10. Database ConnectivityΒΆ
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 aftermax-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 aftermax-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 aftermax-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.