Improving the "Environment variables" administration doc

config/database.yml.env

 <%= ENV['RAILS_ENV'] %>:
+  ## Connection information
+  # Please be aware that the DATABASE_URL environment variable will take
+  # precedence over the following 6 parameters. For more information, see
+  # doc/administration/environment_variables.md
   adapter: <%= ENV['GITLAB_DATABASE_ADAPTER'] || 'postgresql' %>
-  encoding: <%= ENV['GITLAB_DATABASE_ENCODING'] || 'unicode' %>
   database: <%= ENV['GITLAB_DATABASE_DATABASE'] || "gitlab_#{ENV['RAILS_ENV']}" %>
-  pool: <%= ENV['GITLAB_DATABASE_POOL'] || '10' %>
   username: <%= ENV['GITLAB_DATABASE_USERNAME'] || 'root' %>
   password: <%= ENV['GITLAB_DATABASE_PASSWORD'] || '' %>
   host: <%= ENV['GITLAB_DATABASE_HOST'] || 'localhost' %>
   port: <%= ENV['GITLAB_DATABASE_PORT'] || '5432' %>
+
+  ## Behavior information
+  # The following parameters will be used even if you're using the DATABASE_URL
+  # environment variable.
+  encoding: <%= ENV['GITLAB_DATABASE_ENCODING'] || 'unicode' %>
+  pool: <%= ENV['GITLAB_DATABASE_POOL'] || '10' %>

doc/administration/environment_variables.md

@@ -2,7 +2,7 @@
 
 ## Introduction
 
-Commonly people configure GitLab via the gitlab.rb configuration file in the Omnibus package.
+Commonly people configure GitLab via the `gitlab.rb` configuration file in the Omnibus package.
 
 But if you prefer to use environment variables we allow that too.
 
@@ -10,45 +10,45 @@ But if you prefer to use environment variables we allow that too.
 
 Variable | Type | Explanation
 -------- | ---- | -----------
-GITLAB_ROOT_PASSWORD | string | sets the password for the `root` user on installation
-GITLAB_HOST | url | hostname of the GitLab server includes http or https
-RAILS_ENV | production / development / staging / test | Rails environment
-DATABASE_URL | url | For example: postgresql://localhost/blog_development?pool=5
-GITLAB_EMAIL_FROM | email | Email address used in the "From" field in mails sent by GitLab
-GITLAB_EMAIL_DISPLAY_NAME | string | Name used in the "From" field in mails sent by GitLab
-GITLAB_EMAIL_REPLY_TO | email | Email address used in the "Reply-To" field in mails sent by GitLab
-GITLAB_UNICORN_MEMORY_MIN | integer | The minimum memory threshold (in bytes) for the Unicorn worker killer
-GITLAB_UNICORN_MEMORY_MAX | integer | The maximum memory threshold (in bytes) for the Unicorn worker killer
+`GITLAB_ROOT_PASSWORD` | string | Sets the password for the `root` user on installation
+`GITLAB_HOST` | url | Hostname of the GitLab server includes http or https
+`RAILS_ENV` | production / development / staging / test | Rails environment
+`DATABASE_URL` | url | For example: postgresql://localhost/blog_development
+`GITLAB_EMAIL_FROM` | email | Email address used in the "From" field in mails sent by GitLab
+`GITLAB_EMAIL_DISPLAY_NAME` | string | Name used in the "From" field in mails sent by GitLab
+`GITLAB_EMAIL_REPLY_TO` | email | Email address used in the "Reply-To" field in mails sent by GitLab
+`GITLAB_UNICORN_MEMORY_MIN` | integer | The minimum memory threshold (in bytes) for the Unicorn worker killer
+`GITLAB_UNICORN_MEMORY_MAX` | integer | The maximum memory threshold (in bytes) for the Unicorn worker killer
 
 ## Complete database variables
 
-As explained in the [Heroku documentation](https://devcenter.heroku.com/articles/rails-database-connection-behavior) the DATABASE_URL doesn't let you set:
+The recommended way of specifying your database connection information is to set
+the `DATABASE_URL` environment variable. This variable only holds connection
+information (adapter, database, username, password, host and port), but not
+behavior information (encoding, pool). If you don't want to use `DATABASE_URL`
+and/or want to set database behavior information, you will have to:
 
-- adapter
-- database
-- username
-- password
-- host
-- port
+- copy our template `config/database.yml` file: `cp config/database.yml.env config/database.yml`
+- set a value for some `GITLAB_DATABASE_XXX` variables
 
-To do so please `cp config/database.yml.env config/database.yml` and use the following variables:
+The list of `GITLAB_DATABASE_XXX` variables that you can set is as follow:
 
-Variable | Default
---- | ---
-GITLAB_DATABASE_ADAPTER | postgresql
-GITLAB_DATABASE_ENCODING | unicode
-GITLAB_DATABASE_DATABASE | gitlab_#{ENV['RAILS_ENV']
-GITLAB_DATABASE_POOL | 10
-GITLAB_DATABASE_USERNAME | root
-GITLAB_DATABASE_PASSWORD |
-GITLAB_DATABASE_HOST | localhost
-GITLAB_DATABASE_PORT | 5432
+Variable | Default value | Overridden by `DATABASE_URL`?
+--- | --- | ---
+`GITLAB_DATABASE_ADAPTER` | `postgresql` | Yes
+`GITLAB_DATABASE_DATABASE` | `gitlab_#{ENV['RAILS_ENV']` | Yes
+`GITLAB_DATABASE_USERNAME` | `root` | Yes
+`GITLAB_DATABASE_PASSWORD` | None | Yes
+`GITLAB_DATABASE_HOST` | `localhost` | Yes
+`GITLAB_DATABASE_PORT` | `5432` | Yes
+`GITLAB_DATABASE_ENCODING` | `unicode` | No
+`GITLAB_DATABASE_POOL` | `10` | No
 
 ## Adding more variables
 
 We welcome merge requests to make more settings configurable via variables.
-Please make changes in the file config/initializers/1_settings.rb
-Please stick to the naming scheme "GITLAB_#{name 1_settings.rb in upper case}".
+Please make changes in the `config/initializers/1_settings.rb` file.
+Please stick to the naming scheme `GITLAB_#{name in 1_settings.rb in upper case}`.
 
 ## Omnibus configuration