If a client process fails to heartbeat to GAB, the process is killed. If the process hangs in the kernel and cannot be killed, GAB halts the system. If the -k option is used in the gabconfig command, GAB tries to kill the client process until successful, which may have an impact on the entire cluster. If the -b option is used in gabconfig, GAB does not try to kill the client process. Instead, it panics the system when the client process fails to heartbeat. This option cannot be turned off once set.

HAD heartbeats with GAB at regular intervals. The heartbeat timeout is specified by HAD when it registers with GAB; the default is 15 seconds. If HAD gets stuck within the kernel and cannot heartbeat with GAB within the specified timeout, GAB tries to kill HAD by sending a SIGABRT signal. If it does not succeed, GAB sends a SIGKILL and closes the port. By default, GAB tries to kill HAD five times before closing the port. The number of times GAB tries to kill HAD is a kernel tunable parameter, gab_kill_ntries, and is configurable. The minimum value for this tunable is 3 and the maximum is 10.

This is an indication to other nodes that HAD on this node has been killed. Should HAD recover from its stuck state, it first processes pending signals. Here it will receive the SIGKILL first and get killed.

After sending a SIGKILL, GAB waits for a specific amount of time for HAD to get killed. If HAD survives beyond this time limit, GAB panics the system. This time limit is a kernel tunable parameter, gab_isolate_time and is configurable. The minimum value for this timer is 16 seconds and maximum is 4 minutes.