Reasons behind the migration
We have migrated to the official mongo-c-driver binding for providing the MongoDB destination driver in syslog-ng 3.8. Previously in syslog-ng 3.7.x and earlier, libmongo-client provided this binding, mandating its own special syntax.
This change will facilitate future-proof and more fine-grained configuration. MongoDB 3 is not officially supported or being tested yet, but this kind of connection should theoretically enable easy MongoDB 3 support in the future.
What to do when using legacy syntax
If you have used legacy syntax in your configuration file, syslog-ng will substitute the given deprecated options to form a URI. Note that certain aspects of semantics could also differ between the two drivers.