8.3. Cartridge Event Example

In this example, an application developer adds a MySQL database cartridge to a PHP application. The publish and subscribe relationship between the cartridges enables the PHP cartridge to set environment variables on its gear so it can connect to the new MySQL cartridge, which is on a different gear.
MySQL Cartridge as Publisher

The MySQL cartridge lists a publish-mysql-connection-info event in the Publishes section of its mysql/metadata/manifest.yml file: 

Publishes:
  publish-mysql-connection-info:
    Type: "NET_TCP:db:mysql"

The MySQL cartridge implements a script in mysql/hooks/publish-mysql-connection-info.
PHP Cartridge as Subscriber

The PHP cartridge lists a set-mysql-connection-info event in the Subscribes section of its php/metadata/manifest.yml file: 

Subscribes:
  set-mysql-connection-info:
    Type: "NET_TCP:db:mysql"

The PHP cartridge implements a script in php/hooks/set-mysql-connection-info.
Cartridge Event Communication Process

OpenShift Enterprise matches the event Type in the PHP cartridge's Subscribes list to the event Type in the MySQL cartridge's Publishes list. In this example, the event Type is "NET_TCP:db:mysql".

The MySQL cartridge's publish-mysql-connection-info script outputs the username, host, port, URL, and password required to connect to the MySQL instance: 
OPENSHIFT_MYSQL_DB_USERNAME=username;
OPENSHIFT_MYSQL_DB_PASSWORD=password;
OPENSHIFT_MYSQL_DB_HOST=hostname;
OPENSHIFT_MYSQL_DB_PORT=port;
OPENSHIFT_MYSQL_DB_URL=url;
OpenShift Enterprise sends the output of the MySQL cartridge's publish-mysql-connection-info to the PHP cartridge's set-mysql-connection-info script using the following format: 
hooks/publish-mysql-connection-info gear_name namespace gear_uuid 'OPENSHIFT_MYSQL_DB_USERNAME=username;OPENSHIFT_MYSQL_DB_PASSWORD=password;OPENSHIFT_MYSQL_DB_HOST=hostname;OPENSHIFT_MYSQL_DB_PORT=port;OPENSHIFT_MYSQL_DB_URL=url;'
Note that the publisher script determines the format of the information input to the subscriber script. When writing subscriber scripts, ensure that they parse the input correctly.