Subprocesses

supervisord‘s primary purpose is to create and manage processes based on data in its configuration file. It does this by creating subprocesses. Each subprocess spawned by supervisor is managed for the entirety of its lifetime by supervisord (supervisord is the parent process of each process it creates). When a child dies, supervisor is notified of its death via the SIGCHLD signal, and it performs the appropriate operation.

Nondaemonizing of Subprocesses

Programs meant to be run under supervisor should not daemonize themselves. Instead, they should run in the foreground. They should not detach from the terminal from which they are started.

The easiest way to tell if a program will run in the foreground is to run the command that invokes the program from a shell prompt. If it gives you control of the terminal back, but continues running, it’s daemonizing itself and that will almost certainly be the wrong way to run it under supervisor. You want to run a command that essentially requires you to press Ctrl-C to get control of the terminal back. If it gives you a shell prompt back after running it without needing to press Ctrl-C, it’s not useful under supervisor. All programs have options to be run in the foreground but there’s no “standard way” to do it; you’ll need to read the documentation for each program.

Below are configuration file examples that are known to start common programs in “foreground” mode under Supervisor.

Examples of Program Configurations

Here are some “real world” program configuration examples:

Apache 2.2.6

[program:apache2]
command=/path/to/httpd -c "ErrorLog /dev/stdout" -DFOREGROUND
redirect_stderr=true

Two Zope 2.X instances and one ZEO server

[program:zeo]
command=/path/to/runzeo
priority=1

[program:zope1]
command=/path/to/instance/home/bin/runzope
priority=2
redirect_stderr=true

[program:zope2]
command=/path/to/another/instance/home/bin/runzope
priority=2
redirect_stderr=true

Postgres 8.X

[program:postgres]
command=/path/to/postmaster
; we use the "fast" shutdown signal SIGINT
stopsignal=INT
redirect_stderr=true

OpenLDAP slapd

[program:slapd]
command=/path/to/slapd -f /path/to/slapd.conf -h ldap://0.0.0.0:8888
redirect_stderr=true

Other Examples

Other examples of shell scripts that could be used to start services under supervisord can be found at http://www.thedjbway.org/services.html. These examples are actually for daemontools but the premise is the same for supervisor.

Another collection of recipes for starting various programs in the foreground is available from http://smarden.org/runit/runscripts.html.

pidproxy Program

Some processes (like mysqld) ignore signals sent to the actual process which is spawned by supervisord. Instead, a “special” thread/process is created by these kinds of programs which is responsible for handling signals. This is problematic because supervisord can only kill a process which it creates itself. If a process created by supervisord creates its own child processes, supervisord cannot kill them.

Fortunately, these types of programs typically write a “pidfile” which contains the “special” process’ PID, and is meant to be read and used in order to kill the process. As a workaround for this case, a special pidproxy program can handle startup of these kinds of processes. The pidproxy program is a small shim that starts a process, and upon the receipt of a signal, sends the signal to the pid provided in a pidfile. A sample configuration program entry for a pidproxy-enabled program is provided below.

[program:mysql]
command=/path/to/pidproxy /path/to/pidfile /path/to/mysqld_safe

The pidproxy program is put into your configuration’s $BINDIR when supervisor is installed (it is a “console script”).

Subprocess Environment

Subprocesses will inherit the environment of the shell used to start the supervisord program. Several environment variables will be set by supervisord itself in the child’s environment also, including SUPERVISOR_ENABLED (a flag indicating the process is under supervisor control), SUPERVISOR_PROCESS_NAME (the config-file-specified process name for this process) and SUPERVISOR_GROUP_NAME (the config-file-specified process group name for the child process).

These environment variables may be overridden within the [supervisord] section config option named environment (applies to all subprocesses) or within the per- [program:x] section environment config option (applies only to the subprocess specified within the [program:x] section). These “environment” settings are additive. In other words, each subprocess’ environment will consist of:

The environment variables set within the shell used to start supervisord...

... added-to/overridden-by ...

... the environment variables set within the “environment” global
config option ...

... added-to/overridden-by ...

... supervisor-specific environment variables
(SUPERVISOR_ENABLED, SUPERVISOR_PROCESS_NAME, SUPERVISOR_GROUP_NAME) ..

... added-to/overridden-by ...

... the environment variables set within the per-process
“environment” config option.

No shell is executed by supervisord when it runs a subprocess, so environment variables such as USER, PATH, HOME, SHELL, LOGNAME, etc. are not changed from their defaults or otherwise reassigned. This is particularly important to note when you are running a program from a supervisord run as root with a user= stanza in the configuration. Unlike cron, supervisord does not attempt to divine and override “fundamental” environment variables like USER, PATH, HOME, and LOGNAME when it performs a setuid to the user defined within the user= program config option. If you need to set environment variables for a particular program that might otherwise be set by a shell invocation for a particular user, you must do it explicitly within the environment= program config option. An example of setting these enviroment variables is as below.

[program:apache2]
command=/home/chrism/bin/httpd -c "ErrorLog /dev/stdout" -DFOREGROUND
user=chrism
environment=HOME="/home/chrism",USER="chrism"

Process States

A process controlled by supervisord will be in one of the below states at any given time. You may see these state names in various user interface elements in clients.

STOPPED (0)

The process has been stopped due to a stop request or has never been started.

STARTING (10)

The process is starting due to a start request.

RUNNING (20)

The process is running.

BACKOFF (30)

The process entered the STARTING state but subsequently exited too quickly to move to the RUNNING state.

STOPPING (40)

The process is stopping due to a stop request.

EXITED (100)

The process exited from the RUNNING state (expectedly or unexpectedly).

FATAL (200)

The process could not be started successfully.

UNKNOWN (1000)

The process is in an unknown state (supervisord programming error).

Each process run under supervisor progresses through these states as per the following directed graph.

Subprocess State Transition Graph

Subprocess State Transition Graph

A process is in the STOPPED state if it has been stopped adminstratively or if it has never been started.

When an autorestarting process is in the BACKOFF state, it will be automatically restarted by supervisord. It will switch between STARTING and BACKOFF states until it becomes evident that it cannot be started because the number of startretries has exceeded the maximum, at which point it will transition to the FATAL state. Each start retry will take progressively more time.

When a process is in the EXITED state, it will automatically restart:

  • never if its autorestart parameter is set to false.
  • unconditionally if its autorestart parameter is set to true.
  • conditionally if its autorestart parameter is set to unexpected. If it exited with an exit code that doesn’t match one of the exit codes defined in the exitcodes configuration parameter for the process, it will be restarted.

A process automatically transitions from EXITED to RUNNING as a result of being configured to autorestart conditionally or unconditionally. The number of transitions between RUNNING and EXITED is not limited in any way: it is possible to create a configuration that endlessly restarts an exited process. This is a feature, not a bug.

An autorestarted process will never be automatically restarted if it ends up in the FATAL state (it must be manually restarted from this state).

A process transitions into the STOPPING state via an administrative stop request, and will then end up in the STOPPED state.

A process that cannot be stopped successfully will stay in the STOPPING state forever. This situation should never be reached during normal operations as it implies that the process did not respond to a final SIGKILL signal sent to it by supervisor, which is “impossible” under UNIX.

State transitions which always require user action to invoke are these:

FATAL -> STARTING

RUNNING -> STOPPING

State transitions which typically, but not always, require user action to invoke are these, with exceptions noted:

STOPPED -> STARTING (except at supervisord startup if process is configured to autostart)

EXITED -> STARTING (except if process is configured to autorestart)

All other state transitions are managed by supervisord automatically.