Merge branch 'query-counts' into 'master'

Track and act upon the number of executed SQL queries

See merge request gitlab-org/gitlab-ce!16466

2 files changed, 66 insertions, 0 deletions

diff --git a/doc/development/README.md b/doc/development/README.md
index 12cca9f84b7..45e9565f9a7 100644
--- a/doc/development/README.md
+++ b/doc/development/README.md
@@ -75,6 +75,7 @@ comments: false
 - [Ordering table columns](ordering_table_columns.md)
 - [Verifying database capabilities](verifying_database_capabilities.md)
 - [Database Debugging and Troubleshooting](database_debugging.md)
+- [Query Count Limits](query_count_limits.md)
 
 ## Testing guides
 
diff --git a/doc/development/query_count_limits.md b/doc/development/query_count_limits.md
new file mode 100644
index 00000000000..ebb6e0c2dac
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@@ -0,0 +1,65 @@
+# Query Count Limits
+
+Each controller or API endpoint is allowed to execute up to 100 SQL queries. In
+a production environment we'll only log an error in case this threshold is
+exceeded, but in a test environment we'll raise an error instead.
+
+## Solving Failing Tests
+
+When a test fails because it executes more than 100 SQL queries there are two
+solutions to this problem:
+
+1. Reduce the number of SQL queries that are executed.
+2. Whitelist the controller or API endpoint.
+
+You should only resort to whitelisting when an existing controller or endpoint
+is to blame as in this case reducing the number of SQL queries can take a lot of
+effort. Newly added controllers and endpoints are not allowed to execute more
+than 100 SQL queries and no exceptions will be made for this rule. _If_ a large
+number of SQL queries is necessary to perform certain work it's best to have
+this work performed by Sidekiq instead of doing this directly in a web request.
+
+## Whitelisting
+
+In the event that you _have_ to whitelist a controller you'll first need to
+create an issue. This issue should (preferably in the title) mention the
+controller or endpoint and include the appropriate labels (`database`,
+`performance`, and at least a team specific label such as `Discussion`).
+
+Once the issue has been created you can whitelist the code in question. For
+Rails controllers it's best to create a `before_action` hook that runs as early
+as possible. The called method in turn should call
+`Gitlab::QueryLimiting.whitelist('issue URL here')`. For example:
+
+```ruby
+class MyController < ApplicationController
+  before_action :whitelist_query_limiting, only: [:show]
+
+  def index
+    # ...
+  end
+
+  def show
+    # ...
+  end
+
+  def whitelist_query_limiting
+    Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/...')
+  end
+end
+```
+
+By using a `before_action` you don't have to modify the controller method in
+question, reducing the likelihood of merge conflicts.
+
+For Grape API endpoints there unfortunately is not a reliable way of running a
+hook before a specific endpoint. This means that you have to add the whitelist
+call directly into the endpoint like so:
+
+```ruby
+get '/projects/:id/foo' do
+  Gitlab::QueryLimiting.whitelist('...')
+
+  # ...
+end
+```