2.9. High availability support The syslog-ng Premium Edition 7 Administrator Guide
Multiple syslog-ng servers can be run in fail-over mode. The syslog-ng application does not include any internal support for this, as clustering support must be implemented on the operating system level. A tool that can be used to create UNIX clusters is Heartbeat (for details, see this page).
Balabit also has a log server appliance called syslog-ng Store Box that supports high-availability. For details, see the syslog-ng Store Box Product Page.