github.com/rs/seamless (go): Package seamless implements a seamless restart strategy for daemons monitored by a service supervisor expecting non-forking daemons like daemontools, runit, systemd etc.  The seamless strategy is to fully rely on the service supervisor to restart the daemon, while providing to the daemon the full control of the restart process. To achieve this, seamless duplicates the daemon at startup in order to establish a supervisor -> launcher -> daemon relationship. The launcher is the first generation of the daemon hijacked by seamless to act as a circuit breaker between the supervisor and the supervised process.  This way, when the supervisor sends a TERM signal to stop the daemon, the launcher intercepts the signal and send an USR2 signal to its child (the actual daemon). In the daemon, seamless intercepts the USR2 signals to initiate the first stage of the seamless restart.  During the first stage, the daemon prepare itself to welcome a new version of itself by creating a PID file (see below) and by for instance closing file descriptors. At this point, the daemon is still supposed to accept requests. Once read, seamless make it send a CHLD signal back to the launcher (its parent). Upon reception, the launcher, immediately die, cutting to link between the supervisor and the daemon, making the supervisor attempting a restart of the daemon while current daemon is still running, detached and unsupervised.  Once the supervisor restarted the daemon, the daemon can start serving traffic in place of the old (still running) daemon by rebinding sockets using SO_REUSEPORT for instance (see different strategies in examples/). This is the second stage of the seamless restart. When ready, the new daemon calls seamless.Started which will look for a PID file, and if found, will send a TERM signal to the old daemon using the PID found in this file.  When the old daemon receives this TERM signal, the third and last stage of the seamless restart is engaged. The OnShutdown function is called so the daemon can gracefully shutdown using Go 1.8 http graceful Shutdown method for instance. This stage can last as long as you decide. When done, the old process can exit in order to conclude the seamless restart.  Seamless does not try to implement the actual graceful shutdown or to manage sockets migration. This task is left to the caller. See the examples directory for different implementations.
Keywords: daemon|golang