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diff --git a/doc/development/query_count_limits.md b/doc/development/query_count_limits.md new file mode 100644 index 00000000000..ebb6e0c2dac --- /dev/null +++ b/doc/development/query_count_limits.md @@ -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 +``` |