# 10. Database Connectivity

The Kea servers (`kea-dhcp4` and `kea-dhcp6`) can be configured to use a variety of database backends for leases, hosts, and configuration. They can be configured to support automatic recovery when connectivity is lost, via the `on-fail` parameter. (The `reconnect-wait-time` and `max-reconnect-tries` parameters are described in Lease Database Configuration and Lease Database Configuration.)

It is important to understand how and when automatic recovery comes into play. Automatic recovery, when configured, only operates after a successful startup or reconfiguration during which connectivity to all backends has been successfully established.

During server startup, the inability to connect to any of the configured backends is always considered fatal. A fatal error is logged and the server exits, based on the idea that the configuration should be valid at startup. Exiting to the operating system allows nanny scripts to detect the problem.

During dynamic reconfiguration, all backends are disconnected and then reconnected using the new configuration. If connectivity to any of the backends cannot be established, the server logs a fatal error but remains up. It is able to process commands but does not serve clients. This allows the configuration to be corrected via the `config-set` or `remote-*` commands, if required.

During normal operations, if connectivity to any of the backends is lost and automatic recovery for that backend is enabled, the server disconnects from the respective backend and then attempts to reconnect. During the recovery process, the server ceases to serve clients according to the `on-fail` configured option but continues to respond to commands.

The `on-fail` parameter configures the actions the server should take when a connection is lost. It can have one of the following values:

- `stop-retry-exit` - indicates that the server should stop the service while it tries to recover the connection, and exit if recovery is not successful after `max-reconnect-tries`.

- `serve-retry-exit` - indicates that the server should not stop the service while it tries to recover the connection, and exit if recovery is not successful after `max-reconnect-tries`.

- `serve-retry-continue` - indicates that the server should not stop the service while it tries to recover the connection, and not exit if recovery is not successful after `max-reconnect-tries`.

If connectivity to all backends is restored, the server returns to normal operations. If the connection cannot be restored and the server is configured to exit, it issues a fatal error before shutdown.

The connection to the database server can optionally be protected by TLS. Corresponding database configuration parameters for Kea servers are:

- The `trust-anchor` specifies the Certification Authority file name or directory path.

- The `cert-file` specifies the client certificate file name.

- The `key-file` specifies the private key file name.

- The `cipher-list` specifies the list of TLS ciphers (the syntax of the content of this parameter is described in the OpenSSL ciphers manual).

These parameters are similar to the parameters of the secure connections with the agent but are interpreted by different backends using database configurations too.

Currently the support for each database is:

- MySQL supports the whole set, additional configuration must be done in the MySQL local setup, for instance certificate revocation list, choice of a specific TLS version, mutual authentication, etc. When a TLS connection was required but the actual connection is in clear text an error log is emitted.

- PostgreSQL only uses the configuration to enable the SSL/TLS support in the client library (libpq). Anything else must be done in the PostgreSQL local configuration.