The PingDirectory server can write log messages to a relational database.
This should work with any database that provides a driver that allows it to integrate with the Java Database Connectivity (JDBC) framework. As with syslog, these log messages are written to the database as they are generated in the server, so attackers do not have a chance to alter them before they are written.
Communication with the database can be secured using whatever facilities are provided by the JDBC driver. In most cases, this includes support for TLS encryption.