Signal Handling

A brief description of the signals handled by Gunicorn. We also document the signals used internally by Gunicorn to communicate with the workers.

Master process

  • QUIT, INT: Quick shutdown
  • TERM: Graceful shutdown. Waits for workers to finish their current requests up to the graceful timeout.
  • HUP: Reload the configuration, start the new worker processes with a new configuration and gracefully shutdown older workers. If the application is not preloaded (using the --preload option), Gunicorn will also load the new version.
  • TTIN: Increment the number of processes by one
  • TTOU: Decrement the number of processes by one
  • USR1: Reopen the log files
  • USR2: Upgrade the Gunicorn on the fly. A separate TERM signal should be used to kill the old process. This signal can also be used to use the new versions of pre-loaded applications.
  • WINCH: Gracefully shutdown the worker processes when Gunicorn is daemonized.

Worker process

Sending signals directly to the worker processes should not normally be needed. If the master process is running, any exited worker will be automatically respawned.

  • QUIT, INT: Quick shutdown
  • TERM: Graceful shutdown
  • USR1: Reopen the log files

Reload the configuration

The HUP signal can be used to reload the Gunicorn configuration on the fly.

2013-06-29 06:26:55 [20682] [INFO] Handling signal: hup
2013-06-29 06:26:55 [20682] [INFO] Hang up: Master
2013-06-29 06:26:55 [20703] [INFO] Booting worker with pid: 20703
2013-06-29 06:26:55 [20702] [INFO] Booting worker with pid: 20702
2013-06-29 06:26:55 [20688] [INFO] Worker exiting (pid: 20688)
2013-06-29 06:26:55 [20687] [INFO] Worker exiting (pid: 20687)
2013-06-29 06:26:55 [20689] [INFO] Worker exiting (pid: 20689)
2013-06-29 06:26:55 [20704] [INFO] Booting worker with pid: 20704

Sending a HUP signal will reload the configuration, start the new worker processes with a new configuration and gracefully shutdown older workers. If the application is not preloaded (using the --preload option), Gunicorn will also load the new version.

Upgrading to a new binary on the fly

If you need to replace the Gunicorn binary with a new one (when upgrading to a new version or adding/removing server modules), you can do it without any service downtime - no incoming requests will be lost. Preloaded applications will also be reloaded.

First, replace the old binary with a new one, then send the USR2 signal to the master process. It renames its .pid file to .oldbin (e.g. /var/run/gunicorn.pid.oldbin), then executes a new binary, which in turn starts a new master process and the new worker processes:

  PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND
20844 benoitc   20   0 54808  11m 3352 S   0.0  0.1   0:00.36 gunicorn: master [test:app]
20849 benoitc   20   0 54808 9.9m 1500 S   0.0  0.1   0:00.02 gunicorn: worker [test:app]
20850 benoitc   20   0 54808 9.9m 1500 S   0.0  0.1   0:00.01 gunicorn: worker [test:app]
20851 benoitc   20   0 54808 9.9m 1500 S   0.0  0.1   0:00.01 gunicorn: worker [test:app]
20854 benoitc   20   0 55748  12m 3348 S   0.0  0.2   0:00.35 gunicorn: master [test:app]
20859 benoitc   20   0 55748  11m 1500 S   0.0  0.1   0:00.01 gunicorn: worker [test:app]
20860 benoitc   20   0 55748  11m 1500 S   0.0  0.1   0:00.00 gunicorn: worker [test:app]
20861 benoitc   20   0 55748  11m 1500 S   0.0  0.1   0:00.01 gunicorn: worker [test:app]

At this point, two instances of Gunicorn are running, handling the incoming requests together. To phase the old instance out, you have to send the WINCH signal to the old master process, and its worker processes will start to gracefully shut down.

At this point you can still revert to the old server because it hasn’t closed its listen sockets yet, by following these steps:

  • Send the HUP signal to the old master process - it will start the worker processes without reloading a configuration file
  • Send the TERM signal to the new master process to gracefully shut down its worker processes
  • Send the QUIT signal to the new master process to force it quit

If for some reason the new worker processes do not quit, send the KILL signal to them after the new master process quits, the old master process removes .oldbin suffix from its .pid file, and everything is exactly as before the upgrade attempt.

If an update is successful and you want to keep the new server, send the TERM signal to the old master process to leave only the new server running:

  PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND
20854 benoitc   20   0 55748  12m 3348 S   0.0  0.2   0:00.45 gunicorn: master [test:app]
20859 benoitc   20   0 55748  11m 1500 S   0.0  0.1   0:00.02 gunicorn: worker [test:app]
20860 benoitc   20   0 55748  11m 1500 S   0.0  0.1   0:00.02 gunicorn: worker [test:app]
20861 benoitc   20   0 55748  11m 1500 S   0.0  0.1   0:00.01 gunicorn: worker [test:app]