For single-instance hosts, this is simple. The mariadb port and unix socket are :3306 and /run/mysqld/mysqld.sock, and the prom-mysql-exporter runs on :9104. For multi-instance hosts, this gets more complicated.
Each section has an 'index' that is not explicitly documented (that i could find, in any case). s1 has index 1, s8 has index 8, m5 has index 15, etc. The mariadb port for a multi-instance host is 3310 + the index of the section. So x1 (which has index 10) runs mariadb on port :3320. The prom-mysql-exporter port is 13310 + the index of the section (so staging with an index of 40 has a prom exporter port of :13350).
Mapping from a section to a mariadb unix socket is straight-forward, with one catch. The socket is /run/mysqld/mysqld.<section name>.sock. The problem, however, is that if the section name has a - in it this will break due to systemd's escaping rules. Currently we do not handle this special-case anywhere.
The above rules are implicitly encoded or relied upon in many places:
- All the multiinstance mariadb puppet profiles. E.g. modules/profile/manifests/mariadb/dbstore_multiinstance.pp (link)
- conftool-data/dbconfig-instance/instances.yaml (link)
- modules/mariadb/files/check_mariadb.py (link)
- modules/profile/files/prometheus/mysqld_exporter_config.py (link)
- modules/profile/files/mariadb/backup_mariadb.py (link)
- modules/profile/files/mariadb/remote_backup_mariadb.py (link)
- Backup configs. E.g. modules/profile/templates/mariadb/backup_config/cumin1001.cnf.erb (link)
Ideally, this mapping would be precisely defined in exactly one place, but even one place per language (puppet and python) would be a massive improvement. Centralising this would also have the benefit that we could fix things like the mapping of sections that contain - to unix sockets in a way that makes systemd happy.