You don't say what version of Moodle you're developing for, but there is already an infrastructure for exactly this which is the recommended approach. It's called Web Services, and configuration and details for it can be found in your Moodle Settings block at Site administration -> Plugins -> Web services.
At present there are only forum-related web service functions for:
However, you should be able to write your own web services using the plugin infrastructure which will allow you to achieve what you intend without accessing the database directly. This has the main advantage that you're using established APIs which are updated when changes are made to core Moodle systems.
Alternatively, you could contribute new functions to the existing plugins and submit them back to the Moodle project so that others can benefit from them.