The Alert Daemon: alertd

The Alert Daemon (alertd) handles the connection with the Gamma-ray Burst Coordinate Network (GCN) to receive burst triggers. alertd acts as a TCP/IP socket server on a port number registered with Scott Bartlemy at Goddard. The port is configurable and site-specific. On start-up, a permanent socket connection with the GCN is established. Each minute the GCN sends a special IM_ALIVE packet. If several minutes pass without receipt of an IM_ALIVE packet, the socket connection is shut down and tries to reestablish itself. In a separate thread, alertd runs a simulated GCN server. This server listens for a temporary socket connection on a configurable port, and can be used to test the response of the daq system to GRB triggers. A program called simalert can be used to mimic GCN packets and establish a socket connection with the daq computer on this second port.

Upon receipt of a GCN packet (via the real GCN or the simulated GCN), alertd parses the packet to determine the GRB serial number, time, and position, as well as monitoring flags. If the new packet has the same type and serial number as the previous packet, it is assumed to be a glitch and is logged and ignored. If the packet passes this cut, the alert type is then checked against a configurable hash table to determine the response priority. Simulated alerts are automatically given lower priority than real GCN alerts. If no alert is currently running, or if the new alert has a higher priority than the current running alert, a SIG_ROTSE is issued and the GRB position is sent to the scheduler for immediate response. It is up to the scheduler to determine whether the burst is currently visible, and if not, to turn off the alert mode in the daq system.