Added Sherlock, a custom profiling tool for GitLab

Sherlock will be a new GitLab specific tool for measuring the
performance of Rails requests (and SideKiq jobs at some point). Some of
the things that are currently tracked:

* SQL queries along with their timings, backtraces and query plans
  (using "EXPLAIN ANALYZE" for PostgreSQL and regular "EXPLAIN" for
  MySQL)
* Timings of application files (including views) on a per line basis
* Some meta data such as the request method, path, total duration, etc

More tracking (e.g. Rugged or gitlab-shell timings) might be added in
the future.

Sherlock will replace any existing tools we have used so far (e.g.
active_record_query_trace and rack-mini-profiler), hence the
corresponding Gems have been removed from the Gemfile.

Sherlock can be enabled by starting Rails as following:

    ENABLE_SHERLOCK=1 bundle exec rails s

Recorded transactions can be found at `/sherlock/transactions`.

29 files changed, 855 insertions, 23 deletions

diff --git a/Gemfile b/Gemfile
index bb31c147b26..99135b57ac2 100644
--- a/Gemfile
+++ b/Gemfile
@@ -214,11 +214,9 @@ group :development do
   gem "annotate", "~> 2.6.0"
   gem "letter_opener", '~> 1.1.2'
   gem 'quiet_assets', '~> 1.0.2'
-  gem 'rack-mini-profiler', '~> 0.9.0', require: false
   gem 'rerun', '~> 0.10.0'
   gem 'bullet', require: false
-  gem 'active_record_query_trace', require: false
-  gem 'rack-lineprof', platform: :mri
+  gem 'rblineprof', platform: :mri, require: false
 
   # Better errors handler
   gem 'better_errors', '~> 1.0.1'
diff --git a/Gemfile.lock b/Gemfile.lock
index 65abc45ff19..5cfb7096d76 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -17,7 +17,6 @@ GEM
       activesupport (= 4.1.12)
       builder (~> 3.1)
       erubis (~> 2.7.0)
-    active_record_query_trace (1.5)
     activemodel (4.1.12)
       activesupport (= 4.1.12)
       builder (~> 3.1)
@@ -489,12 +488,6 @@ GEM
     rack-attack (4.3.0)
       rack
     rack-cors (0.4.0)
-    rack-lineprof (0.0.3)
-      rack (~> 1.5)
-      rblineprof (~> 0.3.6)
-      term-ansicolor (~> 1.3)
-    rack-mini-profiler (0.9.7)
-      rack (>= 1.1.3)
     rack-mount (0.8.3)
       rack (>= 1.0.0)
     rack-oauth2 (1.0.10)
@@ -777,7 +770,6 @@ PLATFORMS
 DEPENDENCIES
   RedCloth (~> 4.2.9)
   ace-rails-ap (~> 2.0.1)
-  active_record_query_trace
   activerecord-deprecated_finders (~> 1.0.3)
   activerecord-session_store (~> 0.1.0)
   acts-as-taggable-on (~> 3.4)
@@ -875,11 +867,10 @@ DEPENDENCIES
   quiet_assets (~> 1.0.2)
   rack-attack (~> 4.3.0)
   rack-cors (~> 0.4.0)
-  rack-lineprof
-  rack-mini-profiler (~> 0.9.0)
   rack-oauth2 (~> 1.0.5)
   rails (= 4.1.12)
   raphael-rails (~> 2.1.2)
+  rblineprof
   rdoc (~> 3.6)
   redcarpet (~> 3.3.3)
   redis-rails (~> 4.0.0)
diff --git a/app/assets/stylesheets/pages/sherlock.scss b/app/assets/stylesheets/pages/sherlock.scss
new file mode 100644
index 00000000000..92d84d9640f
--- /dev/null
+++ b/app/assets/stylesheets/pages/sherlock.scss
@@ -0,0 +1,33 @@
+table .sherlock-code {
+  max-width: 700px;
+}
+
+.sherlock-code {
+  pre {
+    word-wrap: normal;
+  }
+
+  pre code {
+    white-space: pre;
+  }
+}
+
+.sherlock-line-samples-table {
+  margin-bottom: 0px !important;
+
+  thead tr th,
+  tbody tr td {
+    font-size: 13px !important;
+    text-align: right;
+    padding: 0px 10px !important;
+  }
+}
+
+.sherlock-file-sample pre {
+  padding-top: 28px !important;
+}
+
+.sherlock-line-samples-table .slow {
+  color: $red-light;
+  font-weight: bold;
+}
diff --git a/app/controllers/sherlock/application_controller.rb b/app/controllers/sherlock/application_controller.rb
new file mode 100644
index 00000000000..682ca5e3821
--- /dev/null
+++ b/app/controllers/sherlock/application_controller.rb
@@ -0,0 +1,12 @@
+module Sherlock
+  class ApplicationController < ::ApplicationController
+    before_action :find_transaction
+
+    def find_transaction
+      if params[:transaction_id]
+        @transaction = Gitlab::Sherlock.collection.
+          find_transaction(params[:transaction_id])
+      end
+    end
+  end
+end
diff --git a/app/controllers/sherlock/file_samples_controller.rb b/app/controllers/sherlock/file_samples_controller.rb
new file mode 100644
index 00000000000..0c3bc100106
--- /dev/null
+++ b/app/controllers/sherlock/file_samples_controller.rb
@@ -0,0 +1,7 @@
+module Sherlock
+  class FileSamplesController < Sherlock::ApplicationController
+    def show
+      @file_sample = @transaction.find_file_sample(params[:id])
+    end
+  end
+end
diff --git a/app/controllers/sherlock/queries_controller.rb b/app/controllers/sherlock/queries_controller.rb
new file mode 100644
index 00000000000..63b26aab1a4
--- /dev/null
+++ b/app/controllers/sherlock/queries_controller.rb
@@ -0,0 +1,7 @@
+module Sherlock
+  class QueriesController < Sherlock::ApplicationController
+    def show
+      @query = @transaction.find_query(params[:id])
+    end
+  end
+end
diff --git a/app/controllers/sherlock/transactions_controller.rb b/app/controllers/sherlock/transactions_controller.rb
new file mode 100644
index 00000000000..ccc739da879
--- /dev/null
+++ b/app/controllers/sherlock/transactions_controller.rb
@@ -0,0 +1,19 @@
+module Sherlock
+  class TransactionsController < Sherlock::ApplicationController
+    def index
+      @transactions = Gitlab::Sherlock.collection.newest_first
+    end
+
+    def show
+      @transaction = Gitlab::Sherlock.collection.find_transaction(params[:id])
+
+      render_404 unless @transaction
+    end
+
+    def destroy_all
+      Gitlab::Sherlock.collection.clear
+
+      redirect_to(:back)
+    end
+  end
+end
diff --git a/app/views/sherlock/file_samples/show.html.haml b/app/views/sherlock/file_samples/show.html.haml
new file mode 100644
index 00000000000..cfd11e45b6a
--- /dev/null
+++ b/app/views/sherlock/file_samples/show.html.haml
@@ -0,0 +1,55 @@
+- page_title t('sherlock.title'), t('sherlock.transaction'),
+  t('sherlock.file_sample')
+
+- header_title t('sherlock.title'), sherlock_transactions_path
+
+.gray-content-block
+  .pull-right
+    = link_to(sherlock_transaction_path(@transaction), class: 'btn') do
+      %i.fa.fa-arrow-left
+      = t('sherlock.transaction')
+  .oneline
+    = t('sherlock.file_sample')
+    = @file_sample.id
+
+.prepend-top-default
+  %p
+    %span.light
+      #{t('sherlock.time')}:
+    %strong
+      = @file_sample.duration.round(2)
+      = t('sherlock.milliseconds')
+  %p
+    %span.light
+      #{t('sherlock.events')}:
+    %strong
+      = @file_sample.events
+
+%article.file-holder
+  .file-title
+    %i.fa.fa-file-text-o.fa-fw
+    %strong
+      = @file_sample.file
+  .code.file-content.js-syntax-highlight
+    .line-numbers
+      %table.sherlock-line-samples-table
+        %thead
+          %tr
+            %th= t('sherlock.line_capitalized')
+            %th= t('sherlock.events')
+            %th= t('sherlock.time')
+            %th= t('sherlock.percent')
+        %tbody
+          - @file_sample.line_samples.each_with_index do |sample, index|
+            %tr{class: sample.majority_of?(@file_sample.duration) ? 'slow' : ''}
+              %td= index + 1
+              %td= sample.events
+              %td
+                = sample.duration.round(2)
+                = t('sherlock.milliseconds')
+              %td
+                = sample.percentage_of(@file_sample.duration).round
+                = t('sherlock.percent')
+
+    .sherlock-file-sample
+      = highlight(@file_sample.file, @file_sample.source)
diff --git a/app/views/sherlock/queries/_backtrace.html.haml b/app/views/sherlock/queries/_backtrace.html.haml
new file mode 100644
index 00000000000..5c9294c0ab5
--- /dev/null
+++ b/app/views/sherlock/queries/_backtrace.html.haml
@@ -0,0 +1,27 @@
+.prepend-top-default
+  .panel.panel-default
+    .panel-heading
+      %strong
+        = t('sherlock.application_backtrace')
+    %ul.well-list
+      - @query.application_backtrace.each do |location|
+        %li
+          = location.path
+          %small.light
+            = t('sherlock.line')
+            = location.line
+
+  .panel.panel-default
+    .panel-heading
+      %strong
+        = t('sherlock.full_backtrace')
+    %ul.well-list
+      - @query.backtrace.each do |location|
+        %li
+          - if location.application?
+            %strong= location.path
+          - else
+            = location.path
+          %small.light
+            = t('sherlock.line')
+            = location.line
diff --git a/app/views/sherlock/queries/_general.html.haml b/app/views/sherlock/queries/_general.html.haml
new file mode 100644
index 00000000000..549b47430e6
--- /dev/null
+++ b/app/views/sherlock/queries/_general.html.haml
@@ -0,0 +1,50 @@
+.prepend-top-default
+  .panel.panel-default
+    .panel-heading
+      %strong
+        = t('sherlock.general')
+    %ul.well-list
+      %li
+        %span.light
+          #{t('sherlock.time')}:
+        %strong
+          = @query.duration.round(4)
+          = t('sherlock.milliseconds')
+      %li
+        %span.light
+          #{t('sherlock.origin')}:
+        %strong
+          = @query.last_application_frame.path
+        %small.light
+          = t('sherlock.line')
+          = @query.last_application_frame.line
+
+  .panel.panel-default
+    .panel-heading
+      .pull-right
+        %button.js-clipboard-trigger.btn.btn-xs{title: t('sherlock.copy_to_clipboard'), type: :button}
+          %i.fa.fa-clipboard
+        %pre.hidden
+          = @query.formatted_query
+      %strong
+        = t('sherlock.query')
+    %ul.well-list
+      %li
+        .code.js-syntax-highlight.sherlock-code
+          :preserve
+            #{highlight("#{@query.id}.sql", @query.formatted_query)}
+
+  .panel.panel-default
+    .panel-heading
+      .pull-right
+        %button.js-clipboard-trigger.btn.btn-xs{title: t('sherlock.copy_to_clipboard'), type: :button}
+          %i.fa.fa-clipboard
+        %pre.hidden
+          = @query.explain
+      %strong
+        = t('sherlock.query_plan')
+    %ul.well-list
+      %li
+        .code.js-syntax-highlight.sherlock-code
+          %pre
+            %code= @query.explain
diff --git a/app/views/sherlock/queries/show.html.haml b/app/views/sherlock/queries/show.html.haml
new file mode 100644
index 00000000000..4a84348ac82
--- /dev/null
+++ b/app/views/sherlock/queries/show.html.haml
@@ -0,0 +1,26 @@
+- page_title t('sherlock.title'), t('sherlock.transaction'), t('sherlock.query')
+- header_title t('sherlock.title'), sherlock_transactions_path
+
+%ul.center-top-menu
+  %li.active
+    %a(href="#tab-general" data-toggle="tab")
+      = t('sherlock.general')
+  %li
+    %a(href="#tab-backtrace" data-toggle="tab")
+      = t('sherlock.backtrace')
+
+.gray-content-block
+  .pull-right
+    = link_to(sherlock_transaction_path(@transaction), class: 'btn') do
+      %i.fa.fa-arrow-left
+      = t('sherlock.transaction')
+  .oneline
+    = t('sherlock.query')
+    = @query.id
+
+.tab-content
+  .tab-pane.active#tab-general
+    = render(partial: 'general')
+
+  .tab-pane#tab-backtrace
+    = render(partial: 'backtrace')
diff --git a/app/views/sherlock/transactions/_file_samples.html.haml b/app/views/sherlock/transactions/_file_samples.html.haml
new file mode 100644
index 00000000000..0afdbc8dffa
--- /dev/null
+++ b/app/views/sherlock/transactions/_file_samples.html.haml
@@ -0,0 +1,22 @@
+- if @transaction.file_samples.empty?
+  .nothing-here-block
+    = t('sherlock.no_file_samples')
+- else
+  .table-holder
+    %table.table
+      %thead
+        %tr
+          %th= t('sherlock.time_inclusive')
+          %th= t('sherlock.path')
+          %th
+      %tbody
+        - @transaction.sorted_file_samples.each do |sample|
+          %tr
+            %td
+              = sample.duration.round(2)
+              = t('sherlock.milliseconds')
+            %td= sample.relative_path
+            %td
+              = link_to(t('sherlock.view'),
+                sherlock_transaction_file_sample_path(@transaction, sample),
+                class: 'btn btn-xs')
diff --git a/app/views/sherlock/transactions/_general.html.haml b/app/views/sherlock/transactions/_general.html.haml
new file mode 100644
index 00000000000..4287a0c3203
--- /dev/null
+++ b/app/views/sherlock/transactions/_general.html.haml
@@ -0,0 +1,33 @@
+.prepend-top-default
+  .panel.panel-default
+    .panel-heading
+      %strong
+        = t('sherlock.general')
+    %ul.well-list
+      %li
+        %span.light
+          #{t('sherlock.id')}:
+        %strong
+          = @transaction.id
+      %li
+        %span.light
+          #{t('sherlock.type')}:
+        %strong
+          = @transaction.type
+      %li
+        %span.light
+          #{t('sherlock.path')}:
+        %strong
+          = @transaction.path
+      %li
+        %span.light
+          #{t('sherlock.time')}:
+        %strong
+          = @transaction.duration.round(2)
+          = t('sherlock.seconds')
+      %li
+        %span.light
+          #{t('sherlock.finished_at')}:
+        %strong
+          = time_ago_in_words(@transaction.finished_at)
+          = t('sherlock.ago')
diff --git a/app/views/sherlock/transactions/_queries.html.haml b/app/views/sherlock/transactions/_queries.html.haml
new file mode 100644
index 00000000000..b7e0162e80d
--- /dev/null
+++ b/app/views/sherlock/transactions/_queries.html.haml
@@ -0,0 +1,24 @@
+- if @transaction.queries.empty?
+  .nothing-here-block
+    = t('sherlock.no_queries')
+- else
+  .table-holder
+    %table.table#sherlock-queries
+      %thead
+        %tr
+          %th= t('sherlock.time')
+          %th= t('sherlock.query')
+          %td
+      %tbody
+        - @transaction.sorted_queries.each do |query|
+          %tr
+            %td
+              = query.duration.round(2)
+              = t('sherlock.milliseconds')
+            %td
+              .code.js-syntax-highlight.sherlock-code
+                = highlight("#{query.id}.sql", query.formatted_query)
+            %td
+              = link_to(t('sherlock.view'),
+                sherlock_transaction_query_path(@transaction, query),
+                class: 'btn btn-xs')
diff --git a/app/views/sherlock/transactions/index.html.haml b/app/views/sherlock/transactions/index.html.haml
new file mode 100644
index 00000000000..fb31131ba88
--- /dev/null
+++ b/app/views/sherlock/transactions/index.html.haml
@@ -0,0 +1,40 @@
+- page_title t('sherlock.title')
+- header_title t('sherlock.title'), sherlock_transactions_path
+
+.gray-content-block
+  .pull-right
+    = link_to(destroy_all_sherlock_transactions_path,
+      class: 'btn btn-danger',
+      method: :delete) do
+      %i.fa.fa-trash
+      = t('sherlock.delete_all_transactions')
+  .oneline= t('sherlock.introduction')
+
+- if @transactions.empty?
+  .nothing-here-block= t('sherlock.no_transactions')
+- else
+  .table-holder
+    %table.table
+      %thead
+        %tr
+          %th= t('sherlock.type')
+          %th= t('sherlock.path')
+          %th= t('sherlock.time')
+          %th= t('sherlock.queries')
+          %th= t('sherlock.finished_at')
+          %th
+      %tbody
+        - @transactions.each do |trans|
+          %tr
+            %td= trans.type
+            %td= trans.path
+            %td
+              = trans.duration.round(2)
+              = t('sherlock.seconds')
+            %td= trans.queries.length
+            %td
+              = time_ago_in_words(trans.finished_at)
+              = t('sherlock.ago')
+            %td
+              = link_to(sherlock_transaction_path(trans), class: 'btn btn-xs') do
+                = t('sherlock.view')
diff --git a/app/views/sherlock/transactions/show.html.haml b/app/views/sherlock/transactions/show.html.haml
new file mode 100644
index 00000000000..3c8ffb06648
--- /dev/null
+++ b/app/views/sherlock/transactions/show.html.haml
@@ -0,0 +1,36 @@
+- page_title t('sherlock.title'), t('sherlock.transaction')
+- header_title t('sherlock.title'), sherlock_transactions_path
+
+%ul.center-top-menu
+  %li.active
+    %a(href="#tab-general" data-toggle="tab")
+      = t('sherlock.general')
+  %li
+    %a(href="#tab-queries" data-toggle="tab")
+      = t('sherlock.queries')
+      %span.badge
+        #{@transaction.queries.length}
+  %li
+    %a(href="#tab-file-samples" data-toggle="tab")
+      = t('sherlock.file_samples')
+      %span.badge
+        #{@transaction.file_samples.length}
+
+.gray-content-block
+  .pull-right
+    = link_to(sherlock_transactions_path, class: 'btn') do
+      %i.fa.fa-arrow-left
+      = t('sherlock.all_transactions')
+  .oneline
+    = t('sherlock.transaction')
+    = @transaction.id
+
+.tab-content
+  .tab-pane.active#tab-general
+    = render(partial: 'general')
+
+  .tab-pane#tab-queries
+    = render(partial: 'queries')
+
+  .tab-pane#tab-file-samples
+    = render(partial: 'file_samples')
diff --git a/config/initializers/rack_profiler.rb b/config/initializers/rack_profiler.rb
deleted file mode 100644
index 7710eeac453..00000000000
--- a/config/initializers/rack_profiler.rb
+++ /dev/null
@@ -1,10 +0,0 @@
-if Rails.env.development?
-  require 'rack-mini-profiler'
-
-  # initialization is skipped so trigger it
-  Rack::MiniProfilerRails.initialize!(Gitlab::Application)
-
-  Rack::MiniProfiler.config.position = 'right'
-  Rack::MiniProfiler.config.start_hidden = false
-  Rack::MiniProfiler.config.skip_paths << '/teaspoon'
-end
diff --git a/config/initializers/sherlock.rb b/config/initializers/sherlock.rb
new file mode 100644
index 00000000000..42b0d78c85f
--- /dev/null
+++ b/config/initializers/sherlock.rb
@@ -0,0 +1,5 @@
+if Gitlab::Sherlock.enabled?
+  Gitlab::Application.configure do |config|
+    config.middleware.use(Gitlab::Sherlock::Middleware)
+  end
+end
diff --git a/config/locales/sherlock.en.yml b/config/locales/sherlock.en.yml
new file mode 100644
index 00000000000..5c146b172b1
--- /dev/null
+++ b/config/locales/sherlock.en.yml
@@ -0,0 +1,36 @@
+en:
+  sherlock:
+    title: Sherlock
+    delete_all_transactions: Delete All Transactions
+    introduction: >
+      Below is a list of all transactions recorded by Sherlock. Requests to
+      Sherlock's own routes are ignored.
+    no_transactions: No transactions to show
+    no_queries: No queries to show
+    no_file_samples: No file samples to show
+    all_transactions: All Transactions
+    transaction: Transaction
+    query: Query
+    file_sample: File Sample
+    type: Type
+    path: Path
+    time: Time
+    queries: Queries
+    finished_at: Finished at
+    ago: ago
+    view: View
+    seconds: seconds
+    milliseconds: ms
+    general: General
+    id: ID
+    time_inclusive: Time (inclusive)
+    backtrace: Backtrace
+    application_backtrace: Application Backtrace
+    full_backtrace: Full Backtrace
+    origin: Origin
+    line: line
+    line_capitalized: Line
+    copy_to_clipboard: Copy to clipboard
+    query_plan: Query Plan
+    events: Events
+    percent: '%'
diff --git a/config/routes.rb b/config/routes.rb
index 0458f538eb6..5d1139edf96 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -2,6 +2,19 @@ require 'sidekiq/web'
 require 'api/api'
 
 Gitlab::Application.routes.draw do
+  if Gitlab::Sherlock.enabled?
+    namespace :sherlock do
+      resources :transactions, only: [:index, :show] do
+        resources :queries, only: [:show]
+        resources :file_samples, only: [:show]
+
+        collection do
+          delete :destroy_all
+        end
+      end
+    end
+  end
+
   namespace :ci do
     # CI API
     Ci::API::API.logger Rails.logger
diff --git a/lib/gitlab/sherlock.rb b/lib/gitlab/sherlock.rb
new file mode 100644
index 00000000000..c4b35b24ceb
--- /dev/null
+++ b/lib/gitlab/sherlock.rb
@@ -0,0 +1,20 @@
+require 'securerandom'
+require 'rblineprof' if RUBY_ENGINE == 'ruby'
+
+module Gitlab
+  module Sherlock
+    @collection = Collection.new
+
+    class << self
+      attr_reader :collection
+    end
+
+    def self.enabled?
+      Rails.env.development? && !!ENV['ENABLE_SHERLOCK']
+    end
+
+    def self.enable_line_profiler?
+      RUBY_ENGINE == 'ruby'
+    end
+  end
+end
diff --git a/lib/gitlab/sherlock/collection.rb b/lib/gitlab/sherlock/collection.rb
new file mode 100644
index 00000000000..accdc6469bc
--- /dev/null
+++ b/lib/gitlab/sherlock/collection.rb
@@ -0,0 +1,42 @@
+module Gitlab
+  module Sherlock
+    class Collection
+      include Enumerable
+
+      def initialize
+        @transactions = []
+        @mutex = Mutex.new
+      end
+
+      def add(transaction)
+        synchronize { @transactions << transaction }
+      end
+
+      alias_method :<<, :add
+
+      def each(&block)
+        synchronize { @transactions.each(&block) }
+      end
+
+      def clear
+        synchronize { @transactions.clear }
+      end
+
+      def empty?
+        synchronize { @transactions.empty? }
+      end
+
+      def find_transaction(id)
+        find { |trans| trans.id == id }
+      end
+
+      def newest_first
+        sort { |a, b| b.finished_at <=> a.finished_at }
+      end
+
+      def synchronize(&block)
+        @mutex.synchronize(&block)
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/sherlock/file_sample.rb b/lib/gitlab/sherlock/file_sample.rb
new file mode 100644
index 00000000000..7a220de9abc
--- /dev/null
+++ b/lib/gitlab/sherlock/file_sample.rb
@@ -0,0 +1,27 @@
+module Gitlab
+  module Sherlock
+    class FileSample
+      attr_reader :id, :file, :line_samples, :events, :duration
+
+      def initialize(file, line_samples, duration, events)
+        @id = SecureRandom.uuid
+        @file = file
+        @line_samples = line_samples
+        @duration = duration
+        @events = events
+      end
+
+      def relative_path
+        @relative_path ||= @file.gsub(/^#{Rails.root.to_s}\/?/, '')
+      end
+
+      def to_param
+        @id
+      end
+
+      def source
+        @source ||= File.read(@file)
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/sherlock/line_profiler.rb b/lib/gitlab/sherlock/line_profiler.rb
new file mode 100644
index 00000000000..a191b1e646d
--- /dev/null
+++ b/lib/gitlab/sherlock/line_profiler.rb
@@ -0,0 +1,60 @@
+module Gitlab
+  module Sherlock
+    class LineProfiler
+      # The minimum amount of time that has to be spent in a file for it to be
+      # included in a list of samples.
+      MINIMUM_DURATION = 10.0
+
+      def profile(&block)
+        if RUBY_ENGINE == 'ruby'
+          profile_mri(&block)
+        else
+          raise NotImplementedError,
+            'Line profiling is not supported on this platform'
+        end
+      end
+
+      def profile_mri
+        retval  = nil
+        samples = lineprof(/^#{Rails.root.to_s}/) { retval = yield }
+
+        file_samples = aggregate_rblineprof(samples)
+
+        [retval, file_samples]
+      end
+
+      # Returns an Array of file samples based on the output of rblineprof.
+      def aggregate_rblineprof(lineprof_stats)
+        samples = []
+
+        lineprof_stats.each do |(file, stats)|
+          source_lines = File.read(file).each_line.to_a
+          line_samples = []
+
+          total_duration = microsec_to_millisec(stats[0][0])
+          total_events   = stats[0][2]
+
+          next if total_duration <= MINIMUM_DURATION
+
+          stats[1..-1].each_with_index do |data, index|
+            next unless source_lines[index]
+
+            duration = microsec_to_millisec(data[0])
+            events   = data[2]
+
+            line_samples << LineSample.new(duration, events)
+          end
+
+          samples << FileSample.
+            new(file, line_samples, total_duration, total_events)
+        end
+
+        samples
+      end
+
+      def microsec_to_millisec(microsec)
+        microsec / 1000.0
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/sherlock/line_sample.rb b/lib/gitlab/sherlock/line_sample.rb
new file mode 100644
index 00000000000..38df7a88e4e
--- /dev/null
+++ b/lib/gitlab/sherlock/line_sample.rb
@@ -0,0 +1,20 @@
+module Gitlab
+  module Sherlock
+    class LineSample
+      attr_reader :duration, :events
+
+      def initialize(duration, events)
+        @duration = duration
+        @events = events
+      end
+
+      def percentage_of(total_duration)
+        (duration.to_f / total_duration) * 100.0
+      end
+
+      def majority_of?(total_duration)
+        percentage_of(total_duration) >= 30
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/sherlock/location.rb b/lib/gitlab/sherlock/location.rb
new file mode 100644
index 00000000000..8c0b77dce1a
--- /dev/null
+++ b/lib/gitlab/sherlock/location.rb
@@ -0,0 +1,22 @@
+module Gitlab
+  module Sherlock
+    class Location
+      attr_reader :path, :line
+
+      SHERLOCK_DIR = File.dirname(__FILE__)
+
+      def self.from_ruby_location(location)
+        new(location.path, location.lineno)
+      end
+
+      def initialize(path, line)
+        @path = path
+        @line = line
+      end
+
+      def application?
+        @path.start_with?(Rails.root.to_s) && !path.start_with?(SHERLOCK_DIR)
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/sherlock/middleware.rb b/lib/gitlab/sherlock/middleware.rb
new file mode 100644
index 00000000000..fca7be858eb
--- /dev/null
+++ b/lib/gitlab/sherlock/middleware.rb
@@ -0,0 +1,36 @@
+module Gitlab
+  module Sherlock
+    # Rack middleware used for tracking request metrics.
+    class Middleware
+      CONTENT_TYPES = /text\/html|application\/json/i
+
+      IGNORE_PATHS = %r{^/sherlock}
+
+      def initialize(app)
+        @app = app
+      end
+
+      def call(env)
+        if instrument?(env)
+          call_with_instrumentation(env)
+        else
+          @app.call(env)
+        end
+      end
+
+      def call_with_instrumentation(env)
+        trans = Transaction.new(env['REQUEST_METHOD'], env['REQUEST_URI'])
+        retval = trans.run { @app.call(env) }
+
+        Sherlock.collection.add(trans)
+
+        retval
+      end
+
+      def instrument?(env)
+        !!(env['HTTP_ACCEPT'] =~ CONTENT_TYPES &&
+           env['REQUEST_URI'] !~ IGNORE_PATHS)
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/sherlock/query.rb b/lib/gitlab/sherlock/query.rb
new file mode 100644
index 00000000000..af76e6fd2bf
--- /dev/null
+++ b/lib/gitlab/sherlock/query.rb
@@ -0,0 +1,96 @@
+module Gitlab
+  module Sherlock
+    class Query
+      attr_reader :id, :query, :started_at, :finished_at, :backtrace
+
+      PREFIX_NEWLINE = /
+        \s+(FROM
+          |(LEFT|RIGHT)?INNER\s+JOIN
+          |(LEFT|RIGHT)?OUTER\s+JOIN
+          |WHERE
+          |AND
+          |GROUP\s+BY
+          |ORDER\s+BY
+          |LIMIT
+          |OFFSET)\s+
+      /ix
+
+      def self.new_with_bindings(query, bindings, started_at, finished_at)
+        bindings.each_with_index do |(column, value), index|
+          quoted_value = ActiveRecord::Base.connection.quote(value)
+
+          query = query.gsub("$#{index + 1}", quoted_value)
+        end
+
+        new(query, started_at, finished_at)
+      end
+
+      def initialize(query, started_at, finished_at)
+        @id = SecureRandom.uuid
+        @query = query
+        @started_at = started_at
+        @finished_at = finished_at
+        @backtrace = caller_locations.map do |loc|
+          Location.from_ruby_location(loc)
+        end
+
+        unless @query.end_with?(';')
+          @query += ';'
+        end
+      end
+
+      def duration
+        @duration ||= (@finished_at - @started_at) * 1000.0
+      end
+
+      def to_param
+        @id
+      end
+
+      def formatted_query
+        @formatted_query ||= format_sql(@query)
+      end
+
+      def last_application_frame
+        @last_application_frame ||= @backtrace.find(&:application?)
+      end
+
+      def application_backtrace
+        @application_backtrace ||= @backtrace.select(&:application?)
+      end
+
+      def explain
+        unless @explain
+          ActiveRecord::Base.connection.transaction do
+            @explain = raw_explain(@query).values.flatten.join("\n")
+
+            # Roll back any queries that mutate data so we don't mess up
+            # anything when running explain on an INSERT, UPDATE, DELETE, etc.
+            raise ActiveRecord::Rollback
+          end
+        end
+
+        @explain
+      end
+
+      private
+
+      def raw_explain(query)
+        if Gitlab::Database.postgresql?
+          explain = "EXPLAIN ANALYZE #{query};"
+        else
+          explain = "EXPLAIN #{query};"
+        end
+
+        ActiveRecord::Base.connection.execute(explain)
+      end
+
+      def format_sql(query)
+        query.each_line.
+          map { |line| line.strip }.
+          join("\n").
+          gsub(PREFIX_NEWLINE) { "\n#{$1} " }
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/sherlock/transaction.rb b/lib/gitlab/sherlock/transaction.rb
new file mode 100644
index 00000000000..5cb3e86aa4e
--- /dev/null
+++ b/lib/gitlab/sherlock/transaction.rb
@@ -0,0 +1,85 @@
+module Gitlab
+  module Sherlock
+    class Transaction
+      attr_reader :id, :type, :path, :queries, :file_samples, :started_at,
+        :finished_at
+
+      def initialize(type, path)
+        @id = SecureRandom.uuid
+        @type = type
+        @path = path
+        @duration = 0
+        @queries = []
+        @file_samples = []
+        @started_at = nil
+        @finished_at = nil
+        @thread = Thread.current
+      end
+
+      def run
+        @started_at = Time.now
+
+        subscriber = subscribe_to_active_record
+
+        retval = profile_lines { yield }
+
+        @finished_at = Time.now
+
+        ActiveSupport::Notifications.unsubscribe(subscriber)
+
+        retval
+      end
+
+      def duration
+        @started_at && @finished_at ? @finished_at - @started_at : 0
+      end
+
+      def to_param
+        @id
+      end
+
+      def sorted_queries
+        @queries.sort { |a, b| b.duration <=> a.duration }
+      end
+
+      def sorted_file_samples
+        @file_samples.sort { |a, b| b.duration <=> a.duration }
+      end
+
+      def find_query(id)
+        @queries.find { |query| query.id == id }
+      end
+
+      def find_file_sample(id)
+        @file_samples.find { |sample| sample.id == id }
+      end
+
+      def track_query(query, bindings, start, finish)
+        @queries << Query.new_with_bindings(query, bindings, start, finish)
+      end
+
+      def profile_lines
+        retval = nil
+
+        if Sherlock.enable_line_profiler?
+          retval, @file_samples = LineProfiler.new.profile { yield }
+        else
+          retval = yield
+        end
+
+        retval
+      end
+
+      def subscribe_to_active_record
+        ActiveSupport::Notifications.subscribe('sql.active_record') do |_, start, finish, _, data|
+          # In case somebody uses a multi-threaded server locally (e.g. Puma) we
+          # _only_ want to track queries that originate from the transaction
+          # thread.
+          next unless Thread.current == @thread
+
+          track_query(data[:sql].strip, data[:binds], start, finish)
+        end
+      end
+    end
+  end
+end