  Providing an early warning of galactic supernova explosions from neutrino
signals is important in studying supernova dynamics and neutrino physics. A
dedicated supernova trigger system has been designed and installed in the data
acquisition system at Daya Bay and integrated into the worldwide Supernova
Early Warning System (SNEWS). Daya Bay's unique feature of eight
identically-designed detectors deployed in three separate experimental halls
makes the trigger system naturally robust against cosmogenic backgrounds,
enabling a prompt analysis of online triggers and a tight control of the
false-alert rate. The trigger system is estimated to be fully sensitive to
1987A-type supernova bursts throughout most of the Milky Way. The significant
gain in sensitivity of the eight-detector configuration over a mass-equivalent
single detector is also estimated. The experience of this online trigger system
is applicable to future projects with spatially distributed detectors.
