') { |f|
          f.write @root.to_dot
        }
        @stack = [@root]
        @root.children.clear
      }
    end

    def start name, id, payload
      fields = payload.find_all { |k,v|
        String === v
      }.map { |k,v| [k, v.sub(/^#{Application.root}/, '')] }

      node   = Node.new name, fields

      @stack.last.children << node
      @stack << node
    end

    def finish name, id, payload
      @stack.pop.finished = Time.now
    end
  end

  ActiveSupport::Notifications.notifier.subscribe(nil, Dot.new)