python supervisord program dependency

Question

I have [program:A], [program:B] in my supervisord.conf

B depend A, means:

A should start before B.

How to ensure this by supervisor?

Answer 1

supervisord does not directly support dependencies. Your options instead are:

• Use priorities. Set priority for A to a low value and it'll be started before B, and shut down after B. The default value for priority is 999.

  If you put the two programs into one group as well, that'd let you start and stop them in tandem, with the priorities regulating their start and stop order.

• Write an event listener that listens for PROCESS_STATE STARTING-to-RUNNING transition and STOPPING events for A, then instruct supervisord to start and stop B according to those events. Have A autostart, but disable autostarting for B, so that the event handler controls it.

Answer 2

If you want to take a shortcut, and skip reading the documentation about event listeners and skip modifying your programs so they understand events, then:

Instead of starting program B (which depends on A) directly, you could start a Bash script that sleeps until A has been started, and then starts B. For example, if you have a PostgreSQL database and a server that shouldn't start before PostgreSQL:

[program:server] autorestart=true command=/.../start-server.sh  [program:postgres] user=postgres autorestart=true command=/usr/lib/postgresql/9.3/bin/postgres ... 

And then inside start-server.sh:

#!/bin/bash  # Wait until PostgreSQL started and listens on port 5432. while [ -z "`netstat -tln | grep 5432`" ]; do   echo 'Waiting for PostgreSQL to start ...'   sleep 1 done echo 'PostgreSQL started.'  # Start server. echo 'Starting server...' /.../really-start-the-server