Deployment

Know which parts of a Sites platform travel with exported configuration and which ones live in each environment's database.

Configuration: exported and imported	Content: lives in each environment
Site types, with their setting defaults, assignable content and assignable roles	Sites, with their settings, configuration overrides, translations and values per environment
The pages under Sites configuration, such as General	Who is a member of which site, with which per-site roles
Roles and their permissions	Which content belongs to which site, and its canonical site
Modules, content types, menus, views, blocks	Menu links, URL aliases, redirects and all other content

Configuration

drush config:export and drush config:import carry site types and the platform settings like any other configuration. Change site types on development and import them on the other environments.

Sites are content

A site is created on the environment where it runs:

• in the admin UI (Create a site);
• with drush sites:create in a deployment script (Drush);
• by copying the production database to the other environments.

A module can also declare sites in code; those ship with the code (Site plugins).

The ID of a site is set once, and settings.php addresses the site by it. Give a site the same ID on every environment.

Values per environment

A database copied from production carries the production hostnames. Give each environment its own values in one of two places.

The Environments tab. Every settings section of a site has an Environments tab, for the settings that the site type lets vary by environment (its Environment overrides column). Environment Context supplies the list of environments: its submodule environment_context_config_split takes them from Config Split, environment_context_environment_indicator from Environment Indicator. Each environment names itself in settings.php, or in the DRUPAL_ENVIRONMENT environment variable:

$settings['environment'] = 'stage';

settings.php. Values keyed by the site's ID win over every other layer (Setting layers):

$settings['acme'] = [
  'hostname' => 'acme.stage.example.com',
];

Site settings lists the keys and which of them can differ per environment. To see what a site resolves to on an environment:

drush sites:info acme --environment=stage

Caches

Give every environment its own cache backend: two environments on one cache serve each other's hostnames (Caching and performance).