diff --git a/CHANGELOG b/CHANGELOG
index 19f288d0c7ec6905483a2b17f73c3fd5dfc82fb0..217dc4e00434c3ddce529c14ac4cbbd2b8a408ed 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,6 +1,7 @@
 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.2.0 (unreleased)
+  - Added a GitLab specific profiling tool called "Sherlock" (see GitLab CE merge request #1749)
   - Fix bug where manually merged branches in a MR would end up with an empty diff (Stan Hu)
   - Force update refs/merge-requests/X/head upon a push to the source branch of a merge request (Stan Hu)
   - Improved performance of finding users by one of their Email addresses
diff --git a/Gemfile b/Gemfile
index 0bac89781601962fe7f160a2ed00775021938bfb..b0a7c9b94587712150164bd81dbcd00492e1c9f9 100644
--- a/Gemfile
+++ b/Gemfile
@@ -215,11 +215,9 @@ 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 dce728baf18eba5d863bb6f97c382021801e49b2..c602d40671175afdc1ffb30117bbe9200a890f63 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)
@@ -491,12 +490,6 @@ 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)
@@ -779,7 +772,6 @@ 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)
@@ -878,11 +870,10 @@ 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 0000000000000000000000000000000000000000..92d84d9640fc4cedf8d03bde1070ac7eda7eabf5
--- /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 0000000000000000000000000000000000000000..682ca5e382155f51ccc84432c29b9ecb13ea296a
--- /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 0000000000000000000000000000000000000000..0c3bc100106049688ed7a7b0155b6de1a4e4e779
--- /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 0000000000000000000000000000000000000000..63b26aab1a4b96475e8ec294585bad0c2ad16867
--- /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 0000000000000000000000000000000000000000..ccc739da879a1d3f9baaa1341622847fac6c63d4
--- /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/layouts/header/_default.html.haml b/app/views/layouts/header/_default.html.haml
index c31b1cbe9a8ab8aaa030154f203a182d4365b46c..c08a7b8074474222551ef746e14cda1269c08bf8 100644
--- a/app/views/layouts/header/_default.html.haml
+++ b/app/views/layouts/header/_default.html.haml
@@ -21,6 +21,11 @@
             %li
               = link_to new_project_path, title: 'New project', data: {toggle: 'tooltip', placement: 'bottom'} do
                 = icon('plus fw')
+          - if Gitlab::Sherlock.enabled?
+            %li
+              = link_to sherlock_transactions_path, title: 'Sherlock Transactions',
+                data: {toggle: 'tooltip', placement: 'bottom'} do
+                = icon('tachometer fw')
           %li
             = link_to destroy_user_session_path, class: 'logout', method: :delete, title: 'Sign out', data: {toggle: 'tooltip', placement: 'bottom'} do
               = icon('sign-out')
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 0000000000000000000000000000000000000000..cfd11e45b6af80d5721f73379ea6074498d7136e
--- /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 0000000000000000000000000000000000000000..5c9294c0ab534e0f2879707e06fa119483aab0e8
--- /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 0000000000000000000000000000000000000000..549b47430e697f761c340f5738da302bda36839e
--- /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 0000000000000000000000000000000000000000..4a84348ac82582f753d40c81826bdb17684bb4b5
--- /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 0000000000000000000000000000000000000000..4349c9b7aceb206e563e056546f1656125cf79cf
--- /dev/null
+++ b/app/views/sherlock/transactions/_file_samples.html.haml
@@ -0,0 +1,24 @@
+- 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.count')
+          %th= t('sherlock.path')
+          %th
+      %tbody
+        - @transaction.sorted_file_samples.each do |sample|
+          %tr
+            %td
+              = sample.duration.round(2)
+              = t('sherlock.milliseconds')
+            %td= @transaction.view_counts.fetch(sample.file, 1)
+            %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 0000000000000000000000000000000000000000..4287a0c32036f49cccd1b7ac1482186d7f1f5254
--- /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 0000000000000000000000000000000000000000..b7e0162e80db835d22bc1b3aa44933ee6c3141ad
--- /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 0000000000000000000000000000000000000000..010e1a2a9023e8d5a5d96fc63f67c0963cfebd4a
--- /dev/null
+++ b/app/views/sherlock/transactions/index.html.haml
@@ -0,0 +1,42 @@
+- 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
+              %span{title: trans.path}
+                = truncate(trans.path, length: 70)
+            %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 0000000000000000000000000000000000000000..3c8ffb066484125b3cb1b2880fb7e3f8e305530c
--- /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 7710eeac45395d17705422af8cc519e289efb64b..0000000000000000000000000000000000000000
--- 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 0000000000000000000000000000000000000000..42b0d78c85fae51d6c800ab2cc66b0e77afb75a7
--- /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 0000000000000000000000000000000000000000..683b09dc3294d5710850802193572470bee0f5d9
--- /dev/null
+++ b/config/locales/sherlock.en.yml
@@ -0,0 +1,37 @@
+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: '%'
+    count: Count
diff --git a/config/routes.rb b/config/routes.rb
index 990a00e3d0bdd6995dbd67fbb80d8b6ce2c8ab4a..7d8a546a64cece182c03d4599a446ad29c400037 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/doc/development/profiling.md b/doc/development/profiling.md
index 80c86ef921e9fed698feda43b3c1fa26bc4cfb0a..e244ad4e881d6caae77437e8f0d515f81a33f8cc 100644
--- a/doc/development/profiling.md
+++ b/doc/development/profiling.md
@@ -4,11 +4,15 @@ To make it easier to track down performance problems GitLab comes with a set of
 profiling tools, some of these are available by default while others need to be
 explicitly enabled.
 
-## rack-mini-profiler
+## Sherlock
 
-This Gem is enabled by default in development only. It allows you to see the
-timings of the various components that made up a web request (e.g. the SQL
-queries executed and their execution timings).
+Sherlock is a custom profiling tool built into GitLab. Sherlock is _only_
+available when running GitLab in development mode _and_ when setting the
+environment variable `ENABLE_SHERLOCK` to a non empty value. For example:
+
+    ENABLE_SHERLOCK=1 bundle exec rails s
+
+Recorded transactions can be found by navigating to `/sherlock/transactions`.
 
 ## Bullet
 
@@ -21,36 +25,3 @@ starting GitLab. For example:
 
 Bullet will log query problems to both the Rails log as well as the Chrome
 console.
-
-## ActiveRecord Query Trace
-
-This Gem adds backtraces for every ActiveRecord query in the Rails console. This
-can be useful to track down where a query was executed. Because this Gem adds
-quite a bit of noise (5-10 extra lines per ActiveRecord query) it's disabled by
-default. To use this Gem you'll need to set `ENABLE_QUERY_TRACE` to a non empty
-file before starting GitLab. For example:
-
-    ENABLE_QUERY_TRACE=true bundle exec rails s
-
-## rack-lineprof
-
-This is a Gem that can trace the execution time of code on a per line basis.
-Because this Gem can add quite a bit of overhead it's disabled by default. To
-enable it, set the environment variable `ENABLE_LINEPROF` to a non-empty value.
-For example:
-
-    ENABLE_LINEPROF=true bundle exec rails s
-
-Once enabled you'll need to add a query string parameter to a request to
-actually profile code execution. The name of the parameter is `lineprof` and
-should be set to a regular expression (minus the starting/ending slash) used to
-select what files to profile. To profile all files containing "foo" somewhere in
-the path you'd use the following parameter:
-
-    ?lineprof=foo
-
-Or when filtering for files containing "foo" and "bar" in their path:
-
-    ?lineprof=foo|bar
-
-Once set the profiling output will be displayed in your terminal.
diff --git a/lib/gitlab/sherlock.rb b/lib/gitlab/sherlock.rb
new file mode 100644
index 0000000000000000000000000000000000000000..c4b35b24ceb72a3b875dd7c63be3fcdd4c848d40
--- /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 0000000000000000000000000000000000000000..66bd6258521deedc0e6781802c161868c87819ae
--- /dev/null
+++ b/lib/gitlab/sherlock/collection.rb
@@ -0,0 +1,49 @@
+module Gitlab
+  module Sherlock
+    # A collection of transactions recorded by Sherlock.
+    #
+    # Method calls for this class are synchronized using a mutex to allow
+    # sharing of a single Collection instance between threads (e.g. when using
+    # Puma as a webserver).
+    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
+
+      private
+
+      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 0000000000000000000000000000000000000000..8a3e1a5e5bf1747ae6c62d4f9880556309a3f549
--- /dev/null
+++ b/lib/gitlab/sherlock/file_sample.rb
@@ -0,0 +1,31 @@
+module Gitlab
+  module Sherlock
+    class FileSample
+      attr_reader :id, :file, :line_samples, :events, :duration
+
+      # file - The full path to the file this sample belongs to.
+      # line_samples - An array of LineSample objects.
+      # duration - The total execution time in milliseconds.
+      # events - The total amount of events.
+      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 0000000000000000000000000000000000000000..152749dcc39f3009db8fc61b6e6b18818c5bb232
--- /dev/null
+++ b/lib/gitlab/sherlock/line_profiler.rb
@@ -0,0 +1,96 @@
+module Gitlab
+  module Sherlock
+    # Class for profiling code on a per line basis.
+    #
+    # The LineProfiler class can be used to profile code on per line basis
+    # without littering your code with Ruby implementation specific profiling
+    # methods.
+    #
+    # This profiler only includes samples taking longer than a given threshold
+    # and those that occur in the actual application (e.g. files from Gems are
+    # ignored).
+    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
+
+      # Profiles the given block.
+      #
+      # Example:
+      #
+      #     profiler = LineProfiler.new
+      #
+      #     retval, samples = profiler.profile do
+      #       "cats are amazing"
+      #     end
+      #
+      #     retval  # => "cats are amazing"
+      #     samples # => [#<Gitlab::Sherlock::FileSample ...>, ...]
+      #
+      # Returns an Array containing the block's return value and an Array of
+      # FileSample objects.
+      def profile(&block)
+        if mri?
+          profile_mri(&block)
+        else
+          raise NotImplementedError,
+            'Line profiling is not supported on this platform'
+        end
+      end
+
+      # Profiles the given block using rblineprof (MRI only).
+      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.
+      #
+      # lineprof_stats - A Hash containing rblineprof statistics on a per file
+      #                  basis.
+      #
+      # Returns an Array of FileSample objects.
+      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
+
+      private
+
+      def microsec_to_millisec(microsec)
+        microsec / 1000.0
+      end
+
+      def mri?
+        RUBY_ENGINE == 'ruby'
+      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 0000000000000000000000000000000000000000..eb1948eb6d6ff1ee5b818e49837da707551a87de
--- /dev/null
+++ b/lib/gitlab/sherlock/line_sample.rb
@@ -0,0 +1,36 @@
+module Gitlab
+  module Sherlock
+    class LineSample
+      attr_reader :duration, :events
+
+      # duration - The execution time in milliseconds.
+      # events - The amount of events.
+      def initialize(duration, events)
+        @duration = duration
+        @events = events
+      end
+
+      # Returns the sample duration percentage relative to the given duration.
+      #
+      # Example:
+      #
+      #     sample.duration            # => 150
+      #     sample.percentage_of(1500) # => 10.0
+      #
+      # total_duration - The total duration to compare with.
+      #
+      # Returns a float
+      def percentage_of(total_duration)
+        (duration.to_f / total_duration) * 100.0
+      end
+
+      # Returns true if the current sample takes up the majority of the given
+      # duration.
+      #
+      # total_duration - The total duration to compare with.
+      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 0000000000000000000000000000000000000000..5ac265618ad2175eeeaee026506b6c6695fa7083
--- /dev/null
+++ b/lib/gitlab/sherlock/location.rb
@@ -0,0 +1,26 @@
+module Gitlab
+  module Sherlock
+    class Location
+      attr_reader :path, :line
+
+      SHERLOCK_DIR = File.dirname(__FILE__)
+
+      # Creates a new Location from a `Thread::Backtrace::Location`.
+      def self.from_ruby_location(location)
+        new(location.path, location.lineno)
+      end
+
+      # path - The full path of the frame as a String.
+      # line - The line number of the frame as a Fixnum.
+      def initialize(path, line)
+        @path = path
+        @line = line
+      end
+
+      # Returns true if the current frame originated from the application.
+      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 0000000000000000000000000000000000000000..687332fc5fc18fb45bd408e84ebe038bc7eb3c90
--- /dev/null
+++ b/lib/gitlab/sherlock/middleware.rb
@@ -0,0 +1,41 @@
+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
+
+      # env - A Hash containing Rack environment details.
+      def call(env)
+        if instrument?(env)
+          call_with_instrumentation(env)
+        else
+          @app.call(env)
+        end
+      end
+
+      def call_with_instrumentation(env)
+        trans = transaction_from_env(env)
+        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
+
+      def transaction_from_env(env)
+        Transaction.new(env['REQUEST_METHOD'], env['REQUEST_URI'])
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/sherlock/query.rb b/lib/gitlab/sherlock/query.rb
new file mode 100644
index 0000000000000000000000000000000000000000..4917c4ae2ac5c1a0a3b95b72d8cd2a25d42709db
--- /dev/null
+++ b/lib/gitlab/sherlock/query.rb
@@ -0,0 +1,114 @@
+module Gitlab
+  module Sherlock
+    class Query
+      attr_reader :id, :query, :started_at, :finished_at, :backtrace
+
+      # SQL identifiers that should be prefixed with newlines.
+      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 # Vim indent breaks when this is on a newline :<
+
+      # Creates a new Query using a String and a separate Array of bindings.
+      #
+      # query - A String containing a SQL query, optionally with numeric
+      #         placeholders (`$1`, `$2`, etc).
+      #
+      # bindings - An Array of ActiveRecord columns and their values.
+      # started_at - The start time of the query as a Time-like object.
+      # finished_at - The completion time of the query as a Time-like object.
+      #
+      # Returns a new Query object.
+      def self.new_with_bindings(query, bindings, started_at, finished_at)
+        bindings.each_with_index do |(_, value), index|
+          quoted_value = ActiveRecord::Base.connection.quote(value)
+
+          query = query.gsub("$#{index + 1}", quoted_value)
+        end
+
+        new(query, started_at, finished_at)
+      end
+
+      # query - The SQL query as a String (without placeholders).
+      # started_at - The start time of the query as a Time-like object.
+      # finished_at - The completion time of the query as a Time-like object.
+      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
+
+      # Returns the query duration in milliseconds.
+      def duration
+        @duration ||= (@finished_at - @started_at) * 1000.0
+      end
+
+      def to_param
+        @id
+      end
+
+      # Returns a human readable version of the query.
+      def formatted_query
+        @formatted_query ||= format_sql(@query)
+      end
+
+      # Returns the last application frame of the backtrace.
+      def last_application_frame
+        @last_application_frame ||= @backtrace.find(&:application?)
+      end
+
+      # Returns an Array of application frames (excluding Gems and the likes).
+      def application_backtrace
+        @application_backtrace ||= @backtrace.select(&:application?)
+      end
+
+      # Returns the query plan as a String.
+      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 0000000000000000000000000000000000000000..d87a4c9bb4a82a5e59dc6dbe87dfe0c6436e6673
--- /dev/null
+++ b/lib/gitlab/sherlock/transaction.rb
@@ -0,0 +1,131 @@
+module Gitlab
+  module Sherlock
+    class Transaction
+      attr_reader :id, :type, :path, :queries, :file_samples, :started_at,
+        :finished_at, :view_counts
+
+      # type - The type of transaction (e.g. "GET", "POST", etc)
+      # path - The path of the transaction (e.g. the HTTP request path)
+      def initialize(type, path)
+        @id = SecureRandom.uuid
+        @type = type
+        @path = path
+        @queries = []
+        @file_samples = []
+        @started_at = nil
+        @finished_at = nil
+        @thread = Thread.current
+        @view_counts = Hash.new(0)
+      end
+
+      # Runs the transaction and returns the block's return value.
+      def run
+        @started_at = Time.now
+
+        retval = with_subscriptions do
+          profile_lines { yield }
+        end
+
+        @finished_at = Time.now
+
+        retval
+      end
+
+      # Returns the duration in seconds.
+      def duration
+        @duration ||= started_at && finished_at ? finished_at - started_at : 0
+      end
+
+      def to_param
+        @id
+      end
+
+      # Returns the queries sorted in descending order by their durations.
+      def sorted_queries
+        @queries.sort { |a, b| b.duration <=> a.duration }
+      end
+
+      # Returns the file samples sorted in descending order by their durations.
+      def sorted_file_samples
+        @file_samples.sort { |a, b| b.duration <=> a.duration }
+      end
+
+      # Finds a query by the given ID.
+      #
+      # id - The query ID as a String.
+      #
+      # Returns a Query object if one could be found, nil otherwise.
+      def find_query(id)
+        @queries.find { |query| query.id == id }
+      end
+
+      # Finds a file sample by the given ID.
+      #
+      # id - The query ID as a String.
+      #
+      # Returns a FileSample object if one could be found, nil otherwise.
+      def find_file_sample(id)
+        @file_samples.find { |sample| sample.id == id }
+      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|
+          next unless same_thread?
+
+          track_query(data[:sql].strip, data[:binds], start, finish)
+        end
+      end
+
+      def subscribe_to_action_view
+        regex = /render_(template|partial)\.action_view/
+
+        ActiveSupport::Notifications.subscribe(regex) do |_, start, finish, _, data|
+          next unless same_thread?
+
+          track_view(data[:identifier])
+        end
+      end
+
+      private
+
+      def track_query(query, bindings, start, finish)
+        @queries << Query.new_with_bindings(query, bindings, start, finish)
+      end
+
+      def track_view(path)
+        @view_counts[path] += 1
+      end
+
+      def with_subscriptions
+        ar_subscriber = subscribe_to_active_record
+        av_subscriber = subscribe_to_action_view
+
+        retval = yield
+
+        ActiveSupport::Notifications.unsubscribe(ar_subscriber)
+        ActiveSupport::Notifications.unsubscribe(av_subscriber)
+
+        retval
+      end
+
+      # In case somebody uses a multi-threaded server locally (e.g. Puma) we
+      # _only_ want to track notifications that originate from the transaction
+      # thread.
+      def same_thread?
+        Thread.current == @thread
+      end
+    end
+  end
+end
diff --git a/spec/lib/gitlab/sherlock/collection_spec.rb b/spec/lib/gitlab/sherlock/collection_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..a8a9d6fc7bc171f771893a605f193b11d6019140
--- /dev/null
+++ b/spec/lib/gitlab/sherlock/collection_spec.rb
@@ -0,0 +1,82 @@
+require 'spec_helper'
+
+describe Gitlab::Sherlock::Collection do
+  let(:collection) { described_class.new }
+
+  let(:transaction) do
+    Gitlab::Sherlock::Transaction.new('POST', '/cat_pictures')
+  end
+
+  describe '#add' do
+    it 'adds a new transaction' do
+      collection.add(transaction)
+
+      expect(collection).to_not be_empty
+    end
+
+    it 'is aliased as <<' do
+      collection << transaction
+
+      expect(collection).to_not be_empty
+    end
+  end
+
+  describe '#each' do
+    it 'iterates over every transaction' do
+      collection.add(transaction)
+
+      expect { |b| collection.each(&b) }.to yield_with_args(transaction)
+    end
+  end
+
+  describe '#clear' do
+    it 'removes all transactions' do
+      collection.add(transaction)
+
+      collection.clear
+
+      expect(collection).to be_empty
+    end
+  end
+
+  describe '#empty?' do
+    it 'returns true for an empty collection' do
+      expect(collection).to be_empty
+    end
+
+    it 'returns false for a collection with a transaction' do
+      collection.add(transaction)
+
+      expect(collection).to_not be_empty
+    end
+  end
+
+  describe '#find_transaction' do
+    it 'returns the transaction for the given ID' do
+      collection.add(transaction)
+
+      expect(collection.find_transaction(transaction.id)).to eq(transaction)
+    end
+
+    it 'returns nil when no transaction could be found' do
+      collection.add(transaction)
+
+      expect(collection.find_transaction('cats')).to be_nil
+    end
+  end
+
+  describe '#newest_first' do
+    it 'returns transactions sorted from new to old' do
+      trans1 = Gitlab::Sherlock::Transaction.new('POST', '/cat_pictures')
+      trans2 = Gitlab::Sherlock::Transaction.new('POST', '/more_cat_pictures')
+
+      allow(trans1).to receive(:finished_at).and_return(Time.utc(2015, 1, 1))
+      allow(trans2).to receive(:finished_at).and_return(Time.utc(2015, 1, 2))
+
+      collection.add(trans1)
+      collection.add(trans2)
+
+      expect(collection.newest_first).to eq([trans2, trans1])
+    end
+  end
+end
diff --git a/spec/lib/gitlab/sherlock/file_sample_spec.rb b/spec/lib/gitlab/sherlock/file_sample_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..f05a59f56f6cb56a76ef3a74a09075861194ea49
--- /dev/null
+++ b/spec/lib/gitlab/sherlock/file_sample_spec.rb
@@ -0,0 +1,54 @@
+require 'spec_helper'
+
+describe Gitlab::Sherlock::FileSample do
+  let(:sample) { described_class.new(__FILE__, [], 150.4, 2) }
+
+  describe '#id' do
+    it 'returns the ID' do
+      expect(sample.id).to be_an_instance_of(String)
+    end
+  end
+
+  describe '#file' do
+    it 'returns the file path' do
+      expect(sample.file).to eq(__FILE__)
+    end
+  end
+
+  describe '#line_samples' do
+    it 'returns the line samples' do
+      expect(sample.line_samples).to eq([])
+    end
+  end
+
+  describe '#events' do
+    it 'returns the total number of events' do
+      expect(sample.events).to eq(2)
+    end
+  end
+
+  describe '#duration' do
+    it 'returns the total execution time' do
+      expect(sample.duration).to eq(150.4)
+    end
+  end
+
+  describe '#relative_path' do
+    it 'returns the relative path' do
+      expect(sample.relative_path).
+        to eq('spec/lib/gitlab/sherlock/file_sample_spec.rb')
+    end
+  end
+
+  describe '#to_param' do
+    it 'returns the sample ID' do
+      expect(sample.to_param).to eq(sample.id)
+    end
+  end
+
+  describe '#source' do
+    it 'returns the contents of the file' do
+      expect(sample.source).to eq(File.read(__FILE__))
+    end
+  end
+end
diff --git a/spec/lib/gitlab/sherlock/line_profiler_spec.rb b/spec/lib/gitlab/sherlock/line_profiler_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..8f2e1299714ee17775b5b9f1d63d4508193a2660
--- /dev/null
+++ b/spec/lib/gitlab/sherlock/line_profiler_spec.rb
@@ -0,0 +1,73 @@
+require 'spec_helper'
+
+describe Gitlab::Sherlock::LineProfiler do
+  let(:profiler) { described_class.new }
+
+  describe '#profile' do
+    it 'runs the profiler when using MRI' do
+      allow(profiler).to receive(:mri?).and_return(true)
+      allow(profiler).to receive(:profile_mri)
+
+      profiler.profile { 'cats' }
+    end
+
+    it 'raises NotImplementedError when profiling an unsupported platform' do
+      allow(profiler).to receive(:mri?).and_return(false)
+
+      expect { profiler.profile { 'cats' } }.to raise_error(NotImplementedError)
+    end
+  end
+
+  describe '#profile_mri' do
+    it 'returns an Array containing the return value and profiling samples' do
+      allow(profiler).to receive(:lineprof).
+        and_yield.
+        and_return({ __FILE__ => [[0, 0, 0, 0]] })
+
+      retval, samples = profiler.profile_mri { 42 }
+
+      expect(retval).to eq(42)
+      expect(samples).to eq([])
+    end
+  end
+
+  describe '#aggregate_rblineprof' do
+    let(:raw_samples) do
+      { __FILE__ => [[30000, 30000, 5, 0], [15000, 15000, 4, 0]] }
+    end
+
+    it 'returns an Array of FileSample objects' do
+      samples = profiler.aggregate_rblineprof(raw_samples)
+
+      expect(samples).to be_an_instance_of(Array)
+      expect(samples[0]).to be_an_instance_of(Gitlab::Sherlock::FileSample)
+    end
+
+    describe 'the first FileSample object' do
+      let(:file_sample) do
+        profiler.aggregate_rblineprof(raw_samples)[0]
+      end
+
+      it 'uses the correct file path' do
+        expect(file_sample.file).to eq(__FILE__)
+      end
+
+      it 'contains a list of line samples' do
+        line_sample = file_sample.line_samples[0]
+
+        expect(line_sample).to be_an_instance_of(Gitlab::Sherlock::LineSample)
+
+        expect(line_sample.duration).to eq(15.0)
+        expect(line_sample.events).to eq(4)
+      end
+
+      it 'contains the total file execution time' do
+        expect(file_sample.duration).to eq(30.0)
+      end
+
+      it 'contains the total amount of file events' do
+        expect(file_sample.events).to eq(5)
+      end
+    end
+  end
+end
diff --git a/spec/lib/gitlab/sherlock/line_sample_spec.rb b/spec/lib/gitlab/sherlock/line_sample_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..5f02f6a3213eb0b998f6b7c2edc6e5b78edbd2d2
--- /dev/null
+++ b/spec/lib/gitlab/sherlock/line_sample_spec.rb
@@ -0,0 +1,33 @@
+require 'spec_helper'
+
+describe Gitlab::Sherlock::LineSample do
+  let(:sample) { described_class.new(150.0, 4) }
+
+  describe '#duration' do
+    it 'returns the duration' do
+      expect(sample.duration).to eq(150.0)
+    end
+  end
+
+  describe '#events' do
+    it 'returns the amount of events' do
+      expect(sample.events).to eq(4)
+    end
+  end
+
+  describe '#percentage_of' do
+    it 'returns the percentage of 1500.0' do
+      expect(sample.percentage_of(1500.0)).to be_within(0.1).of(10.0)
+    end
+  end
+
+  describe '#majority_of' do
+    it 'returns true if the sample takes up the majority of the given duration' do
+      expect(sample.majority_of?(500.0)).to eq(true)
+    end
+
+    it "returns false if the sample doesn't take up the majority of the given duration" do
+      expect(sample.majority_of?(1500.0)).to eq(false)
+    end
+  end
+end
diff --git a/spec/lib/gitlab/sherlock/location_spec.rb b/spec/lib/gitlab/sherlock/location_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..b295a624b35e47c86e67ac37da61ddfb233f27dc
--- /dev/null
+++ b/spec/lib/gitlab/sherlock/location_spec.rb
@@ -0,0 +1,40 @@
+require 'spec_helper'
+
+describe Gitlab::Sherlock::Location do
+  let(:location) { described_class.new(__FILE__, 1) }
+
+  describe 'from_ruby_location' do
+    it 'creates a Location from a Thread::Backtrace::Location' do
+      input  = caller_locations[0]
+      output = described_class.from_ruby_location(input)
+
+      expect(output).to be_an_instance_of(described_class)
+      expect(output.path).to eq(input.path)
+      expect(output.line).to eq(input.lineno)
+    end
+  end
+
+  describe '#path' do
+    it 'returns the file path' do
+      expect(location.path).to eq(__FILE__)
+    end
+  end
+
+  describe '#line' do
+    it 'returns the line number' do
+      expect(location.line).to eq(1)
+    end
+  end
+
+  describe '#application?' do
+    it 'returns true for an application frame' do
+      expect(location.application?).to eq(true)
+    end
+
+    it 'returns false for a non application frame' do
+      loc = described_class.new('/tmp/cats.rb', 1)
+
+      expect(loc.application?).to eq(false)
+    end
+  end
+end
diff --git a/spec/lib/gitlab/sherlock/middleware_spec.rb b/spec/lib/gitlab/sherlock/middleware_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..aa74fc53a7954dd6a2413eab494ec4eef64a8d25
--- /dev/null
+++ b/spec/lib/gitlab/sherlock/middleware_spec.rb
@@ -0,0 +1,79 @@
+require 'spec_helper'
+
+describe Gitlab::Sherlock::Middleware do
+  let(:app) { double(:app) }
+  let(:middleware) { described_class.new(app) }
+
+  describe '#call' do
+    describe 'when instrumentation is enabled' do
+      it 'instruments a request' do
+        allow(middleware).to receive(:instrument?).and_return(true)
+        allow(middleware).to receive(:call_with_instrumentation)
+
+        middleware.call({})
+      end
+    end
+
+    describe 'when instrumentation is disabled' do
+      it "doesn't instrument a request" do
+        allow(middleware).to receive(:instrument).and_return(false)
+        allow(app).to receive(:call)
+
+        middleware.call({})
+      end
+    end
+  end
+
+  describe '#call_with_instrumentation' do
+    it 'instruments a request' do
+      trans = double(:transaction)
+      retval = 'cats are amazing'
+      env = {}
+
+      allow(app).to receive(:call).with(env).and_return(retval)
+      allow(middleware).to receive(:transaction_from_env).and_return(trans)
+      allow(trans).to receive(:run).and_yield.and_return(retval)
+      allow(Gitlab::Sherlock.collection).to receive(:add).with(trans)
+
+      middleware.call_with_instrumentation(env)
+    end
+  end
+
+  describe '#instrument?' do
+    it 'returns false for a text/css request' do
+      env = { 'HTTP_ACCEPT' => 'text/css', 'REQUEST_URI' => '/' }
+
+      expect(middleware.instrument?(env)).to eq(false)
+    end
+
+    it 'returns false for a request to a Sherlock route' do
+      env = {
+        'HTTP_ACCEPT' => 'text/html',
+        'REQUEST_URI' => '/sherlock/transactions'
+      }
+
+      expect(middleware.instrument?(env)).to eq(false)
+    end
+
+    it 'returns true for a request that should be instrumented' do
+      env = {
+        'HTTP_ACCEPT' => 'text/html',
+        'REQUEST_URI' => '/cats'
+      }
+
+      expect(middleware.instrument?(env)).to eq(true)
+    end
+  end
+
+  describe '#transaction_from_env' do
+    it 'returns a Transaction' do
+      env = {
+        'HTTP_ACCEPT' => 'text/html',
+        'REQUEST_URI' => '/cats'
+      }
+
+      expect(middleware.transaction_from_env(env)).
+        to be_an_instance_of(Gitlab::Sherlock::Transaction)
+    end
+  end
+end
diff --git a/spec/lib/gitlab/sherlock/query_spec.rb b/spec/lib/gitlab/sherlock/query_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..a9afef5dc1dc8a7ac579bb7eead3ac44d47b320e
--- /dev/null
+++ b/spec/lib/gitlab/sherlock/query_spec.rb
@@ -0,0 +1,113 @@
+require 'spec_helper'
+
+describe Gitlab::Sherlock::Query do
+  let(:started_at)  { Time.utc(2015, 1, 1) }
+  let(:finished_at) { started_at + 5 }
+
+  let(:query) do
+    described_class.new('SELECT COUNT(*) FROM users', started_at, finished_at)
+  end
+
+  describe 'new_with_bindings' do
+    it 'returns a Query' do
+      sql = 'SELECT COUNT(*) FROM users WHERE id = $1'
+      bindings = [[double(:column), 10]]
+
+      query = described_class.
+        new_with_bindings(sql, bindings, started_at, finished_at)
+
+      expect(query.query).to eq('SELECT COUNT(*) FROM users WHERE id = 10;')
+    end
+  end
+
+  describe '#id' do
+    it 'returns a String' do
+      expect(query.id).to be_an_instance_of(String)
+    end
+  end
+
+  describe '#query' do
+    it 'returns the query with a trailing semi-colon' do
+      expect(query.query).to eq('SELECT COUNT(*) FROM users;')
+    end
+  end
+
+  describe '#started_at' do
+    it 'returns the start time' do
+      expect(query.started_at).to eq(started_at)
+    end
+  end
+
+  describe '#finished_at' do
+    it 'returns the completion time' do
+      expect(query.finished_at).to eq(finished_at)
+    end
+  end
+
+  describe '#backtrace' do
+    it 'returns the backtrace' do
+      expect(query.backtrace).to be_an_instance_of(Array)
+    end
+  end
+
+  describe '#duration' do
+    it 'returns the duration in milliseconds' do
+      expect(query.duration).to be_within(0.1).of(5000.0)
+    end
+  end
+
+  describe '#to_param' do
+    it 'returns the query ID' do
+      expect(query.to_param).to eq(query.id)
+    end
+  end
+
+  describe '#formatted_query' do
+    it 'returns a formatted version of the query' do
+      expect(query.formatted_query).to eq(<<-EOF.strip)
+SELECT COUNT(*)
+FROM users;
+      EOF
+    end
+  end
+
+  describe '#last_application_frame' do
+    it 'returns the last application frame' do
+      frame = query.last_application_frame
+
+      expect(frame).to be_an_instance_of(Gitlab::Sherlock::Location)
+      expect(frame.path).to eq(__FILE__)
+    end
+  end
+
+  describe '#application_backtrace' do
+    it 'returns an Array of application frames' do
+      frames = query.application_backtrace
+
+      expect(frames).to be_an_instance_of(Array)
+      expect(frames).to_not be_empty
+
+      frames.each do |frame|
+        expect(frame.path).to start_with(Rails.root.to_s)
+      end
+    end
+  end
+
+  describe '#explain' do
+    it 'returns the query plan as a String' do
+      lines = [
+        ['Aggregate (cost=123 rows=1)'],
+        ['  -> Index Only Scan using index_cats_are_amazing']
+      ]
+
+      result = double(:result, values: lines)
+
+      allow(query).to receive(:raw_explain).and_return(result)
+
+      expect(query.explain).to eq(<<-EOF.strip)
+Aggregate (cost=123 rows=1)
+  -> Index Only Scan using index_cats_are_amazing
+      EOF
+    end
+  end
+end
diff --git a/spec/lib/gitlab/sherlock/transaction_spec.rb b/spec/lib/gitlab/sherlock/transaction_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..bb49fb65cf8c6ec65bcb5f8a02abe5b17aed2f2b
--- /dev/null
+++ b/spec/lib/gitlab/sherlock/transaction_spec.rb
@@ -0,0 +1,222 @@
+require 'spec_helper'
+
+describe Gitlab::Sherlock::Transaction do
+  let(:transaction) { described_class.new('POST', '/cat_pictures') }
+
+  describe '#id' do
+    it 'returns the transaction ID' do
+      expect(transaction.id).to be_an_instance_of(String)
+    end
+  end
+
+  describe '#type' do
+    it 'returns the type' do
+      expect(transaction.type).to eq('POST')
+    end
+  end
+
+  describe '#path' do
+    it 'returns the path' do
+      expect(transaction.path).to eq('/cat_pictures')
+    end
+  end
+
+  describe '#queries' do
+    it 'returns an Array of queries' do
+      expect(transaction.queries).to be_an_instance_of(Array)
+    end
+  end
+
+  describe '#file_samples' do
+    it 'returns an Array of file samples' do
+      expect(transaction.file_samples).to be_an_instance_of(Array)
+    end
+  end
+
+  describe '#started_at' do
+    it 'returns the start time' do
+      allow(transaction).to receive(:profile_lines).and_yield
+
+      transaction.run { 'cats are amazing' }
+
+      expect(transaction.started_at).to be_an_instance_of(Time)
+    end
+  end
+
+  describe '#finished_at' do
+    it 'returns the completion time' do
+      allow(transaction).to receive(:profile_lines).and_yield
+
+      transaction.run { 'cats are amazing' }
+
+      expect(transaction.finished_at).to be_an_instance_of(Time)
+    end
+  end
+
+  describe '#view_counts' do
+    it 'returns a Hash' do
+      expect(transaction.view_counts).to be_an_instance_of(Hash)
+    end
+
+    it 'sets the default value of a key to 0' do
+      expect(transaction.view_counts['cats.rb']).to be_zero
+    end
+  end
+
+  describe '#run' do
+    it 'runs the transaction' do
+      allow(transaction).to receive(:profile_lines).and_yield
+
+      retval = transaction.run { 'cats are amazing' }
+
+      expect(retval).to eq('cats are amazing')
+    end
+  end
+
+  describe '#duration' do
+    it 'returns the duration in seconds' do
+      start_time = Time.now
+
+      allow(transaction).to receive(:started_at).and_return(start_time)
+      allow(transaction).to receive(:finished_at).and_return(start_time + 5)
+
+      expect(transaction.duration).to be_within(0.1).of(5.0)
+    end
+  end
+
+  describe '#to_param' do
+    it 'returns the transaction ID' do
+      expect(transaction.to_param).to eq(transaction.id)
+    end
+  end
+
+  describe '#sorted_queries' do
+    it 'returns the queries in descending order' do
+      start_time = Time.now
+
+      query1 = Gitlab::Sherlock::Query.new('SELECT 1', start_time, start_time)
+
+      query2 = Gitlab::Sherlock::Query.
+        new('SELECT 2', start_time, start_time + 5)
+
+      transaction.queries << query1
+      transaction.queries << query2
+
+      expect(transaction.sorted_queries).to eq([query2, query1])
+    end
+  end
+
+  describe '#sorted_file_samples' do
+    it 'returns the file samples in descending order' do
+      sample1 = Gitlab::Sherlock::FileSample.new(__FILE__, [], 10.0, 1)
+      sample2 = Gitlab::Sherlock::FileSample.new(__FILE__, [], 15.0, 1)
+
+      transaction.file_samples << sample1
+      transaction.file_samples << sample2
+
+      expect(transaction.sorted_file_samples).to eq([sample2, sample1])
+    end
+  end
+
+  describe '#find_query' do
+    it 'returns a Query when found' do
+      query = Gitlab::Sherlock::Query.new('SELECT 1', Time.now, Time.now)
+
+      transaction.queries << query
+
+      expect(transaction.find_query(query.id)).to eq(query)
+    end
+
+    it 'returns nil when no query could be found' do
+      expect(transaction.find_query('cats')).to be_nil
+    end
+  end
+
+  describe '#find_file_sample' do
+    it 'returns a FileSample when found' do
+      sample = Gitlab::Sherlock::FileSample.new(__FILE__, [], 10.0, 1)
+
+      transaction.file_samples << sample
+
+      expect(transaction.find_file_sample(sample.id)).to eq(sample)
+    end
+
+    it 'returns nil when no file sample could be found' do
+      expect(transaction.find_file_sample('cats')).to be_nil
+    end
+  end
+
+  describe '#profile_lines' do
+    describe 'when line profiling is enabled' do
+      it 'yields the block using the line profiler' do
+        allow(Gitlab::Sherlock).to receive(:enable_line_profiler?).
+          and_return(true)
+
+        allow_any_instance_of(Gitlab::Sherlock::LineProfiler).
+          to receive(:profile).and_return('cats are amazing', [])
+
+        retval = transaction.profile_lines { 'cats are amazing' }
+
+        expect(retval).to eq('cats are amazing')
+      end
+    end
+
+    describe 'when line profiling is disabled' do
+      it 'yields the block' do
+        allow(Gitlab::Sherlock).to receive(:enable_line_profiler?).
+          and_return(false)
+
+        retval = transaction.profile_lines { 'cats are amazing' }
+
+        expect(retval).to eq('cats are amazing')
+      end
+    end
+  end
+
+  describe '#subscribe_to_active_record' do
+    let(:subscription) { transaction.subscribe_to_active_record }
+    let(:time) { Time.now }
+    let(:query_data) { { sql: 'SELECT 1', binds: [] } }
+
+    after do
+      ActiveSupport::Notifications.unsubscribe(subscription)
+    end
+
+    it 'tracks executed queries' do
+      expect(transaction).to receive(:track_query).
+        with('SELECT 1', [], time, time)
+
+      subscription.publish('test', time, time, nil, query_data)
+    end
+
+    it 'only tracks queries triggered from the transaction thread' do
+      expect(transaction).to_not receive(:track_query)
+
+      Thread.new { subscription.publish('test', time, time, nil, query_data) }.
+        join
+    end
+  end
+
+  describe '#subscribe_to_action_view' do
+    let(:subscription) { transaction.subscribe_to_action_view }
+    let(:time) { Time.now }
+    let(:view_data) { { identifier: 'foo.rb' } }
+
+    after do
+      ActiveSupport::Notifications.unsubscribe(subscription)
+    end
+
+    it 'tracks rendered views' do
+      expect(transaction).to receive(:track_view).with('foo.rb')
+
+      subscription.publish('test', time, time, nil, view_data)
+    end
+
+    it 'only tracks views rendered from the transaction thread' do
+      expect(transaction).to_not receive(:track_view)
+
+      Thread.new { subscription.publish('test', time, time, nil, view_data) }.
+        join
+    end
+  end
+end