Automatically change database during acceptance and functional tests

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You should always back up any site you run tests on if you care about the site content.
Now this disclaimer has been made ad nauseam; there's a simple way to use a different database when during tests.
Identifying requests
The first component of this solution is identifying the source of the current HTTP request.
WordPress makes this identification before deciding which database to use.
To provide the WordPress installation with this information, you can set the headers entry of the WPBrowser or WPWebDriver module in the suite configuration file.
As an example here is an acceptance suite configuration file setting two custom headers, X_WPBROWSER_REQUEST and X_TEST_REQUEST, on each request sent by the WPWebDriver module:
actor: AcceptanceTester
modules:
    enabled:
        - WPDb
        - WPBrowser
        - \Helper\Acceptance
    config:
        WPDb:
            dsn: 'mysql:host=localhost;dbname=tests'
            user: 'root'
            password: 'root'
            dump: 'tests/_data/dump.sql'
            populate: true
            cleanup: true
            waitlock: 10
            url: 'http://wp.test'
            urlReplacement: true
            tablePrefix: 'wp_'
        WPBrowser:
            url: 'http://wp.test'
            adminUsername: 'admin'
            adminPassword: 'admin'
            adminPath: '/wp-admin'
            headers:
                X_WPBROWSER_REQUEST: 1
                X_TEST_REQUEST: 1
The two headers are sent on each HTTP request type, not just on GET type requests.
Using a different database to handle test requests
Now that each request made by the WPWebDriver module contains those two headers, it's time for WordPress to check those and change the database to use accordingly.
The database to use is set by the DB_NAME constant that is, in turn, set in the wp-config.php file.
Different setups could involve more complex configurations for the wp-config.php file but, for the sake of simplicity, I assume the default WordPress wp-config.php file structure.
In the example below, the default database name is wordpress, while the name of the test database is tests.
- define( 'DB_NAME', 'wordpress' );
+ if( isset( $_SERVER['HTTP_X_TEST_REQUEST'] ) && $_SERVER['HTTP_X_TEST_REQUEST'] ) {
+     define( 'DB_NAME', 'tests' );
+ } else {
+     define( 'DB_NAME', 'wordpress' );
+ }
The diff shows the replacement done in the WordPress installation wp-config.php file.
For copy-and-paste pleasure, replace the line starting with:
define( 'DB_NAME', 'default_db_name' );
With this snippet:
if( isset( $_SERVER['HTTP_X_TEST_REQUEST'] ) && $_SERVER['HTTP_X_TEST_REQUEST'] ) {
      define( 'DB_NAME', 'test_db_name' );
} else {
      define( 'DB_NAME', 'default_db_name' );
}
Where default_db_name is the name of the database your test WordPress installation normally uses.
Happy, and safer, testing.
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Setting up wp-browser on VVV to test a plugin
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Identifying requests
Using a different database to handle test requests